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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 31, 2018
Entity Information [Line Items]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	0001549922
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Trading Symbol	SMLP
Common units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		73,355,775
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,490,999

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 8,212	$ 1,430
Accounts receivable	80,166	72,301
Other current assets	2,594	4,327
Total current assets	90,972	78,058
Property, plant and equipment, net	1,887,759	1,795,129
Intangible assets, net	286,665	301,345
Goodwill	16,211	16,211
Investment in equity method investees	670,827	690,485
Other noncurrent assets	13,144	13,565
Total assets	2,965,578	2,894,793
Current liabilities:
Trade accounts payable	21,698	16,375
Accrued expenses	19,433	12,499
Due to affiliate	91	1,088
Deferred revenue	10,778	4,000
Ad valorem taxes payable	6,504	8,329
Accrued interest	12,259	12,310
Accrued environmental remediation	3,522	3,130
Other current liabilities	7,412	11,258
Total current liabilities	81,697	68,989
Long-term debt	1,147,005	1,051,192
Deferred Purchase Price Obligation	453,922	362,959
Noncurrent deferred revenue	38,119	12,707
Noncurrent accrued environmental remediation	1,617	2,214
Other noncurrent liabilities	6,410	7,063
Total liabilities	1,728,770	1,505,124
Commitments and contingencies (Note 16)
Series A Preferred Units (300,000 units issued and outstanding at June 30, 2018 and December 31, 2017)	293,616	294,426
Common limited partner capital (73,355,775 units issued and outstanding at June 30, 2018 and 73,085,996 units issued and outstanding at December 31, 2017)	907,099	1,056,510
General Partner interests (1,490,999 units issued and outstanding at June 30, 2018 and December 31, 2017)	25,137	27,920
Noncontrolling interest	10,956	10,813
Total partners' capital	1,236,808	1,389,669
Total liabilities and partners' capital	$ 2,965,578	$ 2,894,793

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Series A preferred unitholders, issued	300,000	300,000
Series A preferred unitholders, outstanding	300,000	300,000
Common limited partner capital (in shares), issued	73,355,775	73,085,996
Common limited partner capital (in shares), outstanding	73,355,775	73,085,996
General partner interests (in shares), issued	1,490,999	1,490,999
General partner interests (in shares), outstanding	1,490,999	1,490,999

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 24,466	$ 24,016	$ 49,070	$ 47,708
General and administrative	13,484	12,949	27,926	27,081
Depreciation and amortization	26,784	28,688	53,461	57,257
Transaction costs	0	119	0	119
Loss (gain) on asset sales, net	62	67	(12)	70
Long-lived asset impairment	587	3	587	287
Total costs and expenses	89,767	74,941	175,702	150,673
Other income	27	64	20	135
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt	0	0	0	(22,020)
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(45,699)	14,420	(51,101)	14,945
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Net income attributable to noncontrolling interest	58	89	143	191
Net (loss) income attributable to SMLP	(49,971)	11,157	(53,901)	10,472
Less net income attributable to General Partner, including IDRs	1,140	2,351	3,198	4,443
Net (loss) income attributable to limited partners	(51,111)	8,806	(57,099)	6,029
Series A Preferred Units
Costs and expenses:
Net (loss) income attributable to limited partners	7,125	0	14,250	0
Common units
Costs and expenses:
Net (loss) income attributable to limited partners	$ (58,236)	$ 8,806	$ (71,349)	$ 6,029
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Diluted (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Weighted-average limited partner units outstanding:
Basic (shares)	73,356	72,532	73,245	72,341
Diluted (shares)	73,356	72,842	73,245	72,708
Gathering Services and Related Fees
Revenues:
Total revenues	$ 89,585	$ 84,801	$ 173,946	$ 202,814
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Other Revenues
Revenues:
Total revenues	$ 6,707	$ 6,396	$ 13,549	$ 13,068

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) $ in Thousands	Total	Adjustment	Noncontrolling interest	Noncontrolling interest Adjustment	Common	Common Adjustment	General Partner	General Partner Adjustment	Limited partners, Series A Preferred Units	Limited partners, Series A Preferred Units Adjustment
Beginning balance at Dec. 31, 2016	$ 1,169,673		$ 11,247		$ 1,129,132		$ 29,294
Net income (loss)	10,663		191		6,029		4,443
Distributions to unitholders	(89,029)		0		(83,044)		(5,985)
Unit-based compensation	3,919		0		3,919		0
Tax withholdings on vested SMLP LTIP awards	(2,051)		0		(2,051)		0
ATM Program issuances, net of costs	17,259		0		17,259		0
Contribution from General Partner	465		0		0		465
Ending balance at Jun. 30, 2017	1,110,899		11,438		1,071,244		28,217
January 1, 2018 impact of Topic 606day 1 adoption	4,214		0		4,130		84		$ 0
Beginning balance at Dec. 31, 2017	1,389,669	$ 1,393,883	10,813	$ 10,813	1,056,510	$ 1,060,640	27,920	$ 28,004	294,426	$ 294,426
Net income (loss)	(53,758)		143		(71,349)		3,198		14,250
Distributions to unitholders	(104,519)		0		(84,204)		(6,065)		(14,250)
Unit-based compensation	3,983		0		3,983		0		0
Tax withholdings on vested SMLP LTIP awards	(1,840)		0		(1,840)		0		0
Other	(941)		0		(131)		0		(810)
Ending balance at Jun. 30, 2018	$ 1,236,808		$ 10,956		$ 907,099		$ 25,137		$ 293,616

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

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+ Details

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income (loss)	$ (53,758)	$ 10,663
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization	53,160	56,955
Amortization of debt issuance costs	2,086	2,072
Deferred Purchase Price Obligation	90,963	15,825
Unit-based and noncash compensation	4,223	3,999
Loss from equity method investees	2,534	4,041
Distributions from equity method investees	17,124	18,003
Loss (gain) on asset sales, net	(12)	70
Long-lived asset impairment	587	287
Early extinguishment of debt	0	22,020
Write-off of debt issuance costs	0	302
Changes in operating assets and liabilities:
Accounts receivable	(6,265)	41,527
Trade accounts payable	(3,483)	(4,372)
Accrued expenses	6,934	(3,111)
Due (to) from affiliate	(997)	212
Deferred revenue, net	3,281	(39,671)
Ad valorem taxes payable	(1,825)	(3,293)
Accrued interest	(51)	(468)
Accrued environmental remediation, net	(1,805)	(924)
Other, net	(2,647)	(2,796)
Net cash provided by operating activities	110,049	121,341
Cash flows from investing activities:
Capital expenditures	(90,394)	(45,912)
Proceeds from asset sale	496	0
Contributions to equity method investees	0	(15,649)
Other, net	(306)	(521)
Net cash provided by (used in) investing activities	(90,204)	(62,082)
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes	0	(17,913)
Proceeds from ATM Program common unit issuances, net of costs	0	17,259
Contribution from General Partner	0	465
Issuance of senior notes	0	500,000
Tender and redemption of senior notes	0	(300,000)
Other, net	(3,423)	(2,268)
Net cash provided by (used in) financing activities	(13,063)	(64,099)
Net change in cash and cash equivalents	6,782	(4,840)
Cash and cash equivalents, beginning of period	1,430	7,428
Cash and cash equivalents, end of period	8,212	2,588
Supplemental cash flow disclosures:
Cash interest paid	30,962	33,382
Less capitalized interest	3,085	918
Interest paid (net of capitalized interest)	27,877	32,464
Cash paid for taxes	175	0
Noncash investing and financing activities
Capital expenditures in trade accounts payable (period-end accruals)	20,598	6,869
Capital expenditures relating to contributions in aid of construction for Topic 606 day 1 adoption	33,123	0
Common units
Cash flows from financing activities:
Distributions to unitholders	(90,269)	(89,029)
Series A Preferred Units
Cash flows from financing activities:
Distributions to unitholders	$ (14,250)	$ 0

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

1. ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

Organization. SMLP, a Delaware limited partnership, was formed in May 2012 and began operations in October 2012 in connection with its IPO of common limited partner units. SMLP is a growth-oriented limited partnership focused on developing, owning and operating midstream energy infrastructure assets that are strategically located in the core producing areas of unconventional resource basins, primarily shale formations, in the continental United States. Our business activities are conducted through various operating subsidiaries, each of which is owned or controlled by our wholly owned subsidiary holding company, Summit Holdings, a Delaware limited liability company. References to the "Partnership," "we," or "our" refer collectively to SMLP and its subsidiaries.

The General Partner, a Delaware limited liability company, manages our operations and activities. Summit Investments, a Delaware limited liability company, is the ultimate owner of our General Partner and has the right to appoint the entire Board of Directors. Summit Investments is controlled by Energy Capital Partners.

In addition to its approximate 2% general partner interest in SMLP (including the IDRs), Summit Investments has indirect ownership interests in our common units. As of June 30, 2018, Summit Investments beneficially owned 25,854,581 SMLP common units and a subsidiary of Energy Capital Partners directly owned 5,915,827 SMLP common units.

Neither SMLP nor its subsidiaries have any employees. All of the personnel that conduct our business are employed by Summit Investments, but these individuals are sometimes referred to as our employees.

Business Operations. We provide natural gas gathering, treating and processing services as well as crude oil and produced water gathering services pursuant to primarily long-term and fee-based agreements with our customers. Our results are driven primarily by the volumes of natural gas that we gather, treat, compress and process as well as by the volumes of crude oil and produced water that we gather. We are the owner-operator of or have significant ownership interests in the following gathering systems:

•

Summit Utica, a natural gas gathering system operating in the Appalachian Basin, which includes the Utica and Point Pleasant shale formations in southeastern Ohio;

•

Ohio Gathering, a natural gas gathering system and a condensate stabilization facility operating in the Appalachian Basin, which includes the Utica and Point Pleasant shale formations in southeastern Ohio;

•

Polar and Divide, crude oil and produced water gathering systems and transmission pipelines located in the Williston Basin, which includes the Bakken and Three Forks shale formations in northwestern North Dakota;

•

Tioga Midstream, crude oil, produced water and associated natural gas gathering systems operating in the Williston Basin, which includes the Bakken and Three Forks shale formations in northwestern North Dakota;

•

Bison Midstream, an associated natural gas gathering system operating in the Williston Basin, which includes the Bakken and Three Forks shale formations in northwestern North Dakota;

•

Grand River, a natural gas gathering and processing system located in the Piceance Basin, which includes the Mesaverde formation and the Mancos and Niobrara shale formations in western Colorado and eastern Utah;

•

Niobrara G&P, an associated natural gas gathering and processing system operating in the DJ Basin, which includes the Niobrara and Codell shale formations in northeastern Colorado;

•

DFW Midstream, a natural gas gathering system operating in the Fort Worth Basin, which includes the Barnett Shale formation in north-central Texas;

•

Mountaineer Midstream, a natural gas gathering system operating in the Appalachian Basin, which includes the Marcellus Shale formation in northern West Virginia; and

•

Summit Permian, an associated natural gas gathering and processing system and interstate natural gas transportation service under development in the northern Delaware Basin in southeastern New Mexico.

In February 2016, the Partnership and SMP Holdings, a wholly owned subsidiary of Summit Investments, entered into a contribution agreement (the "Contribution Agreement") pursuant to which SMP Holdings agreed to contribute to the Partnership substantially all of its limited partner interest in OpCo, a Delaware limited partnership that owns (i) 100% of the issued and outstanding membership interests of Summit Utica, Meadowlark Midstream and Tioga Midstream (collectively, the "Contributed Entities"), each a limited liability company and (ii) a 40% ownership interest in each of OGC and OCC (collectively with OpCo and the Contributed Entities, the “2016 Drop Down Assets”)(the “2016 Drop Down”). The 2016 Drop Down closed in March 2016; concurrent therewith, a subsidiary of Summit Investments retained a 1% noncontrolling interest in OpCo.

In December 2017, the Niobrara associated natural gas gathering and processing assets held by Meadowlark Midstream were contributed to Summit Niobrara, a newly formed entity. Concurrent with this contribution (i) a subsidiary of SMLP purchased the remaining 1% ownership interest in Summit Niobrara held by Summit Epping, LLC; and (ii) 100% of the ownership interests in Summit Niobrara were contributed to Grand River Gathering, LLC (“Grand River”), after which Summit Niobrara became a wholly-owned subsidiary of Grand River.

Summit Marketing, a Delaware limited liability company and a wholly owned subsidiary of Summit Holdings, manages OpCo, a Delaware limited liability partnership, and provides natural gas and crude oil marketing services in and around our gathering systems.

Presentation and Consolidation. We prepare our unaudited condensed consolidated financial statements in accordance with GAAP as established by the FASB. We make estimates and assumptions that affect the reported amounts of assets and liabilities at the balance sheet dates, including fair value measurements, the reported amounts of revenue and expense and the disclosure of contingencies. Although management believes these estimates are reasonable, actual results could differ from its estimates.

These unaudited condensed consolidated financial statements have been prepared pursuant to the rules and the regulations of the SEC. Certain information and note disclosures normally included in the annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to those rules and regulations. We believe that the disclosures made are adequate to make the information not misleading. In the opinion of management, the unaudited condensed consolidated financial statements contain all adjustments, including normal recurring adjustments, which are necessary to fairly present the unaudited condensed consolidated balance sheet as of June 30, 2018, the unaudited condensed consolidated statements of operations for the three and six months ended June 30, 2018 and 2017 and the unaudited condensed consolidated statements of partners’ capital and cash flows for the six months ended June 30, 2018 and 2017. The balance sheet at December 31, 2017 included herein was derived from our audited financial statements, but does not include all disclosures required by GAAP. These unaudited condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto that are included in our annual report on Form 10-K for the year ended December 31, 2017, as filed with the SEC on February 26, 2018 (the "2017 Annual Report"). The results of operations for an interim period are not necessarily indicative of results expected for a full year.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Except for the changes below, there have been no changes to our significant accounting policies since December 31, 2017.

Recent Accounting Pronouncements. Accounting standard setters frequently issue new or revised accounting rules. We review new pronouncements to determine the impact, if any, on our financial statements. Accounting standards that have or could possibly have a material effect on our financial statements are discussed below.

Recently Adopted Accounting Pronouncements. We have recently adopted the following accounting pronouncements:

•

ASU No. 2014-09 Revenue from Contracts with Customers (“Topic 606”). We adopted Topic 606 with a date of initial application of January 1, 2018. We applied Topic 606 by recognizing the cumulative effect of initially applying Topic 606 as an adjustment to the opening balance of partners’ capital at January 1, 2018. The comparative information has not been adjusted and is reported under the accounting standards in effect for those periods.

For contracts where we perform gathering services and earn a per-unit fee which is recognized at a point in time, revenue is recognized over time as the service is performed and results in revenue recognition materially consistent with historical GAAP. In addition, our contracts generally contain forms of variable consideration, which will likely be constrained as the volumes are susceptible to factors outside of our control and influence. As a result of applying the constraint guidance, timing of revenue recognition will be materially consistent with historical GAAP.

Prior to the adoption of Topic 606, contributions in aid of construction were recognized as a reduction to our cost basis of property, plant and equipment and facility fees were recognized as revenue when the amounts were billed. Upon adoption of Topic 606, the contributions in aid of construction amounts previously received were capitalized to property, plant and equipment, net of any accumulated depreciation, and will be depreciated over the remaining useful lives. Any future contributions in aid of construction will be recognized as revenue over the remaining term of the respective contract in accordance with Topic 606. Additionally, facility fees will be deferred and recognized over the contract term.

There are certain percent-of-proceeds contracts within our Williston Basin reportable segment where we previously recognized revenue for services provided to producers in gathering services and related fees. Such amounts which were previously presented gross in gathering services and related fees are presented net within cost of natural gas and NGLs. This change did not have any impact on our net income (loss), cash flows, or the amount we present as segment adjusted EBITDA.

For contracts containing MVC arrangements with banking mechanisms we previously deferred revenue. Under Topic 606, the recognition of revenue was accelerated. This acceleration totaled $16.7 million and is included in the Topic 606 day one adjustment amounts below in deferred revenue.

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Impact on financial statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

•

ASU No. 2017-04 Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"). ASU 2017-04 simplifies the subsequent measurement of goodwill by, among other things, eliminating step two from the goodwill impairment test. ASU 2017-04 is effective for public companies for fiscal years beginning after December 15, 2019 and it specifies the amendments in ASU 2017-04 should be applied on a prospective basis. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. We adopted the provisions of ASU 2017-04 effective January 1, 2018. The adoption of this standard had no impact on our consolidated financial statements.

Accounting Pronouncements Pending Adoption. We have not yet adopted the following accounting pronouncements as of June 30, 2018:

•

ASU No. 2016-02 Leases (Topic 842) ("ASU 2016-02"). ASU 2016-02 requires that lessees recognize all leases on the balance sheet, with the exception of short-term leases. A lease liability will be recorded for the obligation of a lessee to make lease payments arising from a lease. A right-of-use asset will be recorded which represents the lessee’s right to use, or to control the use of, a specified asset for a lease term. ASU 2016-02 is effective for public companies for fiscal years beginning after December 15, 2018, and requires the modified retrospective approach for transition. We are currently evaluating the provisions of ASU 2016-02 to determine its impact on our financial statements and related disclosures and will adopt its provisions effective January 1, 2019. We expect to utilize certain practical expedients including (i) not being required to reassess whether any expired or existing contracts are or contain leases; (ii) not being required to reassess the lease classification for any expired or existing leases (that is, all existing leases that were classified as operating leases in accordance with Topic 840 will be classified as operating leases, and all existing leases that were classified as capital leases in accordance with Topic 840 will be classified as finance leases); and (iii) not being required to reassess initial direct costs for any existing leases.

•

ASU No. 2018-01 Leases: Land Easement Practical Expedient for Transition to Topic 842 (“ASU 2018-01”). ASU 2018-01 provides an optional transition practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance in Topic 840. Upon adoption of Topic 842, an entity that elects this practical expedient should evaluate new or modified land easements under Topic 842 beginning at the date the entity adopts Topic 842. We expect to adopt the optional transition practical expedient of ASU 2018-01 effective January 1, 2019.

REVENUE

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

REVENUE

3. REVENUE

The majority of our revenue is derived from long-term and fee-based contracts with original terms of up to 25 years. We account for revenue in accordance with Topic 606 which we adopted on January 1, 2018, using the modified retrospective method. See Note 2 for further discussion of the adoption, including the impact on our unaudited condensed consolidated financial statements.

We recognize revenue earned from fee-based gathering, treating and processing services in gathering services and related fees. We also earn revenue in the Williston Basin reporting segment from the sale of physical natural gas

purchased from our customers under certain percent-of-proceeds arrangements. Under Topic 606, these gathering fee contracts are presented net within cost of natural gas and NGLs. We sell natural gas that we retain from certain DFW Midstream customers to offset the power expenses of the electric-driven compression on the DFW Midstream system. We also sell condensate retained from our gathering services at Grand River. Revenues from the sale of natural gas and condensate are recognized in natural gas, NGLs and condensate sales; the associated expense is included in operation and maintenance expense. Certain customers reimburse us for costs we incur on their behalf. We record costs incurred and reimbursed by our customers on a gross basis, with the revenue component recognized in other revenues.

The transaction price in our contracts is primarily based on the volume of natural gas, crude oil or produced water transferred by our gathering systems to the customer’s agreed upon delivery point multiplied by the contractual rate. For contracts that include MVCs, variable consideration up to the MVC will be included in the transaction price. For contracts that do not include MVCs, we do not estimate variable consideration because the performance obligations are completed and settled on a daily basis. For contracts containing noncash consideration such as fuel received in-kind, we will measure the transaction price at the point of sale when the volume, mix and market price of the commodities are known.

We have contracts with MVCs that are variable and constrained. Contracts with MVCs are reviewed on a quarterly basis and adjustments to those estimates are made during each respective reporting period, if necessary.

The transaction price is allocated if the contract contains more than one performance obligation such as contracts that include MVCs. The transaction price allocated is based on the MVC for the applicable measurement period.

Performance obligations. The majority of our contracts have a single performance obligation which is either to provide gathering services (an integrated service) or sell natural gas, NGLs and condensate, which are both satisfied when the related natural gas, crude oil and produced water are received and transferred to an agreed upon delivery point. We also have certain contracts with multiple performance obligations. They include an option for the customer to acquire additional services such as contracts containing MVCs. These performance obligations would also be satisfied when the related natural gas, crude oil and produced water are received and transferred to an agreed upon delivery point. In these instances, we allocate the contract’s transaction price to each performance obligation using our best estimate of the standalone selling price of each service in the contract.

Performance obligations for gathering services are generally satisfied over time. We utilize either an output method (i.e., measure of progress) for guaranteed, stand-ready service contracts or an asset / system delivery time estimate for non-guaranteed, as-available service contracts.

Performance obligations for the sale of natural gas, NGLs and condensate are satisfied at a point in time. There are no significant judgments for these transactions because the customer obtains control based on an agreed upon delivery point.

Certain of our gathering and/or processing agreements provide for monthly, annual or multi-year MVCs. Under these MVCs, our customers agree to ship and/or process a minimum volume of production on our gathering systems or to pay a minimum monetary amount over certain periods during the term of the MVC. A customer must make a shortfall payment to us at the end of the contracted measurement period if its actual throughput volumes are less than its MVC for that period. Certain customers are entitled to utilize shortfall payments to offset gathering fees in one or more subsequent contracted measurement periods to the extent that such customer's throughput volumes in a subsequent contracted measurement period exceed its MVC for that contracted measurement period.

We recognize customer obligations under their MVCs as revenue and contract assets when (i) we consider it remote that the customer will utilize shortfall payments to offset gathering or processing fees in excess of its MVCs in subsequent periods; (ii) the customer incurs a shortfall in a contract with no banking mechanism or claw back provision; (iii) the customer’s banking mechanism has expired; or (iv) it is remote that the customer will use its unexercised right.

Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components.

The following table presents estimated revenue expected to be recognized during the remainder of 2018 and over the remaining contract period related to performance obligations that are unsatisfied and are comprised of estimated MVC shortfall payments.

We applied the practical expedient in paragraph 606-10-50-14 of Topic 606 for certain arrangements that we consider optional purchases (i.e., there is no enforceable obligation for the customer to make purchases) and those amounts are excluded from the table.

	2018		2019		2020		2021		2022		Thereafter
	(In thousands)
Gathering services and related fees	$	97,523	$	133,743	$	122,429	$	102,777	$	83,648	$	174,227

Revenue by Category. In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4.

	Reportable Segments
	Three months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	10,422	$	23,106	$	36,170	$	14,080	$	8,050	$	91,828	$	(2,243	)	$	89,585
Natural gas, NGLs and condensate sales		—		7,350		4,675		381		—		12,406		19,485			31,891
Other revenues		—		2,960		2,147		1,694		—		6,801		(94	)		6,707
Total	$	10,422	$	33,416	$	42,992	$	16,155	$	8,050	$	111,035	$	17,148		$	128,183

	Reportable Segments
	Six months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	20,463	$	40,772	$	71,464	$	27,717	$	15,875	$	176,291	$	(2,345	)	$	173,946
Natural gas, NGLs and condensate sales		—		15,196		9,000		926		—		25,122		32,886			58,008
Other revenues		—		5,872		4,115		3,682		—		13,669		(120	)		13,549
Total	$	20,463	$	61,840	$	84,579	$	32,325	$	15,875	$	215,082	$	30,421		$	245,503

Contract balances. Contract assets relate to our rights to consideration for work completed but not billed at the reporting date and consist of the estimated MVC shortfall payments expected from our customers and unbilled activity associated with contributions in aid of construction. Contract assets are transferred to trade receivables when the rights become unconditional. The following table provides information about contract assets from contracts with customers:

	June 30, 2018
	(In thousands)
Contract assets, December 31, 2017	$	—
Net impact of Topic 606 day 1 adoption		3,514
Additions		11,523
Transfers out		(5,704	)
Contract assets, June 30, 2018	$	9,333

Current		9,333
Noncurrent		—
Contract assets, June 30, 2018	$	9,333

As of June 30, 2018, receivables with customers totaled $67.5 million and contract assets totaled $9.3 million which were included in the accounts receivable caption on the unaudited condensed consolidated balance sheet.

Contract liabilities (deferred revenue) relate to the advance consideration received from customers primarily for contributions in aid of construction. We recognize contract liabilities under these arrangements in revenue over the contract period. For the three and six months ended June 30, 2018, we recognized $3.9 million and $5.0 million of gathering services and related fees which was included in the contract liability balance as of the beginning of the period. See Note 9 for additional details.

SEGMENT INFORMATION

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

SEGMENT INFORMATION

4. SEGMENT INFORMATION

As of June 30, 2018, our reportable segments are:

•

the Utica Shale, which is served by Summit Utica;

•

Ohio Gathering, which includes our ownership interest in OGC and OCC;

•

the Williston Basin, which is served by Polar and Divide, Tioga Midstream and Bison Midstream;

•

the Piceance/DJ Basins, which is served by Grand River and Niobrara G&P;

•

the Barnett Shale, which is served by DFW Midstream; and

•

the Marcellus Shale, which is served by Mountaineer Midstream.

Each of our reportable segments provides midstream services in a specific geographic area. Our reportable segments reflect the way in which we internally report the financial information used to make decisions and allocate resources in connection with our operations.

The Ohio Gathering reportable segment includes our investment in OGC and OCC (see Note 8). Income or loss from equity method investees, as reflected on the statements of operations, solely relates to Ohio Gathering and is recognized and disclosed on a one-month lag (see Note 8). No other line items in the statements of operations or cash flows, as disclosed in the tables below, include results for our investment in Ohio Gathering.

Corporate and Other represents those results that are: (i) not specifically attributable to a reportable segment; (ii) not individually reportable; or (iii) not allocated to our reportable segments for the purpose of evaluating their performance, including certain general and administrative expense items, natural gas and crude oil marketing services, and transaction costs.

Assets by reportable segment follow.

	June 30, 2018			December 31, 2017
	(In thousands)
Assets:
Utica Shale	$	212,326		$	212,311
Ohio Gathering		670,827			690,485
Williston Basin		520,799			512,860
Piceance/DJ Basins		828,999			798,722
Barnett Shale		376,969			383,306
Marcellus Shale		213,366			217,362
Total reportable segment assets		2,823,286			2,815,046
Corporate and Other		143,450			79,996
Eliminations		(1,158	)		(249	)
Total assets	$	2,965,578		$	2,894,793

Revenues by reportable segment follow.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Revenues (1):
Utica Shale	$	10,422		$	10,456		$	20,463		$	19,252
Williston Basin		33,416			29,114			61,840			95,999
Piceance/DJ Basins		42,992			33,763			84,579			68,571
Barnett Shale		16,155			20,904			32,325			38,646
Marcellus Shale		8,050			7,365			15,875			14,269
Total reportable segments revenue		111,035			101,602			215,082			236,737
Corporate and Other		19,422			1,362			33,598			3,148
Eliminations		(2,274	)		(1,172	)		(3,177	)		(2,288	)
Total revenues	$	128,183		$	101,792		$	245,503		$	237,597

(1) Excludes revenues earned by Ohio Gathering due to equity method accounting.

Counterparties accounting for more than 10% of total revenues were as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins		10	%	*				11	%	*
Counterparty B - Piceance/DJ Basins	*				10	%	*			*
Counterparty C - Barnett Shale		11	%		11	%	*			*
Counterparty D - Utica Shale	*				10	%	*			*
Counterparty E - Williston Basin	*			*			*				21	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments.

(2) Excludes revenues earned by Ohio Gathering due to equity method accounting.

* Less than 10%

Depreciation and amortization, including the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	2,033	$	1,748	$	3,886	$	3,395
Williston Basin		5,622		8,385		11,231		16,766
Piceance/DJ Basins		12,450		12,225		25,005		24,436
Barnett Shale (2)		3,759		3,762		7,516		7,524
Marcellus Shale		2,274		2,263		4,546		4,526
Total reportable segment depreciation and amortization		26,138		28,383		52,184		56,647
Corporate and Other		496		154		976		308
Total depreciation and amortization	$	26,634	$	28,537	$	53,160	$	56,955

(1) Excludes depreciation and amortization recognized by Ohio Gathering due to equity method accounting.

(2) Includes the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues.

Cash paid for capital expenditures by reportable segment follow.

	Six months ended June 30,
	2018		2017
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	1,846	$	16,473
Williston Basin		10,966		11,085
Piceance/DJ Basins		24,827		11,934
Barnett Shale		349		(399	)
Marcellus Shale		545		407
Total reportable segment capital expenditures		38,533		39,500
Corporate and Other		51,861		6,412
Total cash paid for capital expenditures	$	90,394	$	45,912

(1) Excludes cash paid for capital expenditures by Ohio Gathering due to equity method accounting.

During the six months ended June 30, 2018, Corporate included cash paid of $1.0 million for corporate purposes; the remainder represents capital expenditures for Summit Permian.

We assess the performance of our reportable segments based on segment adjusted EBITDA. We define segment adjusted EBITDA as total revenues less total costs and expenses; plus (i) other income excluding interest income, (ii) our proportional adjusted EBITDA for equity method investees (as defined below), (iii) depreciation and amortization, (iv) adjustments related to MVC shortfall payments, (v) adjustments related to capital reimbursement activity, (vi) unit-based and noncash compensation, (vii) change in the Deferred Purchase Price Obligation fair value, (viii) early extinguishment of debt expense, (ix) impairments and (x) other noncash expenses or losses, less other noncash income or gains. We define proportional adjusted EBITDA for our equity method investees as the product of (i) total revenues less total expenses, excluding impairments and other noncash income or expense items and (ii) amortization for deferred contract costs; multiplied by our ownership interest in Ohio Gathering during the respective period.

For the purpose of evaluating segment performance, we exclude the effect of Corporate and Other revenues and expenses, such as certain general and administrative expenses (including compensation-related expenses and professional services fees), natural gas and crude oil marketing services, transaction costs, interest expense, change

in the Deferred Purchase Price Obligation fair value, early extinguishment of debt expense and income tax expense or benefit from segment adjusted EBITDA.

Segment adjusted EBITDA by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	9,223	$	9,533	$	17,938	$	17,445
Ohio Gathering		8,935		9,606		19,412		18,679
Williston Basin		19,030		17,155		35,000		34,964
Piceance/DJ Basins		27,673		27,274		56,908		56,248
Barnett Shale		11,093		12,998		20,952		25,086
Marcellus Shale		6,543		5,446		13,219		11,093
Total of reportable segments' measures of profit	$	82,497	$	82,012	$	163,429	$	163,515

A reconciliation of income or loss before income taxes and income or loss from equity method investees to total of reportable segments' measures of profit or loss follows.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Reconciliation of (loss) income before income taxes and loss from equity method investees to total of reportable segments' measures of profit:
(Loss) income before income taxes and loss from equity method investees	$	(45,699	)	$	14,420		$	(51,101	)	$	14,945
Add:
Corporate and Other		9,002			9,435			19,625			19,528
Interest expense		14,837			17,553			29,959			34,269
Early extinguishment of debt		—			—			—			22,020
Deferred Purchase Price Obligation		69,305			(5,058	)		90,963			15,825
Depreciation and amortization		26,634			28,537			53,160			56,955
Proportional adjusted EBITDA for equity method investees		8,935			9,606			19,412			18,679
Adjustments related to MVC shortfall payments		(3,542	)		5,578			(3,542	)		(23,062	)
Adjustments related to capital reimbursement activity		115			—			155			—
Unit-based and noncash compensation		2,261			1,871			4,223			3,999
Loss (gain) on asset sales, net		62			67			(12	)		70
Long-lived asset impairment		587			3			587			287
Total of reportable segments' measures of profit	$	82,497		$	82,012		$	163,429		$	163,515

For the three and six months ended June 30, 2017, we included adjustments related to MVC shortfall payments in our calculation of segment adjusted EBITDA to account for (i) the net increases or decreases in deferred revenue for MVC shortfall payments and (ii) our inclusion of expected annual or multi-year MVC shortfall payments. With respect to the impact of a net change in deferred revenue for MVC shortfall payments, we treated increases in deferred revenue balances as a favorable adjustment to segment adjusted EBITDA, while decreases in deferred revenue balances were treated as an unfavorable adjustment to segment adjusted EBITDA. We also included a proportional amount of any historical and expected MVC shortfall payments in each quarter prior to the quarter in which we actually recognized the shortfall payment.

For the three and six months ended June 30, 2018, in accordance with Topic 606, adjustments related to MVC shortfall payments are recognized in gathering services and related fees (see Note 3).

In accordance with Topic 606, contributions in aid of construction are recognized over the remaining term of the respective contract. We include adjustments related to capital reimbursement activity in our calculation of segment adjusted EBITDA to account for revenue recognized from contributions in aid of construction.

Adjustments related to MVC shortfall payments by reportable segment follow.

	Three months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Three months ended June 30, 2017
	Williston Basin		Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—	$	(1,186	)	$	—		$	(1,186	)
Expected MVC shortfall adjustments		1,982		6,522			(1,740	)		6,764
Total adjustments related to MVC shortfall payments	$	1,982	$	5,336		$	(1,740	)	$	5,578

	Six months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Six months ended June 30, 2017
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	(37,693	)	$	(1,978	)	$	—		$	(39,671	)
Expected MVC shortfall adjustments		3,964			13,067			(422	)		16,609
Total adjustments related to MVC shortfall payments	$	(33,729	)	$	11,089		$	(422	)	$	(23,062	)

PROPERTY, PLANT, AND EQUIPMENT, NET

6 Months Ended

Jun. 30, 2018

Property Plant And Equipment [Abstract]

PROPERTY, PLANT, AND EQUIPMENT, NET

5. PROPERTY, PLANT AND EQUIPMENT, NET

Details on property, plant and equipment follow.

	June 30, 2018		December 31, 2017
	(In thousands)
Gathering and processing systems and related equipment	$	2,023,832	$	1,973,722
Construction in progress		160,239		78,850
Land and line fill		11,735		11,735
Other		41,802		40,262
Total		2,237,608		2,104,569
Less accumulated depreciation		349,849		309,440
Property, plant and equipment, net	$	1,887,759	$	1,795,129

Depreciation expense and capitalized interest follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation expense	$	18,657	$	18,607	$	37,214	$	37,098
Capitalized interest		1,863		450		3,085		918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

6. AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities), all of which are subject to amortization, follow.

	June 30, 2018
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(13,128	)	$	11,067
Contract intangibles		278,448		(130,891	)		147,557
Rights-of-way		162,339		(34,298	)		128,041
Total intangible assets	$	464,982	$	(178,317	)	$	286,665

Unfavorable gas gathering contract	$	10,962	$	(10,153	)	$	809

	December 31, 2017
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(12,350	)	$	11,845
Contract intangibles		278,448		(117,821	)		160,627
Rights-of-way		159,986		(31,113	)		128,873
Total intangible assets	$	462,629	$	(161,284	)	$	301,345

Unfavorable gas gathering contract	$	10,962	$	(9,074	)	$	1,888

We recognized amortization expense in other revenues as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(388	)	$	(388	)	$	(777	)	$	(777	)
Amortization expense – unfavorable gas gathering contract		538			539			1,078			1,079

We recognized amortization expense in costs and expenses as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Amortization expense – contract intangibles	$	6,535	$	8,551	$	13,070	$	17,102
Amortization expense – rights-of-way		1,592		1,530		3,177		3,057

The estimated aggregate annual amortization expected to be recognized for the remainder of 2018 and each of the four succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	17,053	$	809
2019		33,217		—
2020		33,041		—
2021		29,349		—
2022		26,282		—

GOODWILL

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

GOODWILL

7. GOODWILL

We evaluate goodwill for impairment annually on September 30. We also evaluate goodwill whenever events or circumstances indicate that it is more likely than not that the fair value of a reporting unit is less than its carrying value, including goodwill. There have been no impairments of goodwill during the six months ended June 30, 2018.

Fair Value Measurement. Our impairment determinations, in the context of (i) our annual impairment evaluations and (ii) our other-than-annual impairment evaluations involved significant assumptions and judgments, as discussed in the 2017 Annual Report. Differing assumptions regarding any of these inputs could have a significant effect on the various valuations. As such, the fair value measurements utilized within these models are classified as non-recurring Level 3 measurements in the fair value hierarchy because they are not observable from objective sources. Due to the volatility of the inputs used, we cannot predict the likelihood of any future impairment.

EQUITY METHOD INVESTMENTS

6 Months Ended

Jun. 30, 2018

Equity Method Investments And Joint Ventures [Abstract]

EQUITY METHOD INVESTMENTS

8. EQUITY METHOD INVESTMENTS

Ohio Gathering owns, operates and is currently developing midstream infrastructure consisting of a liquids-rich natural gas gathering system, a dry natural gas gathering system and a condensate stabilization facility in the Utica Shale in southeastern Ohio. Ohio Gathering provides gathering services pursuant to primarily long-term, fee-based gathering agreements, which include acreage dedications.

A reconciliation of our 40% ownership interest in Ohio Gathering to our investment per Ohio Gathering's books and records follows (in thousands).

Investment in equity method investees, June 30, 2018	$	670,827
June cash distributions		2,580
Basis difference		(123,508	)
Investment in equity method investees, net of basis difference, May 31, 2018	$	549,899

For the three and six months ended June 30, 2018, there were no contributions to Ohio Gathering.

Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information). Results include gross asset impairments of $12.3 million for the three and six months ending June 30, 2018 and $8.7 million for the three and six months ending June 30, 2017.

	Three months ended May 31, 2018						Three months ended May 31, 2017
	OGC			OCC			OGC			OCC
	(In thousands)
Total revenues	$	34,123		$	2,070		$	31,083		$	2,004
Total operating expenses		35,518			1,958			33,221			1,836
Net (loss) income		(1,396	)		(59	)		(139	)		23

	Six months ended May 31, 2018				Six months ended May 31, 2017
	OGC		OCC		OGC		OCC
	(In thousands)
Total revenues	$	69,083	$	4,559	$	68,158	$	4,057
Total operating expenses		62,293		4,099		60,326		4,309
Net income (loss)		6,784		121		7,834		(1,192	)

DEFERRED REVENUE

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

DEFERRED REVENUE

9. DEFERRED REVENUE

A rollforward of current deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total current
	(In thousands)
Current deferred revenue, December 31, 2017, as reported	$	—	$	—	$	4,000	$	—	$	—	$	4,000
Net impact of Topic 606 day 1 adoption		18		1,017		3,396		1,619		38		6,088
Current deferred revenue, January 1, 2018		18		1,017		7,396		1,619		38		10,088
Additions		9		1,088		10,996		814		19		12,926
Less revenue recognized		9		643		10,762		803		19		12,236
Current deferred revenue, June 30, 2018	$	18	$	1,462	$	7,630	$	1,630	$	38	$	10,778

A rollforward of noncurrent deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, December 31, 2017, as reported	$	—	$	—	$	12,707	$	—	$	—	$	12,707
Net impact of Topic 606 day 1 adoption		39		4,215		10,017		8,217		333		22,821
Noncurrent deferred revenue, January 1, 2018		39		4,215		22,724		8,217		333		35,528
Additions		—		1,622		5,200		1,583		—		8,405
Less reclassification to current deferred revenue		9		1,017		3,955		814		19		5,814
Noncurrent deferred revenue, June 30, 2018	$	30	$	4,820	$	23,969	$	8,986	$	314	$	38,119

DEBT

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

DEBT

10. DEBT

Debt consisted of the following:

	June 30, 2018			December 31, 2017
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.35% at June 30, 2018 and 4.07% at December 31, 2017) due May 2022	$	356,000		$	261,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,588	)		(2,910	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			500,000
Less unamortized debt issuance costs (1)		(6,407	)		(6,898	)
Total long-term debt	$	1,147,005		$	1,051,192

(1) Issuance costs are being amortized over the life of the notes.

Revolving Credit Facility. Summit Holdings has a senior secured revolving credit facility that allows for revolving loans, letters of credit and swing line loans. The Revolving Credit Facility has a $1.25 billion borrowing capacity, matures in May 2022, and includes a $250.0 million accordion feature.

Borrowings under the Revolving Credit Facility bear interest, at the election of Summit Holdings, at a rate based on the alternate base rate (as defined in the credit agreement) plus an applicable margin ranging from 0.75% to 1.75% or the adjusted Eurodollar rate (as defined in the credit agreement) plus an applicable margin ranging from 1.75% to 2.75%, with the commitment fee ranging from 0.30% to 0.50% in each case based on our relative leverage at the time of determination. At June 30, 2018, the applicable margin under LIBOR borrowings was 2.25%, the interest rate was 4.35% and the unused portion of the Revolving Credit Facility totaled $894.0 million (subject to a commitment fee of 0.375%).

As of June 30, 2018, we had $9.8 million of debt issuance costs attributable to our Revolving Credit Facility and related amendments which are included in noncurrent assets on the unaudited condensed consolidated balance sheet.

As of and during the six months ended June 30, 2018, we were in compliance with the Revolving Credit Facility's covenants. There were no defaults or events of default during the six months ended June 30, 2018.

Senior Notes. In July 2014, Summit Holdings and its 100% owned finance subsidiary, Finance Corp. (together with Summit Holdings, the "Co-Issuers") co-issued $300.0 million of 5.5% senior unsecured notes maturing August 15, 2022 (the "5.5% Senior Notes" and, together with the 5.75% Senior Notes (defined below, the “Senior Notes”).

In February 2017, the Co-Issuers completed a public offering of $500.0 million of 5.75% senior unsecured notes (the "5.75% Senior Notes") as described in the 2017 Annual Report. References to the "Senior Notes," refer collectively to the 5.5% Senior Notes and the 5.75% Senior Notes.

In 2017, we executed supplemental indentures and amendments to our Revolving Credit Facility to add three newly formed entities, Summit Permian, Permian Finance and Summit Niobrara, as guarantors. As a result, Bison Midstream and its subsidiaries, Grand River and its subsidiary, DFW Midstream, Summit Marketing, Summit Permian, Permian Finance and Summit Niobrara (collectively the "Guarantor Subsidiaries") and SMLP fully and unconditionally and jointly and severally guarantee the 5.5% Senior Notes and the 5.75% Senior Notes. The Senior Notes are not guaranteed by OpCo, Summit Utica, Meadowlark Midstream and Tioga Midstream (collectively, the "Non-Guarantor Subsidiaries"). There are no significant restrictions on the ability of SMLP or Summit Holdings to obtain funds from its subsidiaries by dividend or loan. Finance Corp. has had no assets or operations since inception in 2013. At no time have the Senior Notes been guaranteed by the Co-Issuers.

As of and during the six months ended June 30, 2018, we were in compliance with the covenants governing our Senior Notes. There were no defaults or events of default during the six months ended June 30, 2018.

FINANCIAL INSTRUMENTS

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

FINANCIAL INSTRUMENTS

11. FINANCIAL INSTRUMENTS

Concentrations of Credit Risk. Financial instruments that potentially subject us to concentrations of credit risk consist of cash and cash equivalents and accounts receivable. We maintain our cash and cash equivalents in bank deposit accounts that frequently exceed federally insured limits. We have not experienced any losses in such accounts and do not believe we are exposed to any significant risk.

Accounts receivable primarily comprise amounts due for the gathering, treating and processing services we provide to our customers and also the sale of natural gas liquids resulting from our processing services. This industry concentration has the potential to impact our overall exposure to credit risk, either positively or negatively, in that our customers may be similarly affected by changes in economic, industry or other conditions. We monitor the creditworthiness of our counterparties and can require letters of credit for receivables from counterparties that are judged to have substandard credit, unless the credit risk can otherwise be mitigated. Our top five customers or counterparties accounted for 46% of total accounts receivable at June 30, 2018, compared with 44% as of December 31, 2017.

Fair Value. The carrying amount of cash and cash equivalents, accounts receivable and trade accounts payable reported on the balance sheet approximates fair value due to their short-term maturities.

The Deferred Purchase Price Obligation's carrying value is its fair value because carrying value represents the present value of the payment expected to be made in 2020. Our calculation of the Deferred Purchase Price Obligation involves significant assumptions and judgments. Differing assumptions regarding any of these inputs could have a material effect on the ultimate cash payment and the Deferred Purchase Price Obligation. As such, its fair value measurement is classified as a recurring Level 3 measurement in the fair value hierarchy because our assumptions and judgments are not observable from objective sources (see Note 17).

The Deferred Purchase Price Obligation represents our only Level 3 financial instrument fair value measurement. A rollforward of our Level 3 liability measured at fair value on a recurring basis follows (in thousands).

Level 3 liability, January 1, 2018	$	362,959
Change in fair value		90,963
Level 3 liability, June 30, 2018	$	453,922

A summary of the estimated fair value of our debt financial instruments follows.

	June 30, 2018				December 31, 2017
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings 5.5% Senior Notes ($300.0 million principal)	$	297,412	$	293,875	$	297,090	$	301,750
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,593		474,375		493,102		501,667

The carrying value on the balance sheet of the Revolving Credit Facility is its fair value due to its floating interest rate. The fair value for the Senior Notes is based on an average of nonbinding broker quotes as of June 30, 2018 and December 31, 2017. The use of different market assumptions or valuation methodologies may have a material effect on the estimated fair value of the Senior Notes.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

PARTNERS' CAPITAL

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

PARTNERS' CAPITAL

12. PARTNERS' CAPITAL

A rollforward of the number of common limited partner and General Partner units follows.

	Limited partners
	Series A Preferred Units		Common		General Partner
Units, January 1, 2018		300,000		73,085,996		1,490,999
Net units issued under the SMLP LTIP		—		269,779		—
Units, June 30, 2018		300,000		73,355,775		1,490,999

At-the-market Program. In 2017, we executed a new equity distribution agreement and filed a prospectus and a prospectus supplement with the SEC for the issuance and sale from time to time of SMLP common units having an aggregate offering price of up to $150.0 million (the "ATM Program"). These sales will be made (i) pursuant to the terms of the equity distribution agreement between us and the sales agents named therein and (ii) by means of ordinary brokers' transactions at market prices, in block transactions or as otherwise agreed between us and the sales agents. Sales of our common units may be made in negotiated transactions or transactions that are deemed to be at-the-market offerings as defined by SEC rules.

During the three and six months ended June 30, 2018, there were no transactions under the ATM Program. Following the effectiveness of the new ATM registration statement and after taking into account the aggregate sales price of common units sold under the ATM Program through June 30, 2018, we have the capacity to issue additional common units under the ATM Program up to an aggregate $132.3 million.

Series A Preferred Units. In 2017, we issued 300,000 Series A Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units (the “Series A Preferred Units”) representing limited partner interests in the Partnership at a price to the public of $1,000 per unit as described in the 2017 Annual Report.

Noncontrolling Interest. We have recorded Summit Investments' indirect retained ownership interest in OpCo and its subsidiaries as a noncontrolling interest in the unaudited condensed consolidated financial statements.

Cash Distributions Paid and Declared. We paid the following per-unit distributions during the three and six months ended June 30:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
Per-unit distributions to unitholders	$	0.575	$	0.575	$	1.150	$	1.150

On July 26, 2018, the Board of Directors of our General Partner declared a distribution of $0.575 per unit for the quarterly period ended June 30, 2018. This distribution, which totaled $45.2 million, will be paid on August 14, 2018 to unitholders of record at the close of business on August 7, 2018.

Incentive Distribution Rights. Our general partner also currently holds IDRs that entitle it to receive increasing percentage allocations, up to a maximum of 50%, of the cash we distribute from operating surplus in excess of $0.46 per unit per quarter. Our payment of IDRs as reported in distributions to unitholders – general partner in the statement of partners' capital during the three and six months ended June 30 follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
IDR payments	$	2,136	$	2,106	$	4,264	$	4,206

For the purposes of calculating net income attributable to General Partner in the statements of operations and partners' capital, the financial impact of IDRs is recognized in respect of the quarter for which the distributions were declared. For the purposes of calculating distributions to unitholders in the statements of partners' capital and cash flows, IDR payments are recognized in the quarter in which they are paid.

EARNINGS PER UNIT

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

EARNINGS PER UNIT

13. EARNINGS PER UNIT

The following table details the components of EPU.

	Three months ended June 30,					Six months ended June 30,
	2018			2017		2018			2017
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net (loss) income among limited partner interests:
Net (loss) income attributable to limited partners	$	(51,111	)	$	8,806	$	(57,099	)	$	6,029
Less net income attributable to Series A Preferred Units		7,125			—		14,250			—
Net (loss) income attributable to common limited partners	$	(58,236	)	$	8,806	$	(71,349	)	$	6,029

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		73,356			72,532		73,245			72,341
Effect of nonvested phantom units		—			310		—			367
Weighted-average common units outstanding – diluted		73,356			72,842		73,245			72,708

(Loss) earnings per limited partner unit:
Common unit – basic	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08
Common unit – diluted	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		1			—		3			—

UNIT-BASED AND NONCASH COMPENSATION

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

UNIT-BASED AND NONCASH COMPENSATION

14. UNIT-BASED AND NONCASH COMPENSATION

SMLP Long-Term Incentive Plan. The SMLP LTIP provides for equity awards to eligible officers, employees, consultants and directors of our General Partner and its affiliates. Items to note:

•

In March 2018, we granted 515,358 phantom units and associated distribution equivalent rights to employees in connection with our annual incentive compensation award cycle. These awards had a grant date fair value of $15.25 and vest ratably over a three-year period.

•

Also in March 2018, 328,388 phantom units vested.

•

As of June 30, 2018, approximately 3.2 million common units remained available for future issuance under the SMLP LTIP.

RELATED-PARTY TRANSACTIONS

6 Months Ended

Jun. 30, 2018

RELATED-PARTY TRANSACTIONS

15. RELATED-PARTY TRANSACTIONS

Acquisitions. See Notes 11 and 16 of the 2017 Annual Report.

Reimbursement of Expenses from General Partner. Our General Partner and its affiliates do not receive a management fee or other compensation in connection with the management of our business, but will be reimbursed for expenses incurred on our behalf. Under our Partnership Agreement, we reimburse our General Partner and its affiliates for certain expenses incurred on our behalf, including, without limitation, salary, bonus, incentive compensation and other amounts paid to our General Partner's employees and executive officers who perform services necessary to run our business. Our Partnership Agreement provides that our General Partner will determine in good faith the expenses that are allocable to us. The "Due to affiliate" line item on the consolidated balance sheet represents the payables to our General Partner for expenses incurred by it and paid on our behalf.

Expenses incurred by the General Partner and reimbursed by us under our Partnership Agreement were as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Operation and maintenance expense	$	7,114	$	6,731	$	14,737	$	13,612
General and administrative expense		7,481		7,895		15,598		16,190

COMMITMENTS AND CONTINGENCIES

6 Months Ended

Jun. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

16. COMMITMENTS AND CONTINGENCIES

Operating Leases. We and Summit Investments lease certain office space and equipment to support our operations. We have determined that our leases are operating leases. We recognize total rent expense incurred or allocated to us in general and administrative expenses. Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Rent expense	$	956	$	923	$	1,978	$	1,802

Environmental Matters. Although we believe that we are in material compliance with applicable environmental regulations, the risk of environmental remediation costs and liabilities are inherent in pipeline ownership and operation. Furthermore, we can provide no assurances that significant environmental remediation costs and liabilities will not be incurred by the Partnership in the future. We are currently not aware of any material contingent liabilities that exist with respect to environmental matters, except as noted below.

As described in the 2017 Annual Report, in 2015, Summit Investments learned of the rupture of a four-inch produced water gathering pipeline on the Meadowlark Midstream system near Williston, North Dakota. The incident, which was covered by Summit Investments' insurance policies, was subject to maximum coverage of $25.0 million from its pollution liability insurance policy and $200.0 million from its property and business interruption insurance policy. Summit Investments exhausted the $25.0 million pollution liability policy in 2015. We submitted property and business interruption claim requests to the insurers and reached a settlement in January 2017. In connection therewith, we recognized $2.6 million of business interruption recoveries and $0.4 million of property recoveries.

A rollforward of the aggregate accrued environmental remediation liabilities follows.

	Total
	(In thousands)
Accrued environmental remediation, January 1, 2018	$	5,344
Payments made		(1,805	)
Additional accruals		1,600
Accrued environmental remediation, June 30, 2018	$	5,139

As of June 30, 2018, we have recognized (i) a current liability for remediation effort expenditures expected to be incurred within the next 12 months and (ii) a noncurrent liability for estimated remediation expenditures and fines expected to be incurred subsequent to June 30, 2019. Each of these amounts represent our best estimate for costs expected to be incurred. Neither of these amounts has been discounted to its present value.

Legal Proceedings. The Partnership is involved in various litigation and administrative proceedings arising in the normal course of business. In the opinion of management, any liabilities that may result from these claims or those arising in the normal course of business would not individually or in the aggregate have a material adverse effect on the Partnership's financial position or results of operations.

As described in the 2017 Annual Report, in 2015 and 2016, the U.S. Department of Justice (“DOJ”) issued grand jury subpoenas to Summit Investments, the Partnership, our General Partner and Meadowlark Midstream requesting certain materials related to the incident. SMLP has continued to exchange information with the DOJ and is cooperating with the investigation. While we cannot predict the ultimate outcome of this matter with certainty for Summit Investments or Meadowlark Midstream, especially as it relates to any material liability as a result of any governmental proceeding related to the incident, we believe at this time that it is unlikely that SMLP or its General Partner will be subject to any material liability as a result of any governmental proceeding related to the rupture.

ACQUISITIONS AND DROP DOWN TRANSACTIONS

6 Months Ended

Jun. 30, 2018

Business Combinations [Abstract]

ACQUISITIONS AND DROP DOWN TRANSACTIONS

17. ACQUISITIONS AND DROP DOWN TRANSACTIONS

2016 Drop Down. In 2016, SMLP acquired a controlling interest in OpCo, the entity which owns the 2016 Drop Down Assets. These assets include certain natural gas, crude oil and produced water gathering systems located in the Utica Shale, the Williston Basin and the DJ Basin, as well as ownership interests in a natural gas gathering system and a condensate stabilization facility, both located in the Utica Shale.

The net consideration paid and recognized in connection with the 2016 Drop Down (i) consisted of a cash payment to SMP Holdings of $360.0 million funded with borrowings under our Revolving Credit Facility and a $0.6 million working capital adjustment received in June 2016 (the “Initial Payment”) and (ii) includes the Deferred Purchase Price Obligation payment due in 2020.

The present value of the Deferred Purchase Price Obligation is reflected as a liability on our balance sheet until paid. As of June 30, 2018, Remaining Consideration was estimated to be $538.4 million and the net present value, as recognized on the consolidated balance sheet, was $453.9 million, using a discount rate of 10.25%. Any subsequent changes to the estimated future payment obligation will be calculated using a discounted cash flow model with a commensurate risk-adjusted discount rate. Such changes and the impact on the liability due to the passage of time will be recorded as a change in the Deferred Purchase Price Obligation fair value on the consolidated statements of operations in the period of the change.

We currently expect that the Deferred Purchase Price Obligation will be financed with a combination of (i) net proceeds from the issuance of equity securities by us, (ii) the net proceeds from the issuance of senior unsecured debt by us, (iii) borrowings under our Revolving Credit Facility and/or (iv) other internally generated sources of cash.

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

18. CONDENSED CONSOLIDATING FINANCIAL INFORMATION

The Senior Notes are fully and unconditionally guaranteed, jointly and severally, on a senior unsecured basis by SMLP and the Guarantor Subsidiaries (see Note 10).

The following supplemental condensed consolidating financial information reflects SMLP's separate accounts, the combined accounts of the Co-Issuers, the combined accounts of the Guarantor Subsidiaries, the combined accounts of the Non-Guarantor Subsidiaries and the consolidating adjustments for the dates and periods indicated. For purposes of the following consolidating information each of SMLP and the Co-Issuers account for their subsidiary investments, if any, under the equity method of accounting.

Condensed Consolidating Balance Sheets. Balance sheets as of June 30, 2018 and December 31, 2017 follow.

	June 30, 2018
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	2,616	$	92	$	4,998	$	506	$	—		$	8,212
Accounts receivable		22		—		68,274		11,870		—			80,166
Other current assets		1,057		—		1,273		264		—			2,594
Due from affiliate		—		—		507,396		51,852		(559,248	)		—
Total current assets		3,695		92		581,941		64,492		(559,248	)		90,972
Property, plant and equipment, net		4,314		—		1,537,468		345,977		—			1,887,759
Intangible assets, net		—		—		263,416		23,249		—			286,665
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		670,827		—			670,827
Other noncurrent assets		3,288		9,762		94		—		—			13,144
Investment in subsidiaries		2,062,096		3,396,771		—		—		(5,458,867	)		—
Total assets	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

Liabilities and Partners' Capital
Trade accounts payable	$	141	$	—	$	18,280	$	3,277	$	—		$	21,698
Accrued expenses		803		—		17,030		1,600		—			19,433
Due to affiliate		374,074		185,265		—		—		(559,248	)		91
Deferred revenue		—		—		10,341		437		—			10,778
Ad valorem taxes payable		9		—		6,224		271		—			6,504
Accrued interest		—		12,259		—		—		—			12,259
Accrued environmental remediation		—		—		—		3,522		—			3,522
Other current liabilities		3,477		—		3,573		362		—			7,412
Total current liabilities		378,504		197,524		55,448		9,469		(559,248	)		81,697
Long-term debt		—		1,147,005		—		—		—			1,147,005
Deferred Purchase Price Obligation		453,922		—		—		—		—			453,922
Noncurrent deferred revenue		—		—		36,416		1,703		—			38,119
Noncurrent accrued environmental remediation		—		—		—		1,617		—			1,617
Other noncurrent liabilities		4,159		—		2,197		54		—			6,410
Total liabilities		836,585		1,344,529		94,061		12,843		(559,248	)		1,728,770

Total partners' capital		1,236,808		2,062,096		2,305,069		1,091,702		(5,458,867	)		1,236,808
Total liabilities and partners' capital	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

	December 31, 2017
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	126	$	75	$	1,138	$	91	$	—		$	1,430
Accounts receivable		22		—		65,842		6,437		—			72,301
Other current assets		1,188		—		2,697		442		—			4,327
Due from affiliate		—		—		493,067		22,030		(515,097	)		—
Total current assets		1,336		75		562,744		29,000		(515,097	)		78,058
Property, plant and equipment, net		4,206		—		1,442,333		348,590		—			1,795,129
Intangible assets, net		—		—		278,958		22,387		—			301,345
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		690,485		—			690,485
Other noncurrent assets		2,547		10,913		105		—		—			13,565
Investment in subsidiaries		2,019,700		3,324,464		—		—		(5,344,164	)		—
Total assets	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Liabilities and Partners' Capital
Trade accounts payable	$	209	$	—	$	11,283	$	4,883	$	—		$	16,375
Accrued expenses		928		—		10,592		979		—			12,499
Due to affiliate		263,935		252,250		—		—		(515,097	)		1,088
Deferred revenue		—		—		4,000		—		—			4,000
Ad valorem taxes payable		—		—		7,809		520		—			8,329
Accrued interest		—		12,310		—		—		—			12,310
Accrued environmental remediation		—		—		—		3,130		—			3,130
Other current liabilities		6,395		—		4,385		478		—			11,258
Total current liabilities		271,467		264,560		38,069		9,990		(515,097	)		68,989
Long-term debt		—		1,051,192		—		—		—			1,051,192
Deferred Purchase Price Obligation		362,959		—		—		—		—			362,959
Deferred revenue		—		—		12,707		—		—			12,707
Noncurrent accrued environmental remediation		—		—		—		2,214		—			2,214
Other noncurrent liabilities		3,694		—		3,293		76		—			7,063
Total liabilities		638,120		1,315,752		54,069		12,280		(515,097	)		1,505,124

Total partners' capital		1,389,669		2,019,700		2,246,282		1,078,182		(5,344,164	)		1,389,669
Total liabilities and partners' capital	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Condensed Consolidating Statements of Operations. For the purposes of the following condensed consolidating statements of operations, we allocate general and administrative expenses recognized at the SMLP parent to the Guarantor Subsidiaries and Non-Guarantor Subsidiaries to reflect what those entities' results would have been had they operated on a stand-alone basis. Statements of operations for the three and six months ended June 30, 2018 and 2017 follow.

	Three months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	73,891		$	15,694		$	—		$	89,585
Natural gas, NGLs and condensate sales		—			—			31,891			—			—			31,891
Other revenues		—			—			6,509			198			—			6,707
Total revenues		—			—			112,291			15,892			—			128,183
Costs and expenses:
Cost of natural gas and NGLs		—			—			24,384			—			—			24,384
Operation and maintenance		—			—			22,221			2,245			—			24,466
General and administrative		—			—			11,907			1,577			—			13,484
Depreciation and amortization		444			—			22,776			3,564			—			26,784
(Gain) loss on asset sales, net		—			—			(1	)		63			—			62
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		444			—			81,874			7,449			—			89,767
Other income		27			—			—			—			—			27
Interest expense		—			(14,837	)		—			—			—			(14,837	)
Deferred Purchase Price Obligation		(69,305	)		—			—			—			—			(69,305	)
(Loss) income before income taxes and loss from equity method investees		(69,722	)		(14,837	)		30,417			8,443			—			(45,699	)
Income tax expense		(294	)		—			—			—			—			(294	)
Loss from equity method investees		—			—			—			(3,920	)		—			(3,920	)
Equity in earnings of consolidated subsidiaries		20,103			34,940			—			—			(55,043	)		—
Net (loss) income	$	(49,913	)	$	20,103		$	30,417		$	4,523		$	(55,043	)	$	(49,913	)

	Three months ended June 30, 2017
	SMLP		Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—	$	—		$	67,679	$	17,122		$	—		$	84,801
Natural gas, NGLs and condensate sales		—		—			10,595		—			—			10,595
Other revenues		—		—			6,332		64			—			6,396
Total revenues		—		—			84,606		17,186			—			101,792
Costs and expenses:
Cost of natural gas and NGLs		—		—			9,099		—			—			9,099
Operation and maintenance		—		—			22,169		1,847			—			24,016
General and administrative		—		—			10,886		2,063			—			12,949
Depreciation and amortization		154		—			25,282		3,252			—			28,688
Transaction costs		119		—			—		—			—			119
Loss on asset sales, net		—		—			67		—			—			67
Long-lived asset impairment		—		—			2		1			—			3
Total costs and expenses		273		—			67,505		7,163			—			74,941
Other income		64		—			—		—			—			64
Interest expense		—		(17,553	)		—		—			—			(17,553	)
Deferred Purchase Price Obligation		5,058		—			—		—			—			5,058
Income (loss) before income taxes and loss from equity method investees		4,849		(17,553	)		17,101		10,023			—			14,420
Income tax benefit		211		—			—		—			—			211
Loss from equity method investees		—		—			—		(3,385	)		—			(3,385	)
Equity in earnings of consolidated subsidiaries		6,186		23,739			—		—			(29,925	)		—
Net income	$	11,246	$	6,186		$	17,101	$	6,638		$	(29,925	)	$	11,246

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	143,357		$	30,589		$	—		$	173,946
Natural gas, NGLs and condensate sales		—			—			58,008			—			—			58,008
Other revenues		—			—			13,236			313			—			13,549
Total revenues		—			—			214,601			30,902			—			245,503
Costs and expenses:
Cost of natural gas and NGLs		—			—			44,670			—			—			44,670
Operation and maintenance		—			—			44,374			4,696			—			49,070
General and administrative		—			—			24,244			3,682			—			27,926
Depreciation and amortization		876			—			45,637			6,948			—			53,461
(Gain) loss on asset sales, net		—			—			(75	)		63			—			(12	)
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		876			—			159,437			15,389			—			175,702
Other income		20			—			—			—			—			20
Interest expense		—			(29,959	)		—			—			—			(29,959	)
Deferred Purchase Price Obligation		(90,963	)		—			—			—			—			(90,963	)
(Loss) income before income taxes and loss from equity method investees		(91,819	)		(29,959	)		55,164			15,513			—			(51,101	)
Income tax expense		(123	)		—			—			—			—			(123	)
Loss from equity method investees		—			—			—			(2,534	)		—			(2,534	)
Equity in earnings of consolidated subsidiaries		38,184			68,143			—			—			(106,327	)		—
Net (loss) income	$	(53,758	)	$	38,184		$	55,164		$	12,979		$	(106,327	)	$	(53,758	)

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	170,192	$	32,622		$	—		$	202,814
Natural gas, NGLs and condensate sales		—			—			21,715		—			—			21,715
Other revenues		—			—			12,932		136			—			13,068
Total revenues		—			—			204,839		32,758			—			237,597
Costs and expenses:
Cost of natural gas and NGLs		—			—			18,151		—			—			18,151
Operation and maintenance		—			—			44,189		3,519			—			47,708
General and administrative		—			—			23,371		3,710			—			27,081
Depreciation and amortization		308			—			50,524		6,425			—			57,257
Transaction costs		119			—			—		—			—			119
Loss on asset sales, net		—			—			70		—			—			70
Long-lived asset impairment		—			—			2		285			—			287
Total costs and expenses		427			—			136,307		13,939			—			150,673
Other income		135			—			—		—			—			135
Interest expense		—			(34,269	)		—		—			—			(34,269	)
Early extinguishment of debt		—			(22,020	)		—		—			—			(22,020	)
Deferred Purchase Price Obligation		(15,825	)		—			—		—			—			(15,825	)
(Loss) income before income taxes and loss from equity method investees		(16,117	)		(56,289	)		68,532		18,819			—			14,945
Income tax expense		(241	)		—			—		—			—			(241	)
Loss from equity method investees		—			—			—		(4,041	)		—			(4,041	)
Equity in earnings of consolidated subsidiaries		27,021			83,310			—		—			(110,331	)		—
Net income	$	10,663		$	27,021		$	68,532	$	14,778		$	(110,331	)	$	10,663

Condensed Consolidating Statements of Cash Flows. Statements of cash flows for the six months ended June 30, 2018 and 2017 follow.

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$	(68	)	$	(27,877	)	$	103,965		$	34,029		$	—		$	110,049

Cash flows from investing activities:
Capital expenditures		(972	)		—			(85,187	)		(4,235	)		—			(90,394	)
Proceeds from asset sales		—			—			—			496			—			496
Other, net		(306	)		—			—			—			—			(306	)
Advances to affiliates		—			(66,985	)		(14,329	)		(29,820	)		111,134			—
Net cash used in investing activities		(1,278	)		(66,985	)		(99,516	)		(33,559	)		111,134			(90,204	)

Cash flows from financing activities:
Distributions to common unitholders		(90,269	)		—			—			—			—			(90,269	)
Distributions to Series A Preferred unitholders		(14,250	)		—			—			—			—			(14,250	)
Borrowings under Revolving Credit Facility		—			148,000			—			—			—			148,000
Repayments under Revolving Credit Facility		—			(53,000	)		—			—			—			(53,000	)
Debt issuance costs		—			(121	)		—			—			—			(121	)
Other, net		(2,779	)		—			(589	)		(55	)		—			(3,423	)
Advances from affiliates		111,134			—			—			—			(111,134	)		—
Net cash provided by (used in) financing activities		3,836			94,879			(589	)		(55	)		(111,134	)		(13,063	)
Net change in cash and cash equivalents		2,490			17			3,860			415			—			6,782
Cash and cash equivalents, beginning of period		126			75			1,138			91			—			1,430
Cash and cash equivalents, end of period	$	2,616		$	92		$	4,998		$	506		$	—		$	8,212

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$	424		$	(32,466	)	$	114,863		$	38,520		$	—		$	121,341

Cash flows from investing activities:
Capital expenditures		(6,412	)		—			(23,578	)		(15,922	)		—			(45,912	)
Contributions to equity method investees		—			—			—			(15,649	)		—			(15,649	)
Other, net		(521	)		—			—			—			—			(521	)
Advances to affiliates		7,635			23,020			(95,361	)		—			64,706			—
Net cash provided by (used in) investing activities		702			23,020			(118,939	)		(31,571	)		64,706			(62,082	)

Cash flows from financing activities:
Distributions to unitholders		(89,029	)		—			—			—			—			(89,029	)
Borrowings under Revolving Credit Facility		—			112,500			—			—			—			112,500
Repayments under Revolving Credit Facility		—			(269,500	)		—			—			—			(269,500	)
Debt issuance costs		—			(15,613	)		—			—			—			(15,613	)
Payment of redemption and call premiums on senior notes		—			(17,913	)		—			—			—			(17,913	)
Proceeds from ATM Program issuances, net of costs		17,259			—			—			—			—			17,259
Contribution from General Partner		465			—			—			—			—			465
Issuance of senior notes		—			500,000			—			—			—			500,000
Tender and redemption of senior notes		—			(300,000	)		—			—			—			(300,000	)
Other, net		(2,632	)		—			372			(8	)		—			(2,268	)
Advances from affiliates		72,341			—			—			(7,635	)		(64,706	)		—
Net cash (used in) provided by financing activities		(1,596	)		9,474			372			(7,643	)		(64,706	)		(64,099	)
Net change in cash and cash equivalents		(470	)		28			(3,704	)		(694	)		—			(4,840	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	228		$	79		$	2,064		$	217		$	—		$	2,588

SUBSEQUENT EVENTS

6 Months Ended

Jun. 30, 2018

Subsequent Events [Abstract]

SUBSEQUENT EVENTS

19. SUBSEQUENT EVENTS

We have evaluated subsequent events for recognition or disclosure in the unaudited condensed consolidated financial statements and no events have occurred that require disclosure.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Accounting Pronouncements

Recently Adopted Accounting Pronouncements. We have recently adopted the following accounting pronouncements:

•

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Impact on financial statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

•

Accounting Pronouncements Pending Adoption. We have not yet adopted the following accounting pronouncements as of June 30, 2018:

•

Basis of Presentation

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Financial Statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

REVENUE (Tables)

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

Schedule of Estimated Revenue Expected to be Recognized and MVC Shortfall Payments

	2018		2019		2020		2021		2022		Thereafter
	(In thousands)
Gathering services and related fees	$	97,523	$	133,743	$	122,429	$	102,777	$	83,648	$	174,227

Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments

Revenue by Category. In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4.

	Reportable Segments
	Three months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	10,422	$	23,106	$	36,170	$	14,080	$	8,050	$	91,828	$	(2,243	)	$	89,585
Natural gas, NGLs and condensate sales		—		7,350		4,675		381		—		12,406		19,485			31,891
Other revenues		—		2,960		2,147		1,694		—		6,801		(94	)		6,707
Total	$	10,422	$	33,416	$	42,992	$	16,155	$	8,050	$	111,035	$	17,148		$	128,183

	Reportable Segments
	Six months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	20,463	$	40,772	$	71,464	$	27,717	$	15,875	$	176,291	$	(2,345	)	$	173,946
Natural gas, NGLs and condensate sales		—		15,196		9,000		926		—		25,122		32,886			58,008
Other revenues		—		5,872		4,115		3,682		—		13,669		(120	)		13,549
Total	$	20,463	$	61,840	$	84,579	$	32,325	$	15,875	$	215,082	$	30,421		$	245,503

Schedule of Information about Contract Assets from Contracts with Customers

The following table provides information about contract assets from contracts with customers:

	June 30, 2018
	(In thousands)
Contract assets, December 31, 2017	$	—
Net impact of Topic 606 day 1 adoption		3,514
Additions		11,523
Transfers out		(5,704	)
Contract assets, June 30, 2018	$	9,333

Current		9,333
Noncurrent		—
Contract assets, June 30, 2018	$	9,333

SEGMENT INFORMATION (Tables)

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

Schedule of assets by reportable segment

Assets by reportable segment follow.

	June 30, 2018			December 31, 2017
	(In thousands)
Assets:
Utica Shale	$	212,326		$	212,311
Ohio Gathering		670,827			690,485
Williston Basin		520,799			512,860
Piceance/DJ Basins		828,999			798,722
Barnett Shale		376,969			383,306
Marcellus Shale		213,366			217,362
Total reportable segment assets		2,823,286			2,815,046
Corporate and Other		143,450			79,996
Eliminations		(1,158	)		(249	)
Total assets	$	2,965,578		$	2,894,793

Schedule of segment reporting information

Revenues by reportable segment follow.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Revenues (1):
Utica Shale	$	10,422		$	10,456		$	20,463		$	19,252
Williston Basin		33,416			29,114			61,840			95,999
Piceance/DJ Basins		42,992			33,763			84,579			68,571
Barnett Shale		16,155			20,904			32,325			38,646
Marcellus Shale		8,050			7,365			15,875			14,269
Total reportable segments revenue		111,035			101,602			215,082			236,737
Corporate and Other		19,422			1,362			33,598			3,148
Eliminations		(2,274	)		(1,172	)		(3,177	)		(2,288	)
Total revenues	$	128,183		$	101,792		$	245,503		$	237,597

(1) Excludes revenues earned by Ohio Gathering due to equity method accounting.

Depreciation and amortization, including the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	2,033	$	1,748	$	3,886	$	3,395
Williston Basin		5,622		8,385		11,231		16,766
Piceance/DJ Basins		12,450		12,225		25,005		24,436
Barnett Shale (2)		3,759		3,762		7,516		7,524
Marcellus Shale		2,274		2,263		4,546		4,526
Total reportable segment depreciation and amortization		26,138		28,383		52,184		56,647
Corporate and Other		496		154		976		308
Total depreciation and amortization	$	26,634	$	28,537	$	53,160	$	56,955

(1) Excludes depreciation and amortization recognized by Ohio Gathering due to equity method accounting.

(2) Includes the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues.

Cash paid for capital expenditures by reportable segment follow.

	Six months ended June 30,
	2018		2017
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	1,846	$	16,473
Williston Basin		10,966		11,085
Piceance/DJ Basins		24,827		11,934
Barnett Shale		349		(399	)
Marcellus Shale		545		407
Total reportable segment capital expenditures		38,533		39,500
Corporate and Other		51,861		6,412
Total cash paid for capital expenditures	$	90,394	$	45,912

(1) Excludes cash paid for capital expenditures by Ohio Gathering due to equity method accounting.

Schedule of counterparties accounting for more than 10% of total revenues

Counterparties accounting for more than 10% of total revenues were as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins		10	%	*				11	%	*
Counterparty B - Piceance/DJ Basins	*				10	%	*			*
Counterparty C - Barnett Shale		11	%		11	%	*			*
Counterparty D - Utica Shale	*				10	%	*			*
Counterparty E - Williston Basin	*			*			*				21	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments.

(2) Excludes revenues earned by Ohio Gathering due to equity method accounting.

* Less than 10%

Reconciliation of net income to adjusted EBITDA

Segment adjusted EBITDA by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	9,223	$	9,533	$	17,938	$	17,445
Ohio Gathering		8,935		9,606		19,412		18,679
Williston Basin		19,030		17,155		35,000		34,964
Piceance/DJ Basins		27,673		27,274		56,908		56,248
Barnett Shale		11,093		12,998		20,952		25,086
Marcellus Shale		6,543		5,446		13,219		11,093
Total of reportable segments' measures of profit	$	82,497	$	82,012	$	163,429	$	163,515

A reconciliation of income or loss before income taxes and income or loss from equity method investees to total of reportable segments' measures of profit or loss follows.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Reconciliation of (loss) income before income taxes and loss from equity method investees to total of reportable segments' measures of profit:
(Loss) income before income taxes and loss from equity method investees	$	(45,699	)	$	14,420		$	(51,101	)	$	14,945
Add:
Corporate and Other		9,002			9,435			19,625			19,528
Interest expense		14,837			17,553			29,959			34,269
Early extinguishment of debt		—			—			—			22,020
Deferred Purchase Price Obligation		69,305			(5,058	)		90,963			15,825
Depreciation and amortization		26,634			28,537			53,160			56,955
Proportional adjusted EBITDA for equity method investees		8,935			9,606			19,412			18,679
Adjustments related to MVC shortfall payments		(3,542	)		5,578			(3,542	)		(23,062	)
Adjustments related to capital reimbursement activity		115			—			155			—
Unit-based and noncash compensation		2,261			1,871			4,223			3,999
Loss (gain) on asset sales, net		62			67			(12	)		70
Long-lived asset impairment		587			3			587			287
Total of reportable segments' measures of profit	$	82,497		$	82,012		$	163,429		$	163,515

Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments

Adjustments related to MVC shortfall payments by reportable segment follow.

	Three months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Three months ended June 30, 2017
	Williston Basin		Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—	$	(1,186	)	$	—		$	(1,186	)
Expected MVC shortfall adjustments		1,982		6,522			(1,740	)		6,764
Total adjustments related to MVC shortfall payments	$	1,982	$	5,336		$	(1,740	)	$	5,578

	Six months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Six months ended June 30, 2017
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	(37,693	)	$	(1,978	)	$	—		$	(39,671	)
Expected MVC shortfall adjustments		3,964			13,067			(422	)		16,609
Total adjustments related to MVC shortfall payments	$	(33,729	)	$	11,089		$	(422	)	$	(23,062	)

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)

6 Months Ended

Jun. 30, 2018

Property Plant And Equipment [Abstract]

Schedule of property, plant, and equipment, net

Details on property, plant and equipment follow.

	June 30, 2018		December 31, 2017
	(In thousands)
Gathering and processing systems and related equipment	$	2,023,832	$	1,973,722
Construction in progress		160,239		78,850
Land and line fill		11,735		11,735
Other		41,802		40,262
Total		2,237,608		2,104,569
Less accumulated depreciation		349,849		309,440
Property, plant and equipment, net	$	1,887,759	$	1,795,129

Schedule of depreciation expense and capitalized interest costs

Depreciation expense and capitalized interest follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation expense	$	18,657	$	18,607	$	37,214	$	37,098
Capitalized interest		1,863		450		3,085		918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Tables)

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible assets and liabilities subject to amortization

Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities), all of which are subject to amortization, follow.

	June 30, 2018
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(13,128	)	$	11,067
Contract intangibles		278,448		(130,891	)		147,557
Rights-of-way		162,339		(34,298	)		128,041
Total intangible assets	$	464,982	$	(178,317	)	$	286,665

Unfavorable gas gathering contract	$	10,962	$	(10,153	)	$	809

	December 31, 2017
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(12,350	)	$	11,845
Contract intangibles		278,448		(117,821	)		160,627
Rights-of-way		159,986		(31,113	)		128,873
Total intangible assets	$	462,629	$	(161,284	)	$	301,345

Unfavorable gas gathering contract	$	10,962	$	(9,074	)	$	1,888

Recognized amortization expense in other revenues and cost and expenses

We recognized amortization expense in other revenues as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(388	)	$	(388	)	$	(777	)	$	(777	)
Amortization expense – unfavorable gas gathering contract		538			539			1,078			1,079

We recognized amortization expense in costs and expenses as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Amortization expense – contract intangibles	$	6,535	$	8,551	$	13,070	$	17,102
Amortization expense – rights-of-way		1,592		1,530		3,177		3,057

Estimated aggregate annual amortization expected to be recognized

The estimated aggregate annual amortization expected to be recognized for the remainder of 2018 and each of the four succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	17,053	$	809
2019		33,217		—
2020		33,041		—
2021		29,349		—
2022		26,282		—

EQUITY METHOD INVESTMENTS (Tables)

6 Months Ended

Jun. 30, 2018

Equity Method Investments And Joint Ventures [Abstract]

Equity Method Investments

A reconciliation of our 40% ownership interest in Ohio Gathering to our investment per Ohio Gathering's books and records follows (in thousands).

Investment in equity method investees, June 30, 2018	$	670,827
June cash distributions		2,580
Basis difference		(123,508	)
Investment in equity method investees, net of basis difference, May 31, 2018	$	549,899

For the three and six months ended June 30, 2018, there were no contributions to Ohio Gathering.

	Three months ended May 31, 2018						Three months ended May 31, 2017
	OGC			OCC			OGC			OCC
	(In thousands)
Total revenues	$	34,123		$	2,070		$	31,083		$	2,004
Total operating expenses		35,518			1,958			33,221			1,836
Net (loss) income		(1,396	)		(59	)		(139	)		23

	Six months ended May 31, 2018				Six months ended May 31, 2017
	OGC		OCC		OGC		OCC
	(In thousands)
Total revenues	$	69,083	$	4,559	$	68,158	$	4,057
Total operating expenses		62,293		4,099		60,326		4,309
Net income (loss)		6,784		121		7,834		(1,192	)

DEFERRED REVENUE (Tables)

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

Rollforward of deferred revenue

A rollforward of current deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total current
	(In thousands)
Current deferred revenue, December 31, 2017, as reported	$	—	$	—	$	4,000	$	—	$	—	$	4,000
Net impact of Topic 606 day 1 adoption		18		1,017		3,396		1,619		38		6,088
Current deferred revenue, January 1, 2018		18		1,017		7,396		1,619		38		10,088
Additions		9		1,088		10,996		814		19		12,926
Less revenue recognized		9		643		10,762		803		19		12,236
Current deferred revenue, June 30, 2018	$	18	$	1,462	$	7,630	$	1,630	$	38	$	10,778

A rollforward of noncurrent deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, December 31, 2017, as reported	$	—	$	—	$	12,707	$	—	$	—	$	12,707
Net impact of Topic 606 day 1 adoption		39		4,215		10,017		8,217		333		22,821
Noncurrent deferred revenue, January 1, 2018		39		4,215		22,724		8,217		333		35,528
Additions		—		1,622		5,200		1,583		—		8,405
Less reclassification to current deferred revenue		9		1,017		3,955		814		19		5,814
Noncurrent deferred revenue, June 30, 2018	$	30	$	4,820	$	23,969	$	8,986	$	314	$	38,119

DEBT (Tables)

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Schedule of debt and capital leases

Debt consisted of the following:

	June 30, 2018			December 31, 2017
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.35% at June 30, 2018 and 4.07% at December 31, 2017) due May 2022	$	356,000		$	261,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,588	)		(2,910	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			500,000
Less unamortized debt issuance costs (1)		(6,407	)		(6,898	)
Total long-term debt	$	1,147,005		$	1,051,192

(1) Issuance costs are being amortized over the life of the notes.

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21521-112644

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

FINANCIAL INSTRUMENTS (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

A rollforward of our Level 3 liability measured at fair value on a recurring basis follows (in thousands).

Level 3 liability, January 1, 2018	$	362,959
Change in fair value		90,963
Level 3 liability, June 30, 2018	$	453,922

Summary of the estimated fair value of debt instruments

A summary of the estimated fair value of our debt financial instruments follows.

	June 30, 2018				December 31, 2017
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings 5.5% Senior Notes ($300.0 million principal)	$	297,412	$	293,875	$	297,090	$	301,750
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,593		474,375		493,102		501,667

PARTNERS' CAPITAL (Tables)

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

Schedule of partner units activity

A rollforward of the number of common limited partner and General Partner units follows.

	Limited partners
	Series A Preferred Units		Common		General Partner
Units, January 1, 2018		300,000		73,085,996		1,490,999
Net units issued under the SMLP LTIP		—		269,779		—
Units, June 30, 2018		300,000		73,355,775		1,490,999

Details of cash distributions

We paid the following per-unit distributions during the three and six months ended June 30:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
Per-unit distributions to unitholders	$	0.575	$	0.575	$	1.150	$	1.150

Our payment of IDRs as reported in distributions to unitholders – general partner in the statement of partners' capital during the three and six months ended June 30 follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
IDR payments	$	2,136	$	2,106	$	4,264	$	4,206

EARNINGS PER UNIT (Tables)

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Schedule of earnings per limited partner unit

The following table details the components of EPU.

	Three months ended June 30,					Six months ended June 30,
	2018			2017		2018			2017
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net (loss) income among limited partner interests:
Net (loss) income attributable to limited partners	$	(51,111	)	$	8,806	$	(57,099	)	$	6,029
Less net income attributable to Series A Preferred Units		7,125			—		14,250			—
Net (loss) income attributable to common limited partners	$	(58,236	)	$	8,806	$	(71,349	)	$	6,029

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		73,356			72,532		73,245			72,341
Effect of nonvested phantom units		—			310		—			367
Weighted-average common units outstanding – diluted		73,356			72,842		73,245			72,708

(Loss) earnings per limited partner unit:
Common unit – basic	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08
Common unit – diluted	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		1			—		3			—

RELATED-PARTY TRANSACTIONS (Tables)

6 Months Ended

Jun. 30, 2018

Schedule of related party transactions

Expenses incurred by the General Partner and reimbursed by us under our Partnership Agreement were as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Operation and maintenance expense	$	7,114	$	6,731	$	14,737	$	13,612
General and administrative expense		7,481		7,895		15,598		16,190

COMMITMENTS AND CONTINGENCIES (Tables)

6 Months Ended

Jun. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of total rent expense related to operating leases

Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Rent expense	$	956	$	923	$	1,978	$	1,802

Schedule of accrued environmental remediation

A rollforward of the aggregate accrued environmental remediation liabilities follows.

	Total
	(In thousands)
Accrued environmental remediation, January 1, 2018	$	5,344
Payments made		(1,805	)
Additional accruals		1,600
Accrued environmental remediation, June 30, 2018	$	5,139

CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Tables)

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Condensed Balance Sheet

Balance sheets as of June 30, 2018 and December 31, 2017 follow.

	June 30, 2018
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	2,616	$	92	$	4,998	$	506	$	—		$	8,212
Accounts receivable		22		—		68,274		11,870		—			80,166
Other current assets		1,057		—		1,273		264		—			2,594
Due from affiliate		—		—		507,396		51,852		(559,248	)		—
Total current assets		3,695		92		581,941		64,492		(559,248	)		90,972
Property, plant and equipment, net		4,314		—		1,537,468		345,977		—			1,887,759
Intangible assets, net		—		—		263,416		23,249		—			286,665
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		670,827		—			670,827
Other noncurrent assets		3,288		9,762		94		—		—			13,144
Investment in subsidiaries		2,062,096		3,396,771		—		—		(5,458,867	)		—
Total assets	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

Liabilities and Partners' Capital
Trade accounts payable	$	141	$	—	$	18,280	$	3,277	$	—		$	21,698
Accrued expenses		803		—		17,030		1,600		—			19,433
Due to affiliate		374,074		185,265		—		—		(559,248	)		91
Deferred revenue		—		—		10,341		437		—			10,778
Ad valorem taxes payable		9		—		6,224		271		—			6,504
Accrued interest		—		12,259		—		—		—			12,259
Accrued environmental remediation		—		—		—		3,522		—			3,522
Other current liabilities		3,477		—		3,573		362		—			7,412
Total current liabilities		378,504		197,524		55,448		9,469		(559,248	)		81,697
Long-term debt		—		1,147,005		—		—		—			1,147,005
Deferred Purchase Price Obligation		453,922		—		—		—		—			453,922
Noncurrent deferred revenue		—		—		36,416		1,703		—			38,119
Noncurrent accrued environmental remediation		—		—		—		1,617		—			1,617
Other noncurrent liabilities		4,159		—		2,197		54		—			6,410
Total liabilities		836,585		1,344,529		94,061		12,843		(559,248	)		1,728,770

Total partners' capital		1,236,808		2,062,096		2,305,069		1,091,702		(5,458,867	)		1,236,808
Total liabilities and partners' capital	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

	December 31, 2017
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	126	$	75	$	1,138	$	91	$	—		$	1,430
Accounts receivable		22		—		65,842		6,437		—			72,301
Other current assets		1,188		—		2,697		442		—			4,327
Due from affiliate		—		—		493,067		22,030		(515,097	)		—
Total current assets		1,336		75		562,744		29,000		(515,097	)		78,058
Property, plant and equipment, net		4,206		—		1,442,333		348,590		—			1,795,129
Intangible assets, net		—		—		278,958		22,387		—			301,345
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		690,485		—			690,485
Other noncurrent assets		2,547		10,913		105		—		—			13,565
Investment in subsidiaries		2,019,700		3,324,464		—		—		(5,344,164	)		—
Total assets	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Liabilities and Partners' Capital
Trade accounts payable	$	209	$	—	$	11,283	$	4,883	$	—		$	16,375
Accrued expenses		928		—		10,592		979		—			12,499
Due to affiliate		263,935		252,250		—		—		(515,097	)		1,088
Deferred revenue		—		—		4,000		—		—			4,000
Ad valorem taxes payable		—		—		7,809		520		—			8,329
Accrued interest		—		12,310		—		—		—			12,310
Accrued environmental remediation		—		—		—		3,130		—			3,130
Other current liabilities		6,395		—		4,385		478		—			11,258
Total current liabilities		271,467		264,560		38,069		9,990		(515,097	)		68,989
Long-term debt		—		1,051,192		—		—		—			1,051,192
Deferred Purchase Price Obligation		362,959		—		—		—		—			362,959
Deferred revenue		—		—		12,707		—		—			12,707
Noncurrent accrued environmental remediation		—		—		—		2,214		—			2,214
Other noncurrent liabilities		3,694		—		3,293		76		—			7,063
Total liabilities		638,120		1,315,752		54,069		12,280		(515,097	)		1,505,124

Total partners' capital		1,389,669		2,019,700		2,246,282		1,078,182		(5,344,164	)		1,389,669
Total liabilities and partners' capital	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Condensed Statement of Operations

Statements of operations for the three and six months ended June 30, 2018 and 2017 follow.

	Three months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	73,891		$	15,694		$	—		$	89,585
Natural gas, NGLs and condensate sales		—			—			31,891			—			—			31,891
Other revenues		—			—			6,509			198			—			6,707
Total revenues		—			—			112,291			15,892			—			128,183
Costs and expenses:
Cost of natural gas and NGLs		—			—			24,384			—			—			24,384
Operation and maintenance		—			—			22,221			2,245			—			24,466
General and administrative		—			—			11,907			1,577			—			13,484
Depreciation and amortization		444			—			22,776			3,564			—			26,784
(Gain) loss on asset sales, net		—			—			(1	)		63			—			62
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		444			—			81,874			7,449			—			89,767
Other income		27			—			—			—			—			27
Interest expense		—			(14,837	)		—			—			—			(14,837	)
Deferred Purchase Price Obligation		(69,305	)		—			—			—			—			(69,305	)
(Loss) income before income taxes and loss from equity method investees		(69,722	)		(14,837	)		30,417			8,443			—			(45,699	)
Income tax expense		(294	)		—			—			—			—			(294	)
Loss from equity method investees		—			—			—			(3,920	)		—			(3,920	)
Equity in earnings of consolidated subsidiaries		20,103			34,940			—			—			(55,043	)		—
Net (loss) income	$	(49,913	)	$	20,103		$	30,417		$	4,523		$	(55,043	)	$	(49,913	)

	Three months ended June 30, 2017
	SMLP		Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—	$	—		$	67,679	$	17,122		$	—		$	84,801
Natural gas, NGLs and condensate sales		—		—			10,595		—			—			10,595
Other revenues		—		—			6,332		64			—			6,396
Total revenues		—		—			84,606		17,186			—			101,792
Costs and expenses:
Cost of natural gas and NGLs		—		—			9,099		—			—			9,099
Operation and maintenance		—		—			22,169		1,847			—			24,016
General and administrative		—		—			10,886		2,063			—			12,949
Depreciation and amortization		154		—			25,282		3,252			—			28,688
Transaction costs		119		—			—		—			—			119
Loss on asset sales, net		—		—			67		—			—			67
Long-lived asset impairment		—		—			2		1			—			3
Total costs and expenses		273		—			67,505		7,163			—			74,941
Other income		64		—			—		—			—			64
Interest expense		—		(17,553	)		—		—			—			(17,553	)
Deferred Purchase Price Obligation		5,058		—			—		—			—			5,058
Income (loss) before income taxes and loss from equity method investees		4,849		(17,553	)		17,101		10,023			—			14,420
Income tax benefit		211		—			—		—			—			211
Loss from equity method investees		—		—			—		(3,385	)		—			(3,385	)
Equity in earnings of consolidated subsidiaries		6,186		23,739			—		—			(29,925	)		—
Net income	$	11,246	$	6,186		$	17,101	$	6,638		$	(29,925	)	$	11,246

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	143,357		$	30,589		$	—		$	173,946
Natural gas, NGLs and condensate sales		—			—			58,008			—			—			58,008
Other revenues		—			—			13,236			313			—			13,549
Total revenues		—			—			214,601			30,902			—			245,503
Costs and expenses:
Cost of natural gas and NGLs		—			—			44,670			—			—			44,670
Operation and maintenance		—			—			44,374			4,696			—			49,070
General and administrative		—			—			24,244			3,682			—			27,926
Depreciation and amortization		876			—			45,637			6,948			—			53,461
(Gain) loss on asset sales, net		—			—			(75	)		63			—			(12	)
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		876			—			159,437			15,389			—			175,702
Other income		20			—			—			—			—			20
Interest expense		—			(29,959	)		—			—			—			(29,959	)
Deferred Purchase Price Obligation		(90,963	)		—			—			—			—			(90,963	)
(Loss) income before income taxes and loss from equity method investees		(91,819	)		(29,959	)		55,164			15,513			—			(51,101	)
Income tax expense		(123	)		—			—			—			—			(123	)
Loss from equity method investees		—			—			—			(2,534	)		—			(2,534	)
Equity in earnings of consolidated subsidiaries		38,184			68,143			—			—			(106,327	)		—
Net (loss) income	$	(53,758	)	$	38,184		$	55,164		$	12,979		$	(106,327	)	$	(53,758	)

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	170,192	$	32,622		$	—		$	202,814
Natural gas, NGLs and condensate sales		—			—			21,715		—			—			21,715
Other revenues		—			—			12,932		136			—			13,068
Total revenues		—			—			204,839		32,758			—			237,597
Costs and expenses:
Cost of natural gas and NGLs		—			—			18,151		—			—			18,151
Operation and maintenance		—			—			44,189		3,519			—			47,708
General and administrative		—			—			23,371		3,710			—			27,081
Depreciation and amortization		308			—			50,524		6,425			—			57,257
Transaction costs		119			—			—		—			—			119
Loss on asset sales, net		—			—			70		—			—			70
Long-lived asset impairment		—			—			2		285			—			287
Total costs and expenses		427			—			136,307		13,939			—			150,673
Other income		135			—			—		—			—			135
Interest expense		—			(34,269	)		—		—			—			(34,269	)
Early extinguishment of debt		—			(22,020	)		—		—			—			(22,020	)
Deferred Purchase Price Obligation		(15,825	)		—			—		—			—			(15,825	)
(Loss) income before income taxes and loss from equity method investees		(16,117	)		(56,289	)		68,532		18,819			—			14,945
Income tax expense		(241	)		—			—		—			—			(241	)
Loss from equity method investees		—			—			—		(4,041	)		—			(4,041	)
Equity in earnings of consolidated subsidiaries		27,021			83,310			—		—			(110,331	)		—
Net income	$	10,663		$	27,021		$	68,532	$	14,778		$	(110,331	)	$	10,663

Condensed Cash Flow Statement

Statements of cash flows for the six months ended June 30, 2018 and 2017 follow.

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$	(68	)	$	(27,877	)	$	103,965		$	34,029		$	—		$	110,049

Cash flows from investing activities:
Capital expenditures		(972	)		—			(85,187	)		(4,235	)		—			(90,394	)
Proceeds from asset sales		—			—			—			496			—			496
Other, net		(306	)		—			—			—			—			(306	)
Advances to affiliates		—			(66,985	)		(14,329	)		(29,820	)		111,134			—
Net cash used in investing activities		(1,278	)		(66,985	)		(99,516	)		(33,559	)		111,134			(90,204	)

Cash flows from financing activities:
Distributions to common unitholders		(90,269	)		—			—			—			—			(90,269	)
Distributions to Series A Preferred unitholders		(14,250	)		—			—			—			—			(14,250	)
Borrowings under Revolving Credit Facility		—			148,000			—			—			—			148,000
Repayments under Revolving Credit Facility		—			(53,000	)		—			—			—			(53,000	)
Debt issuance costs		—			(121	)		—			—			—			(121	)
Other, net		(2,779	)		—			(589	)		(55	)		—			(3,423	)
Advances from affiliates		111,134			—			—			—			(111,134	)		—
Net cash provided by (used in) financing activities		3,836			94,879			(589	)		(55	)		(111,134	)		(13,063	)
Net change in cash and cash equivalents		2,490			17			3,860			415			—			6,782
Cash and cash equivalents, beginning of period		126			75			1,138			91			—			1,430
Cash and cash equivalents, end of period	$	2,616		$	92		$	4,998		$	506		$	—		$	8,212

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$	424		$	(32,466	)	$	114,863		$	38,520		$	—		$	121,341

Cash flows from investing activities:
Capital expenditures		(6,412	)		—			(23,578	)		(15,922	)		—			(45,912	)
Contributions to equity method investees		—			—			—			(15,649	)		—			(15,649	)
Other, net		(521	)		—			—			—			—			(521	)
Advances to affiliates		7,635			23,020			(95,361	)		—			64,706			—
Net cash provided by (used in) investing activities		702			23,020			(118,939	)		(31,571	)		64,706			(62,082	)

Cash flows from financing activities:
Distributions to unitholders		(89,029	)		—			—			—			—			(89,029	)
Borrowings under Revolving Credit Facility		—			112,500			—			—			—			112,500
Repayments under Revolving Credit Facility		—			(269,500	)		—			—			—			(269,500	)
Debt issuance costs		—			(15,613	)		—			—			—			(15,613	)
Payment of redemption and call premiums on senior notes		—			(17,913	)		—			—			—			(17,913	)
Proceeds from ATM Program issuances, net of costs		17,259			—			—			—			—			17,259
Contribution from General Partner		465			—			—			—			—			465
Issuance of senior notes		—			500,000			—			—			—			500,000
Tender and redemption of senior notes		—			(300,000	)		—			—			—			(300,000	)
Other, net		(2,632	)		—			372			(8	)		—			(2,268	)
Advances from affiliates		72,341			—			—			(7,635	)		(64,706	)		—
Net cash (used in) provided by financing activities		(1,596	)		9,474			372			(7,643	)		(64,706	)		(64,099	)
Net change in cash and cash equivalents		(470	)		28			(3,704	)		(694	)		—			(4,840	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	228		$	79		$	2,064		$	217		$	—		$	2,588

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION - Narrative (Details) - shares	1 Months Ended	6 Months Ended
	Feb. 29, 2016	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2016
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		73,355,775	73,085,996
Summit Midstream Niobrara, LLC \| Summit Midstream Partners, LP
Organization And Business Operations [Line Items]
Cost method ownership interest			1.00%
SMP Holdings \| Contributed Entities
Organization And Business Operations [Line Items]
Percent of issued and outstanding membership interest	100.00%
SMP Holdings \| OGC
Organization And Business Operations [Line Items]
Percentage of ownership interest	40.00%
Subsidiary of Summit Investments
Organization And Business Operations [Line Items]
Ownership interest				1.00%
Grand River Gathering, LLC
Organization And Business Operations [Line Items]
Percentage of ownership interest			100.00%
Common units \| SMP Holdings
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		25,854,581
Common units \| Energy Capital Partners
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		5,915,827
General Partner
Organization And Business Operations [Line Items]
General partner interest		2.00%
General Partner \| Summit Investments \| Summit Midstream Partners, LP
Organization And Business Operations [Line Items]
General partner interest		2.00%

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Acceleration totaled, recognition of revenue	$ 16.7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606 (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Property, plant and equipment, net	$ 1,887,759	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	10,778	10,088	4,000
Deferred revenue, noncurrent	38,119	35,528	12,707
Partners' Capital	932,236	1,088,644
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Property, plant and equipment, net	1,852,126		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	11,133		12,707
Partners' Capital	921,898		$ 1,084,430
Accounting Standard Update 2014-09 \| Adjustments Due to Topic 606
Assets
Property, plant and equipment, net	35,633	33,123
Liabilities
Deferred revenue	6,707	6,088
Deferred revenue, noncurrent	26,986	22,821
Partners' Capital	$ 10,338	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable	$ 80,166
Property, plant and equipment, net	1,887,759	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	10,778	10,088	4,000
Deferred revenue, noncurrent	38,119	35,528	12,707
Partners' Capital	932,236	1,088,644
Balances Without Adoption of Topic 606
Assets
Accounts receivable	71,768
Property, plant and equipment, net	1,852,126		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	11,133		12,707
Partners' Capital	921,898		$ 1,084,430
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Assets
Accounts receivable	8,398
Property, plant and equipment, net	35,633	33,123
Liabilities
Deferred revenue	6,707	6,088
Deferred revenue, noncurrent	26,986	22,821
Partners' Capital	$ 10,338	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Gathering services and related fees	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,784	$ 28,688	$ 53,461	$ 57,257
Balances Without Adoption of Topic 606
Costs and expenses:
Cost of natural gas and NGLs	$ 27,559
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,472
Gathering Services and Related Fees
Revenues:
Gathering services and related fees	89,585	$ 84,801	$ 173,946	$ 202,814
Gathering Services and Related Fees \| Balances Without Adoption of Topic 606
Revenues:
Gathering services and related fees	86,892
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Costs and expenses:
Cost of natural gas and NGLs	$ (3,175)
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 312
Accounting Standard Update 2014-09 \| Gathering Services and Related Fees \| Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees	$ 2,693

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (49,913)	$ 11,246	$ (53,758)	$ 10,663
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization	$ 26,634	$ 28,537	53,160	56,955
Changes in operating assets and liabilities:
Accounts receivable			(6,265)	41,527
Deferred revenue, net			3,281	$ (39,671)
Balances Without Adoption of Topic 606
Cash flows from operating activities:
Net loss			(61,385)
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization			52,545
Changes in operating assets and liabilities:
Accounts receivable			2,133
Deferred revenue, net			3,125
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Cash flows from operating activities:
Net loss			7,627
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization			615
Changes in operating assets and liabilities:
Accounts receivable			(8,398)
Deferred revenue, net			$ 156

REVENUE - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
REVENUE - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue From Contract With Customer [Line Items]
Revenue, performance obligation, description of payment terms		Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components.
Receivables with customers	$ 67,500	$ 67,500
Contract assets included in accounts receivable	9,333	9,333	$ 0
Gathering Services and Related Fees
Revenue From Contract With Customer [Line Items]
Contract liabilities, revenue	$ 3,900	$ 5,000
Maximum
Revenue From Contract With Customer [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	25 years	25 years

REVENUE - Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	10,422		20,463
Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	33,416		61,840
Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	42,992		84,579
Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	16,155		32,325
Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	8,050		15,875
Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	111,035		215,082
All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	17,148		30,421
Gathering Services and Related Fees
Disaggregation Of Revenue [Line Items]
Total revenues	89,585		173,946
Gathering Services and Related Fees \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	10,422		20,463
Gathering Services and Related Fees \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	23,106		40,772
Gathering Services and Related Fees \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	36,170		71,464
Gathering Services and Related Fees \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	14,080		27,717
Gathering Services and Related Fees \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	8,050		15,875
Gathering Services and Related Fees \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	91,828		176,291
Gathering Services and Related Fees \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	(2,243)		(2,345)
Natural Gas, NGLs and Condensate Sales
Disaggregation Of Revenue [Line Items]
Total revenues	31,891		58,008
Natural Gas, NGLs and Condensate Sales \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Natural Gas, NGLs and Condensate Sales \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	7,350		15,196
Natural Gas, NGLs and Condensate Sales \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	4,675		9,000
Natural Gas, NGLs and Condensate Sales \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	381		926
Natural Gas, NGLs and Condensate Sales \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Natural Gas, NGLs and Condensate Sales \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	12,406		25,122
Natural Gas, NGLs and Condensate Sales \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	19,485		32,886
Other Revenues
Disaggregation Of Revenue [Line Items]
Total revenues	6,707		13,549
Other Revenues \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Other Revenues \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	2,960		5,872
Other Revenues \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	2,147		4,115
Other Revenues \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	1,694		3,682
Other Revenues \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Other Revenues \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	6,801		13,669
Other Revenues \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	$ (94)		$ (120)

REVENUE - Schedule of Information about Contract Assets from Contracts with Customers (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Contract With Customer Asset And Liability [Abstract]
Contract assets, December 31, 2017	$ 0
Net impact of Topic 606 day 1 adoption	3,514
Additions	11,523
Transfers out	(5,704)
Contract assets, June 30, 2018	9,333
Current	9,333
Noncurrent	$ 0

SEGMENT INFORMATION - Assets, Revenues, Depreciation and Amortization, and Capital Expenditures by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 2,965,578		$ 2,965,578		$ 2,894,793
Revenues	128,183	$ 101,792	245,503	$ 237,597
Depreciation and amortization	26,634	28,537	53,160	56,955
Capital expenditures			90,394	45,912
Corporate
Segment Reporting Information [Line Items]
Capital expenditures			1,000
Reportable Segments
Segment Reporting Information [Line Items]
Assets	2,823,286		2,823,286		2,815,046
Revenues	111,035	101,602	215,082	236,737
Depreciation and amortization	26,138	28,383	52,184	56,647
Capital expenditures			38,533	39,500
Reportable Segments \| Utica Shale
Segment Reporting Information [Line Items]
Assets	212,326		212,326		212,311
Revenues	10,422	10,456	20,463	19,252
Depreciation and amortization	2,033	1,748	3,886	3,395
Capital expenditures			1,846	16,473
Reportable Segments \| Ohio Gathering
Segment Reporting Information [Line Items]
Assets	670,827		670,827		690,485
Reportable Segments \| Williston Basin
Segment Reporting Information [Line Items]
Assets	520,799		520,799		512,860
Revenues	33,416	29,114	61,840	95,999
Depreciation and amortization	5,622	8,385	11,231	16,766
Capital expenditures			10,966	11,085
Reportable Segments \| Piceance/DJ Basins
Segment Reporting Information [Line Items]
Assets	828,999		828,999		798,722
Revenues	42,992	33,763	84,579	68,571
Depreciation and amortization	12,450	12,225	25,005	24,436
Capital expenditures			24,827	11,934
Reportable Segments \| Barnett Shale
Segment Reporting Information [Line Items]
Assets	376,969		376,969		383,306
Revenues	16,155	20,904	32,325	38,646
Depreciation and amortization	3,759	3,762	7,516	7,524
Capital expenditures			349	(399)
Reportable Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Assets	213,366		213,366		217,362
Revenues	8,050	7,365	15,875	14,269
Depreciation and amortization	2,274	2,263	4,546	4,526
Capital expenditures			545	407
Corporate and Other
Segment Reporting Information [Line Items]
Assets	143,450		143,450		79,996
Revenues	19,422	1,362	33,598	3,148
Depreciation and amortization	496	154	976	308
Capital expenditures			51,861	6,412
Eliminations
Segment Reporting Information [Line Items]
Assets	(1,158)		(1,158)		$ (249)
Revenues	$ (2,274)	$ (1,172)	$ (3,177)	$ (2,288)

SEGMENT INFORMATION - Concentration Risk (Details) - Revenue - Customer concentration	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Counterparty A - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	10.00%	0.00%	11.00%	0.00%
Counterparty B - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	10.00%	0.00%	0.00%
Counterparty C - Barnett Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	11.00%	11.00%	0.00%	0.00%
Counterparty D - Utica Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	10.00%	0.00%	0.00%
Counterparty E - Williston Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	0.00%	0.00%	21.00%

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 82,497	$ 82,012	$ 163,429	$ 163,515
Utica Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	9,223	9,533	17,938	17,445
Ohio Gathering
Segment Reporting Information [Line Items]
Adjusted EBITDA	8,935	9,606	19,412	18,679
Williston Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	19,030	17,155	35,000	34,964
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Adjusted EBITDA	27,673	27,274	56,908	56,248
Barnett Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	11,093	12,998	20,952	25,086
Marcellus Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 6,543	$ 5,446	$ 13,219	$ 11,093

SEGMENT INFORMATION - Reconciliation of Net Income to Adjusted EBITDA (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting [Abstract]
(Loss) income before income taxes and loss from equity method investees	$ (45,699,000)	$ 14,420,000	$ (51,101,000)	$ 14,945,000
Add:
Corporate and Other	9,002,000	9,435,000	19,625,000	19,528,000
Interest expense	14,837,000	17,553,000	29,959,000	34,269,000
Early extinguishment of debt	0	0	0	22,020,000
Deferred Purchase Price Obligation	69,305,000	(5,058,000)	90,963,000	15,825,000
Depreciation and amortization	26,634,000	28,537,000	53,160,000	56,955,000
Proportional adjusted EBITDA for equity method investees	8,935,000	9,606,000	19,412,000	18,679,000
Adjustments related to MVC shortfall payments	(3,542,000)	5,578,000	(3,542,000)	(23,062,000)
Adjustments related to capital reimbursement activity	115,000	0	155,000	0
Unit-based and noncash compensation	2,261,000	1,871,000	4,223,000	3,999,000
Loss (gain) on asset sales, net	62,000	67,000	(12,000)	70,000
Long-lived asset impairment	587,000	3,000	587,000	287,000
Total of reportable segments' measures of profit	$ 82,497,000	$ 82,012,000	$ 163,429,000	$ 163,515,000

SEGMENT INFORMATION - Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	$ 0	$ (1,186,000)	$ 0	$ (39,671,000)
Expected MVC shortfall adjustments	(3,542,000)	6,764,000	(3,542,000)	16,609,000
Total adjustments related to MVC shortfall payments	(3,542,000)	5,578,000	(3,542,000)	(23,062,000)
Williston Basin
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	0	0	(37,693,000)
Expected MVC shortfall adjustments	(3,386,000)	1,982,000	(3,386,000)	3,964,000
Total adjustments related to MVC shortfall payments	(3,386,000)	1,982,000	(3,386,000)	(33,729,000)
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	(1,186,000)	0	(1,978,000)
Expected MVC shortfall adjustments	(93,000)	6,522,000	(93,000)	13,067,000
Total adjustments related to MVC shortfall payments	(93,000)	5,336,000	(93,000)	11,089,000
Barnett Shale
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	0	0	0
Expected MVC shortfall adjustments	(63,000)	(1,740,000)	(63,000)	(422,000)
Total adjustments related to MVC shortfall payments	$ (63,000)	$ (1,740,000)	$ (63,000)	$ (422,000)

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Property, Plant, and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,237,608		$ 2,104,569
Less accumulated depreciation	349,849		309,440
Property, plant and equipment, net	1,887,759	$ 1,828,252	1,795,129
Gathering and processing systems and related equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,023,832		1,973,722
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	160,239		78,850
Land and line fill
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,735		11,735
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 41,802		$ 40,262

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Depreciation Expense and Capitalized Interest (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 18,657	$ 18,607	$ 37,214	$ 37,098
Capitalized interest	$ 1,863	$ 450	$ 3,085	$ 918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 464,982	$ 462,629
Accumulated amortization	(178,317)	(161,284)
Net	286,665	301,345
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(13,128)	(12,350)
Net	11,067	11,845
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(130,891)	(117,821)
Net	147,557	160,627
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	162,339	159,986
Accumulated amortization	(34,298)	(31,113)
Net	128,041	128,873
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	(10,153)	(9,074)
Net	$ 809	$ 1,888

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Recognized Amortization Expense in Other Revenues and Cost and Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Revenue \| Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ (388)	$ (388)	$ (777)	$ (777)
Other Revenue \| Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Amortization expense	538	539	1,078	1,079
Costs And Expenses \| Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization expense	6,535	8,551	13,070	17,102
Costs And Expenses \| Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 1,592	$ 1,530	$ 3,177	$ 3,057

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Intangible assets
2018	$ 17,053
2019	33,217
2020	33,041
2021	29,349
2022	26,282
Unfavorable gas gathering contract
2018	809
2019	0
2020	0
2021	0
2022	$ 0

GOODWILL - Narrative (Details)	6 Months Ended
GOODWILL - Narrative (Details)	Jun. 30, 2018 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill impairment	$ 0

EQUITY METHOD INVESTMENTS - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Equity Method Investments [Line Items]
Contributions to acquire equity method investments			$ 0	$ 15,649,000
OGC
Schedule Of Equity Method Investments [Line Items]
Contributions to acquire equity method investments	$ 0		0
OGC and OCC
Schedule Of Equity Method Investments [Line Items]
Asset impairment	$ 12,300,000	$ 8,700,000	$ 12,300,000	$ 8,700,000
Principal Owner \| OGC
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest	40.00%		40.00%

EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	$ 670,827	$ 670,827
June cash distributions		17,124	$ 18,003
Investment in equity method investees, net of basis difference, May 31, 2018		690,485
OGC \| Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	670,827	$ 670,827
June cash distributions	2,580
Basis difference	(123,508)
Investment in equity method investees, net of basis difference, May 31, 2018	$ 549,899

EQUITY METHOD INVESTMENTS - Statements of Operations Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2018	May 31, 2017
OGC
Schedule Of Equity Method Investments [Line Items]
Total revenues	$ 34,123	$ 31,083	$ 69,083	$ 68,158
Total operating expenses	35,518	33,221	62,293	60,326
Net (loss) income	(1,396)	(139)	6,784	7,834
OCC
Schedule Of Equity Method Investments [Line Items]
Total revenues	2,070	2,004	4,559	4,057
Total operating expenses	1,958	1,836	4,099	4,309
Net (loss) income	$ (59)	$ 23	$ 121	$ (1,192)

DEFERRED REVENUE - Rollforward of deferred revenue (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 4,000
Current deferred revenue, Additions	12,926
Current deferred revenue, Less revenue recognized	12,236
Current deferred revenue, ending balance	10,778
Noncurrent deferred revenue, beginning balance	12,707
Noncurrent deferred revenue, Additions	8,405
Noncurrent deferred revenue, Less reclassification to current deferred revenue	5,814
Noncurrent deferred revenue, ending balance	38,119
Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	10,088
Noncurrent deferred revenue, beginning balance	35,528
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	6,088
Noncurrent deferred revenue, beginning balance	22,821
Utica Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	9
Current deferred revenue, Less revenue recognized	9
Current deferred revenue, ending balance	18
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	9
Noncurrent deferred revenue, ending balance	30
Utica Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Noncurrent deferred revenue, beginning balance	39
Utica Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Noncurrent deferred revenue, beginning balance	39
Williston Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	1,088
Current deferred revenue, Less revenue recognized	643
Current deferred revenue, ending balance	1,462
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	1,622
Noncurrent deferred revenue, Less reclassification to current deferred revenue	1,017
Noncurrent deferred revenue, ending balance	4,820
Williston Basin \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Noncurrent deferred revenue, beginning balance	4,215
Williston Basin \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Noncurrent deferred revenue, beginning balance	4,215
Piceance/DJ Basins
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	4,000
Current deferred revenue, Additions	10,996
Current deferred revenue, Less revenue recognized	10,762
Current deferred revenue, ending balance	7,630
Noncurrent deferred revenue, beginning balance	12,707
Noncurrent deferred revenue, Additions	5,200
Noncurrent deferred revenue, Less reclassification to current deferred revenue	3,955
Noncurrent deferred revenue, ending balance	23,969
Piceance/DJ Basins \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	7,396
Noncurrent deferred revenue, beginning balance	22,724
Piceance/DJ Basins \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	3,396
Noncurrent deferred revenue, beginning balance	10,017
Barnett Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	814
Current deferred revenue, Less revenue recognized	803
Current deferred revenue, ending balance	1,630
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	1,583
Noncurrent deferred revenue, Less reclassification to current deferred revenue	814
Noncurrent deferred revenue, ending balance	8,986
Barnett Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Noncurrent deferred revenue, beginning balance	8,217
Barnett Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Noncurrent deferred revenue, beginning balance	8,217
Marcellus Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	19
Current deferred revenue, Less revenue recognized	19
Current deferred revenue, ending balance	38
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	19
Noncurrent deferred revenue, ending balance	314
Marcellus Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Noncurrent deferred revenue, beginning balance	333
Marcellus Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Noncurrent deferred revenue, beginning balance	$ 333

DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,147,005	$ 1,051,192
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,147,005	1,051,192
SMP Holdings \| Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	356,000	261,000
SMP Holdings \| Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(2,588)	(2,910)
SMP Holdings \| Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	500,000
Less unamortized debt issuance costs	$ (6,407)	$ (6,898)

DEBT - Components of Long-Term Debt (Parenthetical) (Details) - SMP Holdings	Jun. 30, 2018	Dec. 31, 2017
Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate	4.35%	4.07%
Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Stated interest rate	5.50%	5.50%
Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Stated interest rate	5.75%	5.75%

DEBT - Revolving Credit Facility (Narrative) (Details) - Revolving credit facility	6 Months Ended
	Jun. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Borrowing capacity	$ 1,250,000,000
Revolving credit facility maturity date	2022-05
Accordion feature	$ 250,000,000.0
Commitment fee on unused portion of the facility	0.375%
Interest rate at period end	4.35%
Unused portion under the facility	$ 894,000,000.0
Debt issuance costs	9,800,000
Debt defaults	$ 0
LIBOR
Line of Credit Facility [Line Items]
Applicable margin	2.25%
Minimum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility	0.30%
Minimum \| ABR
Line of Credit Facility [Line Items]
Applicable margin	0.75%
Minimum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin	1.75%
Maximum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility	0.50%
Maximum \| ABR
Line of Credit Facility [Line Items]
Applicable margin	1.75%
Maximum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin	2.75%

DEBT - Senior Notes (Narrative) (Details) - USD ($)		1 Months Ended
DEBT - Senior Notes (Narrative) (Details) - USD ($)	Jul. 31, 2014	Jul. 31, 2014	Feb. 08, 2017
5.75% Senior Notes
Debt Instrument [Line Items]
Stated interest rate	5.75%	5.75%	5.75%
Debt face amount			$ 500,000,000.0
Summit Holdings and Finance Corporation \| Senior Notes Due August 2022
Debt Instrument [Line Items]
Senior unsecured notes	$ 300,000,000.0	$ 300,000,000.0
Stated interest rate	5.50%	5.50%
Maturity date	Aug. 15, 2022
Finance Corp \| Summit Holdings
Debt Instrument [Line Items]
Cumulative percentage ownership in subsidiary		100.00%

FINANCIAL INSTRUMENTS - Concentration Risk - Narrative (Details) - Customer	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
CONCENTRATIONS OF RISK
Number of top customers	5
Five Largest Customers \| Accounts receivable \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	46.00%	44.00%

FINANCIAL INSTRUMENTS - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 liability, beginning of period	$ 362,959
Change in fair value	90,963
Level 3 liability, end of period	$ 453,922

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying value \| 5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 297,412	$ 297,090
Carrying value \| Senior Notes 5.75%
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	493,593	493,102
Fair Value \| 5.5% Senior Notes \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	293,875	301,750
Fair Value \| Senior Notes 5.75% \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 474,375	$ 501,667

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Parenthetical) (Details) - Summit Holdings - USD ($)	Jun. 30, 2018	Dec. 31, 2017
5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate	5.50%	5.50%
Debt face amount	$ 300,000,000.0	$ 300,000,000.0
Senior Notes 5.75%
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate	5.75%
Debt face amount	$ 500,000,000.0

PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details)	6 Months Ended
	Jun. 30, 2018 shares
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	300,000
Units, ending balance (in shares)	300,000
Net units issued under the SMLP LTIP (in shares)	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	73,085,996
Units, ending balance (in shares)	73,355,775
Net units issued under the SMLP LTIP (in shares)	269,779
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,490,999
Units, ending balance (in shares)	1,490,999
Net units issued under the SMLP LTIP (in shares)	0

PARTNERS' CAPITAL - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 26, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 $ / shares	Jun. 30, 2018 USD ($) Transaction $ / shares shares	Jun. 30, 2017 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares
Partners Capital [Line Items]
Preferred units issued \| shares		300,000		300,000		300,000
Per-unit annual distributions to unitholders (in dollars per unit)		$ 0.575	$ 0.575	$ 1.150	$ 1.150
Surplus in excess (in dollars per share)				$ 0.46
Subsequent Event
Partners Capital [Line Items]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.575
Cash paid to unit holders \| $	$ 45.2
Cash distribution date	Aug. 14, 2018
Cash distribution date of record	Aug. 07, 2018
Series A Preferred Units
Partners Capital [Line Items]
Preferred units issued \| shares						300,000
Interest in partnership per unit						$ 1,000
Common \| Public Offering
Partners Capital [Line Items]
Aggregate offering price \| $						$ 150.0
Amount of stock sold (in shares) \| shares		0		0
Aggregate offering price \| $		$ 132.3		$ 132.3
Transactions under ATM Program \| Transaction				0
General Partner
Partners Capital [Line Items]
Percentage interest in distributions (maximum)				50.00%

PARTNERS' CAPITAL - Cash Distributions Paid and Declared (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.575	$ 0.575	$ 1.150	$ 1.150

PARTNERS' CAPITAL - Incentive Distribution Rights (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
IDR payments	$ 2,136	$ 2,106	$ 4,264	$ 4,206

EARNINGS PER UNIT (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ (51,111)	$ 8,806	$ (57,099)	$ 6,029
Phantom Units
(Loss) earnings per limited partner unit:
Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU (in shares)	1	0	3	0
Series A Preferred Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ 7,125	$ 0	$ 14,250	$ 0
Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ (58,236)	$ 8,806	$ (71,349)	$ 6,029
Weighted-average units outstanding (in shares)	73,356	72,532	73,245	72,341
Effect of nonvested phantom units (in shares)	0	310	0	367
Weighted-average units outstanding – diluted (shares)	73,356	72,842	73,245	72,708
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Diluted (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08

UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) - SMLP LTIP - $ / shares	1 Months Ended
	Mar. 31, 2018	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Units remaining available (in shares)		3,200,000
Phantom Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted units (in shares)	515,358
Grant date fair value (in dollars per share)	$ 15.25
Vesting period (years)	3 years
Units vested (in shares)	328,388

RELATED-PARTY TRANSACTIONS - Expenses Incurred by the General Partner and Reimbursed (Details) - General Partner - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operation and maintenance expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 7,114	$ 6,731	$ 14,737	$ 13,612
General and administrative expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 7,481	$ 7,895	$ 15,598	$ 16,190

COMMITMENTS AND CONTINGENCIES - Rent Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 956	$ 923	$ 1,978	$ 1,802

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2017	Jun. 30, 2018	Dec. 31, 2015
Maximum \| Meadowlark Midstream Gathering System \| Principal Owner
Loss Contingencies [Line Items]
Coverage from pollution liability insurance policy		$ 25,000,000.0	$ 25,000,000.0
Coverage from property and business interruption insurance policy		$ 200,000,000.0
Business Interruption
Loss Contingencies [Line Items]
Recoveries recognized	$ 2,600,000
Property Recoveries
Loss Contingencies [Line Items]
Recoveries recognized	$ 400,000

COMMITMENTS AND CONTINGENCIES - Environmental Matters (Details) - Meadowlark Midstream Gathering System $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 5,344
Payments made	(1,805)
Additional accruals	1,600
Accrued environmental remediation, ending balance	$ 5,139

ACQUISITIONS AND DROP DOWN TRANSACTIONS - 2016 Drop Down (Details) - USD ($) $ in Thousands		1 Months Ended
	Mar. 03, 2016	Jun. 30, 2016	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Deferred purchase price obligation			$ 453,922	$ 362,959
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Initial payment	$ 360,000
Working capital adjustment		$ 600
Estimated future payment obligation			538,400
Deferred purchase price obligation			$ 453,900
Discount rate			10.25%

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 8,212		$ 1,430	$ 2,588	$ 7,428
Accounts receivable	80,166		72,301
Other current assets	2,594		4,327
Due from affiliate	0		0
Total current assets	90,972		78,058
Property, plant and equipment, net	1,887,759	$ 1,828,252	1,795,129
Intangible assets, net	286,665		301,345
Goodwill	16,211		16,211
Investment in equity method investees	670,827		690,485
Other noncurrent assets	13,144		13,565
Investment in subsidiaries	0		0
Total assets	2,965,578		2,894,793
Liabilities and Partners' Capital
Trade accounts payable	21,698		16,375
Accrued expenses	19,433		12,499
Due to affiliate	91		1,088
Deferred revenue	10,778	10,088	4,000
Ad valorem taxes payable	6,504		8,329
Accrued interest	12,259		12,310
Accrued environmental remediation	3,522		3,130
Other current liabilities	7,412		11,258
Total current liabilities	81,697		68,989
Long-term debt	1,147,005		1,051,192
Deferred Purchase Price Obligation	453,922		362,959
Noncurrent deferred revenue	38,119	$ 35,528	12,707
Noncurrent accrued environmental remediation	1,617		2,214
Other noncurrent liabilities	6,410		7,063
Total liabilities	1,728,770		1,505,124
Total partners' capital	1,236,808		1,389,669	1,110,899	1,169,673
Total liabilities and partners' capital	2,965,578		2,894,793
Consolidating adjustments
Assets
Cash and cash equivalents	0		0	0	0
Accounts receivable	0		0
Other current assets	0		0
Due from affiliate	(559,248)		(515,097)
Total current assets	(559,248)		(515,097)
Property, plant and equipment, net	0		0
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	0		0
Investment in subsidiaries	(5,458,867)		(5,344,164)
Total assets	(6,018,115)		(5,859,261)
Liabilities and Partners' Capital
Trade accounts payable	0		0
Accrued expenses	0		0
Due to affiliate	(559,248)		(515,097)
Deferred revenue	0		0
Ad valorem taxes payable	0		0
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	0		0
Total current liabilities	(559,248)		(515,097)
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	0		0
Total liabilities	(559,248)		(515,097)
Total partners' capital	(5,458,867)		(5,344,164)
Total liabilities and partners' capital	(6,018,115)		(5,859,261)
Summit Midstream Partners, LP \| Reportable Legal Entities
Assets
Cash and cash equivalents	2,616		126	228	698
Accounts receivable	22		22
Other current assets	1,057		1,188
Due from affiliate	0		0
Total current assets	3,695		1,336
Property, plant and equipment, net	4,314		4,206
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	3,288		2,547
Investment in subsidiaries	2,062,096		2,019,700
Total assets	2,073,393		2,027,789
Liabilities and Partners' Capital
Trade accounts payable	141		209
Accrued expenses	803		928
Due to affiliate	374,074		263,935
Deferred revenue	0		0
Ad valorem taxes payable	9		0
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	3,477		6,395
Total current liabilities	378,504		271,467
Long-term debt	0		0
Deferred Purchase Price Obligation	453,922		362,959
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	4,159		3,694
Total liabilities	836,585		638,120
Total partners' capital	1,236,808		1,389,669
Total liabilities and partners' capital	2,073,393		2,027,789
Co-Issuers \| Reportable Legal Entities
Assets
Cash and cash equivalents	92		75	79	51
Accounts receivable	0		0
Other current assets	0		0
Due from affiliate	0		0
Total current assets	92		75
Property, plant and equipment, net	0		0
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	9,762		10,913
Investment in subsidiaries	3,396,771		3,324,464
Total assets	3,406,625		3,335,452
Liabilities and Partners' Capital
Trade accounts payable	0		0
Accrued expenses	0		0
Due to affiliate	185,265		252,250
Deferred revenue	0		0
Ad valorem taxes payable	0		0
Accrued interest	12,259		12,310
Accrued environmental remediation	0		0
Other current liabilities	0		0
Total current liabilities	197,524		264,560
Long-term debt	1,147,005		1,051,192
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	0		0
Total liabilities	1,344,529		1,315,752
Total partners' capital	2,062,096		2,019,700
Total liabilities and partners' capital	3,406,625		3,335,452
Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	4,998		1,138	2,064	5,768
Accounts receivable	68,274		65,842
Other current assets	1,273		2,697
Due from affiliate	507,396		493,067
Total current assets	581,941		562,744
Property, plant and equipment, net	1,537,468		1,442,333
Intangible assets, net	263,416		278,958
Goodwill	16,211		16,211
Investment in equity method investees	0		0
Other noncurrent assets	94		105
Investment in subsidiaries	0		0
Total assets	2,399,130		2,300,351
Liabilities and Partners' Capital
Trade accounts payable	18,280		11,283
Accrued expenses	17,030		10,592
Due to affiliate	0		0
Deferred revenue	10,341		4,000
Ad valorem taxes payable	6,224		7,809
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	3,573		4,385
Total current liabilities	55,448		38,069
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	36,416		12,707
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	2,197		3,293
Total liabilities	94,061		54,069
Total partners' capital	2,305,069		2,246,282
Total liabilities and partners' capital	2,399,130		2,300,351
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	506		91	$ 217	$ 911
Accounts receivable	11,870		6,437
Other current assets	264		442
Due from affiliate	51,852		22,030
Total current assets	64,492		29,000
Property, plant and equipment, net	345,977		348,590
Intangible assets, net	23,249		22,387
Goodwill	0		0
Investment in equity method investees	670,827		690,485
Other noncurrent assets	0		0
Investment in subsidiaries	0		0
Total assets	1,104,545		1,090,462
Liabilities and Partners' Capital
Trade accounts payable	3,277		4,883
Accrued expenses	1,600		979
Due to affiliate	0		0
Deferred revenue	437		0
Ad valorem taxes payable	271		520
Accrued interest	0		0
Accrued environmental remediation	3,522		3,130
Other current liabilities	362		478
Total current liabilities	9,469		9,990
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	1,703		0
Noncurrent accrued environmental remediation	1,617		2,214
Other noncurrent liabilities	54		76
Total liabilities	12,843		12,280
Total partners' capital	1,091,702		1,078,182
Total liabilities and partners' capital	$ 1,104,545		$ 1,090,462

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 24,466	$ 24,016	$ 49,070	$ 47,708
General and administrative	13,484	12,949	27,926	27,081
Depreciation and amortization	26,784	28,688	53,461	57,257
Transaction costs	0	119	0	119
Loss (gain) on asset sales, net	62	67	(12)	70
Long-lived asset impairment	587	3	587	287
Total costs and expenses	89,767	74,941	175,702	150,673
Other income	27	64	20	135
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt	0	0	0	(22,020)
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(45,699)	14,420	(51,101)	14,945
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	0	0	0	0
Transaction costs		0		0
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	0	0	0	0
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	(55,043)	(29,925)	(106,327)	(110,331)
Net (loss) income	(55,043)	(29,925)	(106,327)	(110,331)
Gathering Services and Related Fees
Revenues:
Total revenues	89,585	84,801	173,946	202,814
Gathering Services and Related Fees \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Natural Gas, NGLs and Condensate Sales \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Other Revenues
Revenues:
Total revenues	6,707	6,396	13,549	13,068
Other Revenues \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	444	154	876	308
Transaction costs		119		119
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	444	273	876	427
Other income	27	64	20	135
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(69,722)	4,849	(91,819)	(16,117)
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	20,103	6,186	38,184	27,021
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Summit Midstream Partners, LP \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	0	0	0	0
Transaction costs		0		0
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	0	0	0	0
Other income	0	0	0	0
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt				(22,020)
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	(14,837)	(17,553)	(29,959)	(56,289)
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	34,940	23,739	68,143	83,310
Net (loss) income	20,103	6,186	38,184	27,021
Co-Issuers \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Total revenues	112,291	84,606	214,601	204,839
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 22,221	$ 22,169	$ 44,374	$ 44,189
General and administrative	11,907	10,886	24,244	23,371
Depreciation and amortization	22,776	25,282	45,637	50,524
Transaction costs		0		0
Loss (gain) on asset sales, net	(1)	67	(75)	70
Long-lived asset impairment	587	2	587	2
Total costs and expenses	81,874	67,505	159,437	136,307
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	30,417	17,101	55,164	68,532
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	30,417	17,101	55,164	68,532
Guarantor Subsidiaries \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	73,891	67,679	143,357	170,192
Guarantor Subsidiaries \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Guarantor Subsidiaries \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	6,509	6,332	13,236	12,932
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Total revenues	15,892	17,186	30,902	32,758
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 2,245	$ 1,847	$ 4,696	$ 3,519
General and administrative	1,577	2,063	3,682	3,710
Depreciation and amortization	3,564	3,252	6,948	6,425
Transaction costs		0		0
Loss (gain) on asset sales, net	63	0	63	0
Long-lived asset impairment	0	1	0	285
Total costs and expenses	7,449	7,163	15,389	13,939
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	8,443	10,023	15,513	18,819
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	4,523	6,638	12,979	14,778
Non-Guarantor Subsidiaries \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	15,694	17,122	30,589	32,622
Non-Guarantor Subsidiaries \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Non-Guarantor Subsidiaries \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	$ 198	$ 64	$ 313	$ 136

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Cash Flow Statement (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$ 110,049	$ 121,341
Cash flows from investing activities:
Capital expenditures	(90,394)	(45,912)
Contributions to equity method investees	0	(15,649)
Proceeds from asset sale	496	0
Other, net	(306)	(521)
Advances to affiliates	0	0
Net cash provided by (used in) investing activities	(90,204)	(62,082)
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes	0	(17,913)
Proceeds from ATM Program common unit issuances, net of costs	0	17,259
Contribution from General Partner	0	465
Issuance of senior notes	0	500,000
Tender and redemption of senior notes	0	(300,000)
Other, net	(3,423)	(2,268)
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	(13,063)	(64,099)
Net change in cash and cash equivalents	6,782	(4,840)
Cash and cash equivalents, beginning of period	1,430	7,428
Cash and cash equivalents, end of period	8,212	2,588
Consolidating adjustments
Cash flows from operating activities:
Net cash (used in) provided by operating activities	0	0
Cash flows from investing activities:
Capital expenditures	0	0
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	111,134	64,706
Net cash provided by (used in) investing activities	111,134	64,706
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	0	0
Advances from affiliates	(111,134)	(64,706)
Net cash provided by (used in) financing activities	(111,134)	(64,706)
Net change in cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0	0
Common units
Cash flows from financing activities:
Distributions to unitholders	(90,269)	(89,029)
Common units \| Consolidating adjustments
Cash flows from financing activities:
Distributions to unitholders	0
Series A Preferred Units
Cash flows from financing activities:
Distributions to unitholders	(14,250)	0
Series A Preferred Units \| Consolidating adjustments
Cash flows from financing activities:
Distributions to unitholders	0
Summit Midstream Partners, LP \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(68)	424
Cash flows from investing activities:
Capital expenditures	(972)	(6,412)
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	(306)	(521)
Advances to affiliates	0	7,635
Net cash provided by (used in) investing activities	(1,278)	702
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		17,259
Contribution from General Partner		465
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(2,779)	(2,632)
Advances from affiliates	111,134	72,341
Net cash provided by (used in) financing activities	3,836	(1,596)
Net change in cash and cash equivalents	2,490	(470)
Cash and cash equivalents, beginning of period	126	698
Cash and cash equivalents, end of period	2,616	228
Summit Midstream Partners, LP \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	(90,269)
Summit Midstream Partners, LP \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	(14,250)
Co-Issuers \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(27,877)	(32,466)
Cash flows from investing activities:
Capital expenditures	0	0
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	(66,985)	23,020
Net cash provided by (used in) investing activities	(66,985)	23,020
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes		(17,913)
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		500,000
Tender and redemption of senior notes		(300,000)
Other, net	0	0
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	94,879	9,474
Net change in cash and cash equivalents	17	28
Cash and cash equivalents, beginning of period	75	51
Cash and cash equivalents, end of period	92	79
Co-Issuers \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Co-Issuers \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Guarantor Subsidiaries \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	103,965	114,863
Cash flows from investing activities:
Capital expenditures	(85,187)	(23,578)
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	(14,329)	(95,361)
Net cash provided by (used in) investing activities	(99,516)	(118,939)
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(589)	372
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	(589)	372
Net change in cash and cash equivalents	3,860	(3,704)
Cash and cash equivalents, beginning of period	1,138	5,768
Cash and cash equivalents, end of period	4,998	2,064
Guarantor Subsidiaries \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Guarantor Subsidiaries \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	34,029	38,520
Cash flows from investing activities:
Capital expenditures	(4,235)	(15,922)
Contributions to equity method investees		(15,649)
Proceeds from asset sale	496
Other, net	0	0
Advances to affiliates	(29,820)	0
Net cash provided by (used in) investing activities	(33,559)	(31,571)
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(55)	(8)
Advances from affiliates	0	(7,635)
Net cash provided by (used in) financing activities	(55)	(7,643)
Net change in cash and cash equivalents	415	(694)
Cash and cash equivalents, beginning of period	91	911
Cash and cash equivalents, end of period	506	$ 217
Non-Guarantor Subsidiaries \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Non-Guarantor Subsidiaries \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	$ 0

Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 31, 2018
Entity Information [Line Items]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	0001549922
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Trading Symbol	SMLP
Common units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		73,355,775
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,490,999

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 8,212	$ 1,430
Accounts receivable	80,166	72,301
Other current assets	2,594	4,327
Total current assets	90,972	78,058
Property, plant and equipment, net	1,887,759	1,795,129
Intangible assets, net	286,665	301,345
Goodwill	16,211	16,211
Investment in equity method investees	670,827	690,485
Other noncurrent assets	13,144	13,565
Total assets	2,965,578	2,894,793
Current liabilities:
Trade accounts payable	21,698	16,375
Accrued expenses	19,433	12,499
Due to affiliate	91	1,088
Deferred revenue	10,778	4,000
Ad valorem taxes payable	6,504	8,329
Accrued interest	12,259	12,310
Accrued environmental remediation	3,522	3,130
Other current liabilities	7,412	11,258
Total current liabilities	81,697	68,989
Long-term debt	1,147,005	1,051,192
Deferred Purchase Price Obligation	453,922	362,959
Noncurrent deferred revenue	38,119	12,707
Noncurrent accrued environmental remediation	1,617	2,214
Other noncurrent liabilities	6,410	7,063
Total liabilities	1,728,770	1,505,124
Commitments and contingencies (Note 16)
Series A Preferred Units (300,000 units issued and outstanding at June 30, 2018 and December 31, 2017)	293,616	294,426
Common limited partner capital (73,355,775 units issued and outstanding at June 30, 2018 and 73,085,996 units issued and outstanding at December 31, 2017)	907,099	1,056,510
General Partner interests (1,490,999 units issued and outstanding at June 30, 2018 and December 31, 2017)	25,137	27,920
Noncontrolling interest	10,956	10,813
Total partners' capital	1,236,808	1,389,669
Total liabilities and partners' capital	$ 2,965,578	$ 2,894,793

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Series A preferred unitholders, issued	300,000	300,000
Series A preferred unitholders, outstanding	300,000	300,000
Common limited partner capital (in shares), issued	73,355,775	73,085,996
Common limited partner capital (in shares), outstanding	73,355,775	73,085,996
General partner interests (in shares), issued	1,490,999	1,490,999
General partner interests (in shares), outstanding	1,490,999	1,490,999

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 24,466	$ 24,016	$ 49,070	$ 47,708
General and administrative	13,484	12,949	27,926	27,081
Depreciation and amortization	26,784	28,688	53,461	57,257
Transaction costs	0	119	0	119
Loss (gain) on asset sales, net	62	67	(12)	70
Long-lived asset impairment	587	3	587	287
Total costs and expenses	89,767	74,941	175,702	150,673
Other income	27	64	20	135
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt	0	0	0	(22,020)
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(45,699)	14,420	(51,101)	14,945
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Net income attributable to noncontrolling interest	58	89	143	191
Net (loss) income attributable to SMLP	(49,971)	11,157	(53,901)	10,472
Less net income attributable to General Partner, including IDRs	1,140	2,351	3,198	4,443
Net (loss) income attributable to limited partners	(51,111)	8,806	(57,099)	6,029
Series A Preferred Units
Costs and expenses:
Net (loss) income attributable to limited partners	7,125	0	14,250	0
Common units
Costs and expenses:
Net (loss) income attributable to limited partners	$ (58,236)	$ 8,806	$ (71,349)	$ 6,029
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Diluted (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Weighted-average limited partner units outstanding:
Basic (shares)	73,356	72,532	73,245	72,341
Diluted (shares)	73,356	72,842	73,245	72,708
Gathering Services and Related Fees
Revenues:
Total revenues	$ 89,585	$ 84,801	$ 173,946	$ 202,814
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Other Revenues
Revenues:
Total revenues	$ 6,707	$ 6,396	$ 13,549	$ 13,068

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) $ in Thousands	Total	Adjustment	Noncontrolling interest	Noncontrolling interest Adjustment	Common	Common Adjustment	General Partner	General Partner Adjustment	Limited partners, Series A Preferred Units	Limited partners, Series A Preferred Units Adjustment
Beginning balance at Dec. 31, 2016	$ 1,169,673		$ 11,247		$ 1,129,132		$ 29,294
Net income (loss)	10,663		191		6,029		4,443
Distributions to unitholders	(89,029)		0		(83,044)		(5,985)
Unit-based compensation	3,919		0		3,919		0
Tax withholdings on vested SMLP LTIP awards	(2,051)		0		(2,051)		0
ATM Program issuances, net of costs	17,259		0		17,259		0
Contribution from General Partner	465		0		0		465
Ending balance at Jun. 30, 2017	1,110,899		11,438		1,071,244		28,217
January 1, 2018 impact of Topic 606day 1 adoption	4,214		0		4,130		84		$ 0
Beginning balance at Dec. 31, 2017	1,389,669	$ 1,393,883	10,813	$ 10,813	1,056,510	$ 1,060,640	27,920	$ 28,004	294,426	$ 294,426
Net income (loss)	(53,758)		143		(71,349)		3,198		14,250
Distributions to unitholders	(104,519)		0		(84,204)		(6,065)		(14,250)
Unit-based compensation	3,983		0		3,983		0		0
Tax withholdings on vested SMLP LTIP awards	(1,840)		0		(1,840)		0		0
Other	(941)		0		(131)		0		(810)
Ending balance at Jun. 30, 2018	$ 1,236,808		$ 10,956		$ 907,099		$ 25,137		$ 293,616

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

+ Details

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income (loss)	$ (53,758)	$ 10,663
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization	53,160	56,955
Amortization of debt issuance costs	2,086	2,072
Deferred Purchase Price Obligation	90,963	15,825
Unit-based and noncash compensation	4,223	3,999
Loss from equity method investees	2,534	4,041
Distributions from equity method investees	17,124	18,003
Loss (gain) on asset sales, net	(12)	70
Long-lived asset impairment	587	287
Early extinguishment of debt	0	22,020
Write-off of debt issuance costs	0	302
Changes in operating assets and liabilities:
Accounts receivable	(6,265)	41,527
Trade accounts payable	(3,483)	(4,372)
Accrued expenses	6,934	(3,111)
Due (to) from affiliate	(997)	212
Deferred revenue, net	3,281	(39,671)
Ad valorem taxes payable	(1,825)	(3,293)
Accrued interest	(51)	(468)
Accrued environmental remediation, net	(1,805)	(924)
Other, net	(2,647)	(2,796)
Net cash provided by operating activities	110,049	121,341
Cash flows from investing activities:
Capital expenditures	(90,394)	(45,912)
Proceeds from asset sale	496	0
Contributions to equity method investees	0	(15,649)
Other, net	(306)	(521)
Net cash provided by (used in) investing activities	(90,204)	(62,082)
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes	0	(17,913)
Proceeds from ATM Program common unit issuances, net of costs	0	17,259
Contribution from General Partner	0	465
Issuance of senior notes	0	500,000
Tender and redemption of senior notes	0	(300,000)
Other, net	(3,423)	(2,268)
Net cash provided by (used in) financing activities	(13,063)	(64,099)
Net change in cash and cash equivalents	6,782	(4,840)
Cash and cash equivalents, beginning of period	1,430	7,428
Cash and cash equivalents, end of period	8,212	2,588
Supplemental cash flow disclosures:
Cash interest paid	30,962	33,382
Less capitalized interest	3,085	918
Interest paid (net of capitalized interest)	27,877	32,464
Cash paid for taxes	175	0
Noncash investing and financing activities
Capital expenditures in trade accounts payable (period-end accruals)	20,598	6,869
Capital expenditures relating to contributions in aid of construction for Topic 606 day 1 adoption	33,123	0
Common units
Cash flows from financing activities:
Distributions to unitholders	(90,269)	(89,029)
Series A Preferred Units
Cash flows from financing activities:
Distributions to unitholders	$ (14,250)	$ 0

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

1. ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

•

Summit Utica, a natural gas gathering system operating in the Appalachian Basin, which includes the Utica and Point Pleasant shale formations in southeastern Ohio;

•

Bison Midstream, an associated natural gas gathering system operating in the Williston Basin, which includes the Bakken and Three Forks shale formations in northwestern North Dakota;

•

Niobrara G&P, an associated natural gas gathering and processing system operating in the DJ Basin, which includes the Niobrara and Codell shale formations in northeastern Colorado;

•

DFW Midstream, a natural gas gathering system operating in the Fort Worth Basin, which includes the Barnett Shale formation in north-central Texas;

•

Mountaineer Midstream, a natural gas gathering system operating in the Appalachian Basin, which includes the Marcellus Shale formation in northern West Virginia; and

•

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Except for the changes below, there have been no changes to our significant accounting policies since December 31, 2017.

Recently Adopted Accounting Pronouncements. We have recently adopted the following accounting pronouncements:

•

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Impact on financial statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

•

Accounting Pronouncements Pending Adoption. We have not yet adopted the following accounting pronouncements as of June 30, 2018:

•

REVENUE

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

REVENUE

3. REVENUE

Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components.

	2018		2019		2020		2021		2022		Thereafter
	(In thousands)
Gathering services and related fees	$	97,523	$	133,743	$	122,429	$	102,777	$	83,648	$	174,227

Revenue by Category. In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4.

	Reportable Segments
	Three months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	10,422	$	23,106	$	36,170	$	14,080	$	8,050	$	91,828	$	(2,243	)	$	89,585
Natural gas, NGLs and condensate sales		—		7,350		4,675		381		—		12,406		19,485			31,891
Other revenues		—		2,960		2,147		1,694		—		6,801		(94	)		6,707
Total	$	10,422	$	33,416	$	42,992	$	16,155	$	8,050	$	111,035	$	17,148		$	128,183

	Reportable Segments
	Six months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	20,463	$	40,772	$	71,464	$	27,717	$	15,875	$	176,291	$	(2,345	)	$	173,946
Natural gas, NGLs and condensate sales		—		15,196		9,000		926		—		25,122		32,886			58,008
Other revenues		—		5,872		4,115		3,682		—		13,669		(120	)		13,549
Total	$	20,463	$	61,840	$	84,579	$	32,325	$	15,875	$	215,082	$	30,421		$	245,503

	June 30, 2018
	(In thousands)
Contract assets, December 31, 2017	$	—
Net impact of Topic 606 day 1 adoption		3,514
Additions		11,523
Transfers out		(5,704	)
Contract assets, June 30, 2018	$	9,333

Current		9,333
Noncurrent		—
Contract assets, June 30, 2018	$	9,333

SEGMENT INFORMATION

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

SEGMENT INFORMATION

4. SEGMENT INFORMATION

As of June 30, 2018, our reportable segments are:

•

the Utica Shale, which is served by Summit Utica;

•

Ohio Gathering, which includes our ownership interest in OGC and OCC;

•

the Williston Basin, which is served by Polar and Divide, Tioga Midstream and Bison Midstream;

•

the Piceance/DJ Basins, which is served by Grand River and Niobrara G&P;

•

the Barnett Shale, which is served by DFW Midstream; and

•

the Marcellus Shale, which is served by Mountaineer Midstream.

Assets by reportable segment follow.

	June 30, 2018			December 31, 2017
	(In thousands)
Assets:
Utica Shale	$	212,326		$	212,311
Ohio Gathering		670,827			690,485
Williston Basin		520,799			512,860
Piceance/DJ Basins		828,999			798,722
Barnett Shale		376,969			383,306
Marcellus Shale		213,366			217,362
Total reportable segment assets		2,823,286			2,815,046
Corporate and Other		143,450			79,996
Eliminations		(1,158	)		(249	)
Total assets	$	2,965,578		$	2,894,793

Revenues by reportable segment follow.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Revenues (1):
Utica Shale	$	10,422		$	10,456		$	20,463		$	19,252
Williston Basin		33,416			29,114			61,840			95,999
Piceance/DJ Basins		42,992			33,763			84,579			68,571
Barnett Shale		16,155			20,904			32,325			38,646
Marcellus Shale		8,050			7,365			15,875			14,269
Total reportable segments revenue		111,035			101,602			215,082			236,737
Corporate and Other		19,422			1,362			33,598			3,148
Eliminations		(2,274	)		(1,172	)		(3,177	)		(2,288	)
Total revenues	$	128,183		$	101,792		$	245,503		$	237,597

(1) Excludes revenues earned by Ohio Gathering due to equity method accounting.

Counterparties accounting for more than 10% of total revenues were as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins		10	%	*				11	%	*
Counterparty B - Piceance/DJ Basins	*				10	%	*			*
Counterparty C - Barnett Shale		11	%		11	%	*			*
Counterparty D - Utica Shale	*				10	%	*			*
Counterparty E - Williston Basin	*			*			*				21	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments.

(2) Excludes revenues earned by Ohio Gathering due to equity method accounting.

* Less than 10%

Depreciation and amortization, including the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	2,033	$	1,748	$	3,886	$	3,395
Williston Basin		5,622		8,385		11,231		16,766
Piceance/DJ Basins		12,450		12,225		25,005		24,436
Barnett Shale (2)		3,759		3,762		7,516		7,524
Marcellus Shale		2,274		2,263		4,546		4,526
Total reportable segment depreciation and amortization		26,138		28,383		52,184		56,647
Corporate and Other		496		154		976		308
Total depreciation and amortization	$	26,634	$	28,537	$	53,160	$	56,955

(1) Excludes depreciation and amortization recognized by Ohio Gathering due to equity method accounting.

(2) Includes the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues.

Cash paid for capital expenditures by reportable segment follow.

	Six months ended June 30,
	2018		2017
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	1,846	$	16,473
Williston Basin		10,966		11,085
Piceance/DJ Basins		24,827		11,934
Barnett Shale		349		(399	)
Marcellus Shale		545		407
Total reportable segment capital expenditures		38,533		39,500
Corporate and Other		51,861		6,412
Total cash paid for capital expenditures	$	90,394	$	45,912

(1) Excludes cash paid for capital expenditures by Ohio Gathering due to equity method accounting.

During the six months ended June 30, 2018, Corporate included cash paid of $1.0 million for corporate purposes; the remainder represents capital expenditures for Summit Permian.

in the Deferred Purchase Price Obligation fair value, early extinguishment of debt expense and income tax expense or benefit from segment adjusted EBITDA.

Segment adjusted EBITDA by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	9,223	$	9,533	$	17,938	$	17,445
Ohio Gathering		8,935		9,606		19,412		18,679
Williston Basin		19,030		17,155		35,000		34,964
Piceance/DJ Basins		27,673		27,274		56,908		56,248
Barnett Shale		11,093		12,998		20,952		25,086
Marcellus Shale		6,543		5,446		13,219		11,093
Total of reportable segments' measures of profit	$	82,497	$	82,012	$	163,429	$	163,515

A reconciliation of income or loss before income taxes and income or loss from equity method investees to total of reportable segments' measures of profit or loss follows.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Reconciliation of (loss) income before income taxes and loss from equity method investees to total of reportable segments' measures of profit:
(Loss) income before income taxes and loss from equity method investees	$	(45,699	)	$	14,420		$	(51,101	)	$	14,945
Add:
Corporate and Other		9,002			9,435			19,625			19,528
Interest expense		14,837			17,553			29,959			34,269
Early extinguishment of debt		—			—			—			22,020
Deferred Purchase Price Obligation		69,305			(5,058	)		90,963			15,825
Depreciation and amortization		26,634			28,537			53,160			56,955
Proportional adjusted EBITDA for equity method investees		8,935			9,606			19,412			18,679
Adjustments related to MVC shortfall payments		(3,542	)		5,578			(3,542	)		(23,062	)
Adjustments related to capital reimbursement activity		115			—			155			—
Unit-based and noncash compensation		2,261			1,871			4,223			3,999
Loss (gain) on asset sales, net		62			67			(12	)		70
Long-lived asset impairment		587			3			587			287
Total of reportable segments' measures of profit	$	82,497		$	82,012		$	163,429		$	163,515

For the three and six months ended June 30, 2018, in accordance with Topic 606, adjustments related to MVC shortfall payments are recognized in gathering services and related fees (see Note 3).

Adjustments related to MVC shortfall payments by reportable segment follow.

	Three months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Three months ended June 30, 2017
	Williston Basin		Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—	$	(1,186	)	$	—		$	(1,186	)
Expected MVC shortfall adjustments		1,982		6,522			(1,740	)		6,764
Total adjustments related to MVC shortfall payments	$	1,982	$	5,336		$	(1,740	)	$	5,578

	Six months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Six months ended June 30, 2017
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	(37,693	)	$	(1,978	)	$	—		$	(39,671	)
Expected MVC shortfall adjustments		3,964			13,067			(422	)		16,609
Total adjustments related to MVC shortfall payments	$	(33,729	)	$	11,089		$	(422	)	$	(23,062	)

PROPERTY, PLANT, AND EQUIPMENT, NET

6 Months Ended

Jun. 30, 2018

Property Plant And Equipment [Abstract]

PROPERTY, PLANT, AND EQUIPMENT, NET

5. PROPERTY, PLANT AND EQUIPMENT, NET

Details on property, plant and equipment follow.

	June 30, 2018		December 31, 2017
	(In thousands)
Gathering and processing systems and related equipment	$	2,023,832	$	1,973,722
Construction in progress		160,239		78,850
Land and line fill		11,735		11,735
Other		41,802		40,262
Total		2,237,608		2,104,569
Less accumulated depreciation		349,849		309,440
Property, plant and equipment, net	$	1,887,759	$	1,795,129

Depreciation expense and capitalized interest follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation expense	$	18,657	$	18,607	$	37,214	$	37,098
Capitalized interest		1,863		450		3,085		918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

6. AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities), all of which are subject to amortization, follow.

	June 30, 2018
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(13,128	)	$	11,067
Contract intangibles		278,448		(130,891	)		147,557
Rights-of-way		162,339		(34,298	)		128,041
Total intangible assets	$	464,982	$	(178,317	)	$	286,665

Unfavorable gas gathering contract	$	10,962	$	(10,153	)	$	809

	December 31, 2017
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(12,350	)	$	11,845
Contract intangibles		278,448		(117,821	)		160,627
Rights-of-way		159,986		(31,113	)		128,873
Total intangible assets	$	462,629	$	(161,284	)	$	301,345

Unfavorable gas gathering contract	$	10,962	$	(9,074	)	$	1,888

We recognized amortization expense in other revenues as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(388	)	$	(388	)	$	(777	)	$	(777	)
Amortization expense – unfavorable gas gathering contract		538			539			1,078			1,079

We recognized amortization expense in costs and expenses as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Amortization expense – contract intangibles	$	6,535	$	8,551	$	13,070	$	17,102
Amortization expense – rights-of-way		1,592		1,530		3,177		3,057

The estimated aggregate annual amortization expected to be recognized for the remainder of 2018 and each of the four succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	17,053	$	809
2019		33,217		—
2020		33,041		—
2021		29,349		—
2022		26,282		—

GOODWILL

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

GOODWILL

7. GOODWILL

EQUITY METHOD INVESTMENTS

6 Months Ended

Jun. 30, 2018

Equity Method Investments And Joint Ventures [Abstract]

EQUITY METHOD INVESTMENTS

8. EQUITY METHOD INVESTMENTS

A reconciliation of our 40% ownership interest in Ohio Gathering to our investment per Ohio Gathering's books and records follows (in thousands).

Investment in equity method investees, June 30, 2018	$	670,827
June cash distributions		2,580
Basis difference		(123,508	)
Investment in equity method investees, net of basis difference, May 31, 2018	$	549,899

For the three and six months ended June 30, 2018, there were no contributions to Ohio Gathering.

	Three months ended May 31, 2018						Three months ended May 31, 2017
	OGC			OCC			OGC			OCC
	(In thousands)
Total revenues	$	34,123		$	2,070		$	31,083		$	2,004
Total operating expenses		35,518			1,958			33,221			1,836
Net (loss) income		(1,396	)		(59	)		(139	)		23

	Six months ended May 31, 2018				Six months ended May 31, 2017
	OGC		OCC		OGC		OCC
	(In thousands)
Total revenues	$	69,083	$	4,559	$	68,158	$	4,057
Total operating expenses		62,293		4,099		60,326		4,309
Net income (loss)		6,784		121		7,834		(1,192	)

DEFERRED REVENUE

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

DEFERRED REVENUE

9. DEFERRED REVENUE

A rollforward of current deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total current
	(In thousands)
Current deferred revenue, December 31, 2017, as reported	$	—	$	—	$	4,000	$	—	$	—	$	4,000
Net impact of Topic 606 day 1 adoption		18		1,017		3,396		1,619		38		6,088
Current deferred revenue, January 1, 2018		18		1,017		7,396		1,619		38		10,088
Additions		9		1,088		10,996		814		19		12,926
Less revenue recognized		9		643		10,762		803		19		12,236
Current deferred revenue, June 30, 2018	$	18	$	1,462	$	7,630	$	1,630	$	38	$	10,778

A rollforward of noncurrent deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, December 31, 2017, as reported	$	—	$	—	$	12,707	$	—	$	—	$	12,707
Net impact of Topic 606 day 1 adoption		39		4,215		10,017		8,217		333		22,821
Noncurrent deferred revenue, January 1, 2018		39		4,215		22,724		8,217		333		35,528
Additions		—		1,622		5,200		1,583		—		8,405
Less reclassification to current deferred revenue		9		1,017		3,955		814		19		5,814
Noncurrent deferred revenue, June 30, 2018	$	30	$	4,820	$	23,969	$	8,986	$	314	$	38,119

DEBT

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

DEBT

10. DEBT

Debt consisted of the following:

	June 30, 2018			December 31, 2017
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.35% at June 30, 2018 and 4.07% at December 31, 2017) due May 2022	$	356,000		$	261,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,588	)		(2,910	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			500,000
Less unamortized debt issuance costs (1)		(6,407	)		(6,898	)
Total long-term debt	$	1,147,005		$	1,051,192

(1) Issuance costs are being amortized over the life of the notes.

FINANCIAL INSTRUMENTS

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

FINANCIAL INSTRUMENTS

11. FINANCIAL INSTRUMENTS

Fair Value. The carrying amount of cash and cash equivalents, accounts receivable and trade accounts payable reported on the balance sheet approximates fair value due to their short-term maturities.

Level 3 liability, January 1, 2018	$	362,959
Change in fair value		90,963
Level 3 liability, June 30, 2018	$	453,922

A summary of the estimated fair value of our debt financial instruments follows.

	June 30, 2018				December 31, 2017
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings 5.5% Senior Notes ($300.0 million principal)	$	297,412	$	293,875	$	297,090	$	301,750
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,593		474,375		493,102		501,667

- Definition

+ References + Details

PARTNERS' CAPITAL

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

PARTNERS' CAPITAL

12. PARTNERS' CAPITAL

A rollforward of the number of common limited partner and General Partner units follows.

	Limited partners
	Series A Preferred Units		Common		General Partner
Units, January 1, 2018		300,000		73,085,996		1,490,999
Net units issued under the SMLP LTIP		—		269,779		—
Units, June 30, 2018		300,000		73,355,775		1,490,999

Cash Distributions Paid and Declared. We paid the following per-unit distributions during the three and six months ended June 30:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
Per-unit distributions to unitholders	$	0.575	$	0.575	$	1.150	$	1.150

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
IDR payments	$	2,136	$	2,106	$	4,264	$	4,206

EARNINGS PER UNIT

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

EARNINGS PER UNIT

13. EARNINGS PER UNIT

The following table details the components of EPU.

	Three months ended June 30,					Six months ended June 30,
	2018			2017		2018			2017
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net (loss) income among limited partner interests:
Net (loss) income attributable to limited partners	$	(51,111	)	$	8,806	$	(57,099	)	$	6,029
Less net income attributable to Series A Preferred Units		7,125			—		14,250			—
Net (loss) income attributable to common limited partners	$	(58,236	)	$	8,806	$	(71,349	)	$	6,029

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		73,356			72,532		73,245			72,341
Effect of nonvested phantom units		—			310		—			367
Weighted-average common units outstanding – diluted		73,356			72,842		73,245			72,708

(Loss) earnings per limited partner unit:
Common unit – basic	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08
Common unit – diluted	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		1			—		3			—

UNIT-BASED AND NONCASH COMPENSATION

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

UNIT-BASED AND NONCASH COMPENSATION

14. UNIT-BASED AND NONCASH COMPENSATION

SMLP Long-Term Incentive Plan. The SMLP LTIP provides for equity awards to eligible officers, employees, consultants and directors of our General Partner and its affiliates. Items to note:

•

Also in March 2018, 328,388 phantom units vested.

•

As of June 30, 2018, approximately 3.2 million common units remained available for future issuance under the SMLP LTIP.

RELATED-PARTY TRANSACTIONS

6 Months Ended

Jun. 30, 2018

RELATED-PARTY TRANSACTIONS

15. RELATED-PARTY TRANSACTIONS

Acquisitions. See Notes 11 and 16 of the 2017 Annual Report.

Expenses incurred by the General Partner and reimbursed by us under our Partnership Agreement were as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Operation and maintenance expense	$	7,114	$	6,731	$	14,737	$	13,612
General and administrative expense		7,481		7,895		15,598		16,190

COMMITMENTS AND CONTINGENCIES

6 Months Ended

Jun. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

16. COMMITMENTS AND CONTINGENCIES

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Rent expense	$	956	$	923	$	1,978	$	1,802

A rollforward of the aggregate accrued environmental remediation liabilities follows.

	Total
	(In thousands)
Accrued environmental remediation, January 1, 2018	$	5,344
Payments made		(1,805	)
Additional accruals		1,600
Accrued environmental remediation, June 30, 2018	$	5,139

ACQUISITIONS AND DROP DOWN TRANSACTIONS

6 Months Ended

Jun. 30, 2018

Business Combinations [Abstract]

ACQUISITIONS AND DROP DOWN TRANSACTIONS

17. ACQUISITIONS AND DROP DOWN TRANSACTIONS

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

18. CONDENSED CONSOLIDATING FINANCIAL INFORMATION

The Senior Notes are fully and unconditionally guaranteed, jointly and severally, on a senior unsecured basis by SMLP and the Guarantor Subsidiaries (see Note 10).

Condensed Consolidating Balance Sheets. Balance sheets as of June 30, 2018 and December 31, 2017 follow.

	June 30, 2018
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	2,616	$	92	$	4,998	$	506	$	—		$	8,212
Accounts receivable		22		—		68,274		11,870		—			80,166
Other current assets		1,057		—		1,273		264		—			2,594
Due from affiliate		—		—		507,396		51,852		(559,248	)		—
Total current assets		3,695		92		581,941		64,492		(559,248	)		90,972
Property, plant and equipment, net		4,314		—		1,537,468		345,977		—			1,887,759
Intangible assets, net		—		—		263,416		23,249		—			286,665
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		670,827		—			670,827
Other noncurrent assets		3,288		9,762		94		—		—			13,144
Investment in subsidiaries		2,062,096		3,396,771		—		—		(5,458,867	)		—
Total assets	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

Liabilities and Partners' Capital
Trade accounts payable	$	141	$	—	$	18,280	$	3,277	$	—		$	21,698
Accrued expenses		803		—		17,030		1,600		—			19,433
Due to affiliate		374,074		185,265		—		—		(559,248	)		91
Deferred revenue		—		—		10,341		437		—			10,778
Ad valorem taxes payable		9		—		6,224		271		—			6,504
Accrued interest		—		12,259		—		—		—			12,259
Accrued environmental remediation		—		—		—		3,522		—			3,522
Other current liabilities		3,477		—		3,573		362		—			7,412
Total current liabilities		378,504		197,524		55,448		9,469		(559,248	)		81,697
Long-term debt		—		1,147,005		—		—		—			1,147,005
Deferred Purchase Price Obligation		453,922		—		—		—		—			453,922
Noncurrent deferred revenue		—		—		36,416		1,703		—			38,119
Noncurrent accrued environmental remediation		—		—		—		1,617		—			1,617
Other noncurrent liabilities		4,159		—		2,197		54		—			6,410
Total liabilities		836,585		1,344,529		94,061		12,843		(559,248	)		1,728,770

Total partners' capital		1,236,808		2,062,096		2,305,069		1,091,702		(5,458,867	)		1,236,808
Total liabilities and partners' capital	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

	December 31, 2017
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	126	$	75	$	1,138	$	91	$	—		$	1,430
Accounts receivable		22		—		65,842		6,437		—			72,301
Other current assets		1,188		—		2,697		442		—			4,327
Due from affiliate		—		—		493,067		22,030		(515,097	)		—
Total current assets		1,336		75		562,744		29,000		(515,097	)		78,058
Property, plant and equipment, net		4,206		—		1,442,333		348,590		—			1,795,129
Intangible assets, net		—		—		278,958		22,387		—			301,345
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		690,485		—			690,485
Other noncurrent assets		2,547		10,913		105		—		—			13,565
Investment in subsidiaries		2,019,700		3,324,464		—		—		(5,344,164	)		—
Total assets	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Liabilities and Partners' Capital
Trade accounts payable	$	209	$	—	$	11,283	$	4,883	$	—		$	16,375
Accrued expenses		928		—		10,592		979		—			12,499
Due to affiliate		263,935		252,250		—		—		(515,097	)		1,088
Deferred revenue		—		—		4,000		—		—			4,000
Ad valorem taxes payable		—		—		7,809		520		—			8,329
Accrued interest		—		12,310		—		—		—			12,310
Accrued environmental remediation		—		—		—		3,130		—			3,130
Other current liabilities		6,395		—		4,385		478		—			11,258
Total current liabilities		271,467		264,560		38,069		9,990		(515,097	)		68,989
Long-term debt		—		1,051,192		—		—		—			1,051,192
Deferred Purchase Price Obligation		362,959		—		—		—		—			362,959
Deferred revenue		—		—		12,707		—		—			12,707
Noncurrent accrued environmental remediation		—		—		—		2,214		—			2,214
Other noncurrent liabilities		3,694		—		3,293		76		—			7,063
Total liabilities		638,120		1,315,752		54,069		12,280		(515,097	)		1,505,124

Total partners' capital		1,389,669		2,019,700		2,246,282		1,078,182		(5,344,164	)		1,389,669
Total liabilities and partners' capital	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

	Three months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	73,891		$	15,694		$	—		$	89,585
Natural gas, NGLs and condensate sales		—			—			31,891			—			—			31,891
Other revenues		—			—			6,509			198			—			6,707
Total revenues		—			—			112,291			15,892			—			128,183
Costs and expenses:
Cost of natural gas and NGLs		—			—			24,384			—			—			24,384
Operation and maintenance		—			—			22,221			2,245			—			24,466
General and administrative		—			—			11,907			1,577			—			13,484
Depreciation and amortization		444			—			22,776			3,564			—			26,784
(Gain) loss on asset sales, net		—			—			(1	)		63			—			62
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		444			—			81,874			7,449			—			89,767
Other income		27			—			—			—			—			27
Interest expense		—			(14,837	)		—			—			—			(14,837	)
Deferred Purchase Price Obligation		(69,305	)		—			—			—			—			(69,305	)
(Loss) income before income taxes and loss from equity method investees		(69,722	)		(14,837	)		30,417			8,443			—			(45,699	)
Income tax expense		(294	)		—			—			—			—			(294	)
Loss from equity method investees		—			—			—			(3,920	)		—			(3,920	)
Equity in earnings of consolidated subsidiaries		20,103			34,940			—			—			(55,043	)		—
Net (loss) income	$	(49,913	)	$	20,103		$	30,417		$	4,523		$	(55,043	)	$	(49,913	)

	Three months ended June 30, 2017
	SMLP		Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—	$	—		$	67,679	$	17,122		$	—		$	84,801
Natural gas, NGLs and condensate sales		—		—			10,595		—			—			10,595
Other revenues		—		—			6,332		64			—			6,396
Total revenues		—		—			84,606		17,186			—			101,792
Costs and expenses:
Cost of natural gas and NGLs		—		—			9,099		—			—			9,099
Operation and maintenance		—		—			22,169		1,847			—			24,016
General and administrative		—		—			10,886		2,063			—			12,949
Depreciation and amortization		154		—			25,282		3,252			—			28,688
Transaction costs		119		—			—		—			—			119
Loss on asset sales, net		—		—			67		—			—			67
Long-lived asset impairment		—		—			2		1			—			3
Total costs and expenses		273		—			67,505		7,163			—			74,941
Other income		64		—			—		—			—			64
Interest expense		—		(17,553	)		—		—			—			(17,553	)
Deferred Purchase Price Obligation		5,058		—			—		—			—			5,058
Income (loss) before income taxes and loss from equity method investees		4,849		(17,553	)		17,101		10,023			—			14,420
Income tax benefit		211		—			—		—			—			211
Loss from equity method investees		—		—			—		(3,385	)		—			(3,385	)
Equity in earnings of consolidated subsidiaries		6,186		23,739			—		—			(29,925	)		—
Net income	$	11,246	$	6,186		$	17,101	$	6,638		$	(29,925	)	$	11,246

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	143,357		$	30,589		$	—		$	173,946
Natural gas, NGLs and condensate sales		—			—			58,008			—			—			58,008
Other revenues		—			—			13,236			313			—			13,549
Total revenues		—			—			214,601			30,902			—			245,503
Costs and expenses:
Cost of natural gas and NGLs		—			—			44,670			—			—			44,670
Operation and maintenance		—			—			44,374			4,696			—			49,070
General and administrative		—			—			24,244			3,682			—			27,926
Depreciation and amortization		876			—			45,637			6,948			—			53,461
(Gain) loss on asset sales, net		—			—			(75	)		63			—			(12	)
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		876			—			159,437			15,389			—			175,702
Other income		20			—			—			—			—			20
Interest expense		—			(29,959	)		—			—			—			(29,959	)
Deferred Purchase Price Obligation		(90,963	)		—			—			—			—			(90,963	)
(Loss) income before income taxes and loss from equity method investees		(91,819	)		(29,959	)		55,164			15,513			—			(51,101	)
Income tax expense		(123	)		—			—			—			—			(123	)
Loss from equity method investees		—			—			—			(2,534	)		—			(2,534	)
Equity in earnings of consolidated subsidiaries		38,184			68,143			—			—			(106,327	)		—
Net (loss) income	$	(53,758	)	$	38,184		$	55,164		$	12,979		$	(106,327	)	$	(53,758	)

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	170,192	$	32,622		$	—		$	202,814
Natural gas, NGLs and condensate sales		—			—			21,715		—			—			21,715
Other revenues		—			—			12,932		136			—			13,068
Total revenues		—			—			204,839		32,758			—			237,597
Costs and expenses:
Cost of natural gas and NGLs		—			—			18,151		—			—			18,151
Operation and maintenance		—			—			44,189		3,519			—			47,708
General and administrative		—			—			23,371		3,710			—			27,081
Depreciation and amortization		308			—			50,524		6,425			—			57,257
Transaction costs		119			—			—		—			—			119
Loss on asset sales, net		—			—			70		—			—			70
Long-lived asset impairment		—			—			2		285			—			287
Total costs and expenses		427			—			136,307		13,939			—			150,673
Other income		135			—			—		—			—			135
Interest expense		—			(34,269	)		—		—			—			(34,269	)
Early extinguishment of debt		—			(22,020	)		—		—			—			(22,020	)
Deferred Purchase Price Obligation		(15,825	)		—			—		—			—			(15,825	)
(Loss) income before income taxes and loss from equity method investees		(16,117	)		(56,289	)		68,532		18,819			—			14,945
Income tax expense		(241	)		—			—		—			—			(241	)
Loss from equity method investees		—			—			—		(4,041	)		—			(4,041	)
Equity in earnings of consolidated subsidiaries		27,021			83,310			—		—			(110,331	)		—
Net income	$	10,663		$	27,021		$	68,532	$	14,778		$	(110,331	)	$	10,663

Condensed Consolidating Statements of Cash Flows. Statements of cash flows for the six months ended June 30, 2018 and 2017 follow.

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$	(68	)	$	(27,877	)	$	103,965		$	34,029		$	—		$	110,049

Cash flows from investing activities:
Capital expenditures		(972	)		—			(85,187	)		(4,235	)		—			(90,394	)
Proceeds from asset sales		—			—			—			496			—			496
Other, net		(306	)		—			—			—			—			(306	)
Advances to affiliates		—			(66,985	)		(14,329	)		(29,820	)		111,134			—
Net cash used in investing activities		(1,278	)		(66,985	)		(99,516	)		(33,559	)		111,134			(90,204	)

Cash flows from financing activities:
Distributions to common unitholders		(90,269	)		—			—			—			—			(90,269	)
Distributions to Series A Preferred unitholders		(14,250	)		—			—			—			—			(14,250	)
Borrowings under Revolving Credit Facility		—			148,000			—			—			—			148,000
Repayments under Revolving Credit Facility		—			(53,000	)		—			—			—			(53,000	)
Debt issuance costs		—			(121	)		—			—			—			(121	)
Other, net		(2,779	)		—			(589	)		(55	)		—			(3,423	)
Advances from affiliates		111,134			—			—			—			(111,134	)		—
Net cash provided by (used in) financing activities		3,836			94,879			(589	)		(55	)		(111,134	)		(13,063	)
Net change in cash and cash equivalents		2,490			17			3,860			415			—			6,782
Cash and cash equivalents, beginning of period		126			75			1,138			91			—			1,430
Cash and cash equivalents, end of period	$	2,616		$	92		$	4,998		$	506		$	—		$	8,212

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$	424		$	(32,466	)	$	114,863		$	38,520		$	—		$	121,341

Cash flows from investing activities:
Capital expenditures		(6,412	)		—			(23,578	)		(15,922	)		—			(45,912	)
Contributions to equity method investees		—			—			—			(15,649	)		—			(15,649	)
Other, net		(521	)		—			—			—			—			(521	)
Advances to affiliates		7,635			23,020			(95,361	)		—			64,706			—
Net cash provided by (used in) investing activities		702			23,020			(118,939	)		(31,571	)		64,706			(62,082	)

Cash flows from financing activities:
Distributions to unitholders		(89,029	)		—			—			—			—			(89,029	)
Borrowings under Revolving Credit Facility		—			112,500			—			—			—			112,500
Repayments under Revolving Credit Facility		—			(269,500	)		—			—			—			(269,500	)
Debt issuance costs		—			(15,613	)		—			—			—			(15,613	)
Payment of redemption and call premiums on senior notes		—			(17,913	)		—			—			—			(17,913	)
Proceeds from ATM Program issuances, net of costs		17,259			—			—			—			—			17,259
Contribution from General Partner		465			—			—			—			—			465
Issuance of senior notes		—			500,000			—			—			—			500,000
Tender and redemption of senior notes		—			(300,000	)		—			—			—			(300,000	)
Other, net		(2,632	)		—			372			(8	)		—			(2,268	)
Advances from affiliates		72,341			—			—			(7,635	)		(64,706	)		—
Net cash (used in) provided by financing activities		(1,596	)		9,474			372			(7,643	)		(64,706	)		(64,099	)
Net change in cash and cash equivalents		(470	)		28			(3,704	)		(694	)		—			(4,840	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	228		$	79		$	2,064		$	217		$	—		$	2,588

SUBSEQUENT EVENTS

6 Months Ended

Jun. 30, 2018

Subsequent Events [Abstract]

SUBSEQUENT EVENTS

19. SUBSEQUENT EVENTS

We have evaluated subsequent events for recognition or disclosure in the unaudited condensed consolidated financial statements and no events have occurred that require disclosure.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Accounting Pronouncements

Recently Adopted Accounting Pronouncements. We have recently adopted the following accounting pronouncements:

•

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Impact on financial statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

•

Accounting Pronouncements Pending Adoption. We have not yet adopted the following accounting pronouncements as of June 30, 2018:

•

Basis of Presentation

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606

The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows:

	Balance at December 31, 2017		Adjustments Due to Topic 606		Balance at January 1, 2018
	(In thousands)
Assets
Property, plant and equipment, net	$	1,795,129	$	33,123	$	1,828,252

Liabilities
Deferred revenue, current		4,000		6,088		10,088
Deferred revenue, noncurrent		12,707		22,821		35,528

Partners' Capital (1)		1,084,430		4,214		1,088,644

________

(1) Includes common limited partner capital and general partner interests.

Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Financial Statements

The following tables summarize the impact of Topic 606 adoption on our unaudited condensed consolidated financial statements.

Unaudited condensed consolidated balance sheet

	June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Assets
Accounts receivable	$	80,166	$	71,768	$	8,398
Property, plant and equipment, net		1,887,759		1,852,126		35,633

Liabilities
Deferred revenue, current		10,778		4,071		6,707
Deferred revenue, noncurrent		38,119		11,133		26,986

Partners' Capital (1)		932,236		921,898		10,338

________

(1) Includes common limited partner capital and general partner interests.

Unaudited condensed consolidated statement of operations

	Three months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	89,585	$	86,892	$	2,693
Costs and expenses
Cost of natural gas and NGLs		24,384		27,559		(3,175	)
Depreciation and amortization		26,784		26,472		312

	Six months ended June 30, 2018
	As Reported		Balances Without Adoption of Topic 606		Effect of Change Increase / (Decrease)
	(In thousands)
Revenues
Gathering services and related fees	$	173,946	$	172,195	$	1,751
Costs and expenses
Cost of natural gas and NGLs		44,670		51,161		(6,491	)
Depreciation and amortization		53,461		52,846		615

Unaudited condensed consolidated statement of cash flows

	Six months ended June 30, 2018
	As Reported			Balances Without Adoption of Topic 606			Effect of Change Increase / (Decrease)
	(In thousands)
Cash flows from operating activities:
Net loss	$	(53,758	)	$	(61,385	)	$	7,627
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization		53,160			52,545			615
Changes in operating assets and liabilities:
Accounts receivable		(6,265	)		2,133			(8,398	)
Deferred revenue, net		3,281			3,125			156

REVENUE (Tables)

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

Schedule of Estimated Revenue Expected to be Recognized and MVC Shortfall Payments

	2018		2019		2020		2021		2022		Thereafter
	(In thousands)
Gathering services and related fees	$	97,523	$	133,743	$	122,429	$	102,777	$	83,648	$	174,227

Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments

Revenue by Category. In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4.

	Reportable Segments
	Three months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	10,422	$	23,106	$	36,170	$	14,080	$	8,050	$	91,828	$	(2,243	)	$	89,585
Natural gas, NGLs and condensate sales		—		7,350		4,675		381		—		12,406		19,485			31,891
Other revenues		—		2,960		2,147		1,694		—		6,801		(94	)		6,707
Total	$	10,422	$	33,416	$	42,992	$	16,155	$	8,050	$	111,035	$	17,148		$	128,183

	Reportable Segments
	Six months ended June 30, 2018
	Utica Shale		Williston Basin		Piceance / DJ Basins		Barnett Shale		Marcellus Shale		Total reportable segments		All other segments			Total
	(In thousands)
Major products/services lines
Gathering services and related fees	$	20,463	$	40,772	$	71,464	$	27,717	$	15,875	$	176,291	$	(2,345	)	$	173,946
Natural gas, NGLs and condensate sales		—		15,196		9,000		926		—		25,122		32,886			58,008
Other revenues		—		5,872		4,115		3,682		—		13,669		(120	)		13,549
Total	$	20,463	$	61,840	$	84,579	$	32,325	$	15,875	$	215,082	$	30,421		$	245,503

Schedule of Information about Contract Assets from Contracts with Customers

The following table provides information about contract assets from contracts with customers:

	June 30, 2018
	(In thousands)
Contract assets, December 31, 2017	$	—
Net impact of Topic 606 day 1 adoption		3,514
Additions		11,523
Transfers out		(5,704	)
Contract assets, June 30, 2018	$	9,333

Current		9,333
Noncurrent		—
Contract assets, June 30, 2018	$	9,333

SEGMENT INFORMATION (Tables)

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

Schedule of assets by reportable segment

Assets by reportable segment follow.

	June 30, 2018			December 31, 2017
	(In thousands)
Assets:
Utica Shale	$	212,326		$	212,311
Ohio Gathering		670,827			690,485
Williston Basin		520,799			512,860
Piceance/DJ Basins		828,999			798,722
Barnett Shale		376,969			383,306
Marcellus Shale		213,366			217,362
Total reportable segment assets		2,823,286			2,815,046
Corporate and Other		143,450			79,996
Eliminations		(1,158	)		(249	)
Total assets	$	2,965,578		$	2,894,793

Schedule of segment reporting information

Revenues by reportable segment follow.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Revenues (1):
Utica Shale	$	10,422		$	10,456		$	20,463		$	19,252
Williston Basin		33,416			29,114			61,840			95,999
Piceance/DJ Basins		42,992			33,763			84,579			68,571
Barnett Shale		16,155			20,904			32,325			38,646
Marcellus Shale		8,050			7,365			15,875			14,269
Total reportable segments revenue		111,035			101,602			215,082			236,737
Corporate and Other		19,422			1,362			33,598			3,148
Eliminations		(2,274	)		(1,172	)		(3,177	)		(2,288	)
Total revenues	$	128,183		$	101,792		$	245,503		$	237,597

(1) Excludes revenues earned by Ohio Gathering due to equity method accounting.

Depreciation and amortization, including the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	2,033	$	1,748	$	3,886	$	3,395
Williston Basin		5,622		8,385		11,231		16,766
Piceance/DJ Basins		12,450		12,225		25,005		24,436
Barnett Shale (2)		3,759		3,762		7,516		7,524
Marcellus Shale		2,274		2,263		4,546		4,526
Total reportable segment depreciation and amortization		26,138		28,383		52,184		56,647
Corporate and Other		496		154		976		308
Total depreciation and amortization	$	26,634	$	28,537	$	53,160	$	56,955

(1) Excludes depreciation and amortization recognized by Ohio Gathering due to equity method accounting.

(2) Includes the amortization expense associated with our favorable and unfavorable gas gathering contracts as reported in other revenues.

Cash paid for capital expenditures by reportable segment follow.

	Six months ended June 30,
	2018		2017
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	1,846	$	16,473
Williston Basin		10,966		11,085
Piceance/DJ Basins		24,827		11,934
Barnett Shale		349		(399	)
Marcellus Shale		545		407
Total reportable segment capital expenditures		38,533		39,500
Corporate and Other		51,861		6,412
Total cash paid for capital expenditures	$	90,394	$	45,912

(1) Excludes cash paid for capital expenditures by Ohio Gathering due to equity method accounting.

Schedule of counterparties accounting for more than 10% of total revenues

Counterparties accounting for more than 10% of total revenues were as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins		10	%	*				11	%	*
Counterparty B - Piceance/DJ Basins	*				10	%	*			*
Counterparty C - Barnett Shale		11	%		11	%	*			*
Counterparty D - Utica Shale	*				10	%	*			*
Counterparty E - Williston Basin	*			*			*				21	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments.

(2) Excludes revenues earned by Ohio Gathering due to equity method accounting.

* Less than 10%

Reconciliation of net income to adjusted EBITDA

Segment adjusted EBITDA by reportable segment follows.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	9,223	$	9,533	$	17,938	$	17,445
Ohio Gathering		8,935		9,606		19,412		18,679
Williston Basin		19,030		17,155		35,000		34,964
Piceance/DJ Basins		27,673		27,274		56,908		56,248
Barnett Shale		11,093		12,998		20,952		25,086
Marcellus Shale		6,543		5,446		13,219		11,093
Total of reportable segments' measures of profit	$	82,497	$	82,012	$	163,429	$	163,515

A reconciliation of income or loss before income taxes and income or loss from equity method investees to total of reportable segments' measures of profit or loss follows.

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Reconciliation of (loss) income before income taxes and loss from equity method investees to total of reportable segments' measures of profit:
(Loss) income before income taxes and loss from equity method investees	$	(45,699	)	$	14,420		$	(51,101	)	$	14,945
Add:
Corporate and Other		9,002			9,435			19,625			19,528
Interest expense		14,837			17,553			29,959			34,269
Early extinguishment of debt		—			—			—			22,020
Deferred Purchase Price Obligation		69,305			(5,058	)		90,963			15,825
Depreciation and amortization		26,634			28,537			53,160			56,955
Proportional adjusted EBITDA for equity method investees		8,935			9,606			19,412			18,679
Adjustments related to MVC shortfall payments		(3,542	)		5,578			(3,542	)		(23,062	)
Adjustments related to capital reimbursement activity		115			—			155			—
Unit-based and noncash compensation		2,261			1,871			4,223			3,999
Loss (gain) on asset sales, net		62			67			(12	)		70
Long-lived asset impairment		587			3			587			287
Total of reportable segments' measures of profit	$	82,497		$	82,012		$	163,429		$	163,515

Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments

Adjustments related to MVC shortfall payments by reportable segment follow.

	Three months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Three months ended June 30, 2017
	Williston Basin		Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—	$	(1,186	)	$	—		$	(1,186	)
Expected MVC shortfall adjustments		1,982		6,522			(1,740	)		6,764
Total adjustments related to MVC shortfall payments	$	1,982	$	5,336		$	(1,740	)	$	5,578

	Six months ended June 30, 2018
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	—		$	—		$	—		$	—
Expected MVC shortfall adjustments		(3,386	)		(93	)		(63	)		(3,542	)
Total adjustments related to MVC shortfall payments	$	(3,386	)	$	(93	)	$	(63	)	$	(3,542	)

	Six months ended June 30, 2017
	Williston Basin			Piceance/DJ Basins			Barnett Shale			Total
	(In thousands)
Adjustments related to MVC shortfall payments:
Net change in deferred revenue for MVC shortfall payments	$	(37,693	)	$	(1,978	)	$	—		$	(39,671	)
Expected MVC shortfall adjustments		3,964			13,067			(422	)		16,609
Total adjustments related to MVC shortfall payments	$	(33,729	)	$	11,089		$	(422	)	$	(23,062	)

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)

6 Months Ended

Jun. 30, 2018

Property Plant And Equipment [Abstract]

Schedule of property, plant, and equipment, net

Details on property, plant and equipment follow.

	June 30, 2018		December 31, 2017
	(In thousands)
Gathering and processing systems and related equipment	$	2,023,832	$	1,973,722
Construction in progress		160,239		78,850
Land and line fill		11,735		11,735
Other		41,802		40,262
Total		2,237,608		2,104,569
Less accumulated depreciation		349,849		309,440
Property, plant and equipment, net	$	1,887,759	$	1,795,129

Schedule of depreciation expense and capitalized interest costs

Depreciation expense and capitalized interest follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Depreciation expense	$	18,657	$	18,607	$	37,214	$	37,098
Capitalized interest		1,863		450		3,085		918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Tables)

6 Months Ended

Jun. 30, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible assets and liabilities subject to amortization

Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities), all of which are subject to amortization, follow.

	June 30, 2018
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(13,128	)	$	11,067
Contract intangibles		278,448		(130,891	)		147,557
Rights-of-way		162,339		(34,298	)		128,041
Total intangible assets	$	464,982	$	(178,317	)	$	286,665

Unfavorable gas gathering contract	$	10,962	$	(10,153	)	$	809

	December 31, 2017
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(12,350	)	$	11,845
Contract intangibles		278,448		(117,821	)		160,627
Rights-of-way		159,986		(31,113	)		128,873
Total intangible assets	$	462,629	$	(161,284	)	$	301,345

Unfavorable gas gathering contract	$	10,962	$	(9,074	)	$	1,888

Recognized amortization expense in other revenues and cost and expenses

We recognized amortization expense in other revenues as follows:

	Three months ended June 30,						Six months ended June 30,
	2018			2017			2018			2017
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(388	)	$	(388	)	$	(777	)	$	(777	)
Amortization expense – unfavorable gas gathering contract		538			539			1,078			1,079

We recognized amortization expense in costs and expenses as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Amortization expense – contract intangibles	$	6,535	$	8,551	$	13,070	$	17,102
Amortization expense – rights-of-way		1,592		1,530		3,177		3,057

Estimated aggregate annual amortization expected to be recognized

The estimated aggregate annual amortization expected to be recognized for the remainder of 2018 and each of the four succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	17,053	$	809
2019		33,217		—
2020		33,041		—
2021		29,349		—
2022		26,282		—

EQUITY METHOD INVESTMENTS (Tables)

6 Months Ended

Jun. 30, 2018

Equity Method Investments And Joint Ventures [Abstract]

Equity Method Investments

A reconciliation of our 40% ownership interest in Ohio Gathering to our investment per Ohio Gathering's books and records follows (in thousands).

Investment in equity method investees, June 30, 2018	$	670,827
June cash distributions		2,580
Basis difference		(123,508	)
Investment in equity method investees, net of basis difference, May 31, 2018	$	549,899

For the three and six months ended June 30, 2018, there were no contributions to Ohio Gathering.

	Three months ended May 31, 2018						Three months ended May 31, 2017
	OGC			OCC			OGC			OCC
	(In thousands)
Total revenues	$	34,123		$	2,070		$	31,083		$	2,004
Total operating expenses		35,518			1,958			33,221			1,836
Net (loss) income		(1,396	)		(59	)		(139	)		23

	Six months ended May 31, 2018				Six months ended May 31, 2017
	OGC		OCC		OGC		OCC
	(In thousands)
Total revenues	$	69,083	$	4,559	$	68,158	$	4,057
Total operating expenses		62,293		4,099		60,326		4,309
Net income (loss)		6,784		121		7,834		(1,192	)

DEFERRED REVENUE (Tables)

6 Months Ended

Jun. 30, 2018

Revenue From Contract With Customer [Abstract]

Rollforward of deferred revenue

A rollforward of current deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total current
	(In thousands)
Current deferred revenue, December 31, 2017, as reported	$	—	$	—	$	4,000	$	—	$	—	$	4,000
Net impact of Topic 606 day 1 adoption		18		1,017		3,396		1,619		38		6,088
Current deferred revenue, January 1, 2018		18		1,017		7,396		1,619		38		10,088
Additions		9		1,088		10,996		814		19		12,926
Less revenue recognized		9		643		10,762		803		19		12,236
Current deferred revenue, June 30, 2018	$	18	$	1,462	$	7,630	$	1,630	$	38	$	10,778

A rollforward of noncurrent deferred revenue follows.

	Utica Shale		Williston Basin		Piceance/DJ Basins		Barnett Shale		Marcellus Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, December 31, 2017, as reported	$	—	$	—	$	12,707	$	—	$	—	$	12,707
Net impact of Topic 606 day 1 adoption		39		4,215		10,017		8,217		333		22,821
Noncurrent deferred revenue, January 1, 2018		39		4,215		22,724		8,217		333		35,528
Additions		—		1,622		5,200		1,583		—		8,405
Less reclassification to current deferred revenue		9		1,017		3,955		814		19		5,814
Noncurrent deferred revenue, June 30, 2018	$	30	$	4,820	$	23,969	$	8,986	$	314	$	38,119

DEBT (Tables)

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Schedule of debt and capital leases

Debt consisted of the following:

	June 30, 2018			December 31, 2017
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.35% at June 30, 2018 and 4.07% at December 31, 2017) due May 2022	$	356,000		$	261,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,588	)		(2,910	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			500,000
Less unamortized debt issuance costs (1)		(6,407	)		(6,898	)
Total long-term debt	$	1,147,005		$	1,051,192

(1) Issuance costs are being amortized over the life of the notes.

- Definition

+ References

+ Details

FINANCIAL INSTRUMENTS (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

A rollforward of our Level 3 liability measured at fair value on a recurring basis follows (in thousands).

Level 3 liability, January 1, 2018	$	362,959
Change in fair value		90,963
Level 3 liability, June 30, 2018	$	453,922

Summary of the estimated fair value of debt instruments

A summary of the estimated fair value of our debt financial instruments follows.

	June 30, 2018				December 31, 2017
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings 5.5% Senior Notes ($300.0 million principal)	$	297,412	$	293,875	$	297,090	$	301,750
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,593		474,375		493,102		501,667

PARTNERS' CAPITAL (Tables)

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

Schedule of partner units activity

A rollforward of the number of common limited partner and General Partner units follows.

	Limited partners
	Series A Preferred Units		Common		General Partner
Units, January 1, 2018		300,000		73,085,996		1,490,999
Net units issued under the SMLP LTIP		—		269,779		—
Units, June 30, 2018		300,000		73,355,775		1,490,999

Details of cash distributions

We paid the following per-unit distributions during the three and six months ended June 30:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
Per-unit distributions to unitholders	$	0.575	$	0.575	$	1.150	$	1.150

Our payment of IDRs as reported in distributions to unitholders – general partner in the statement of partners' capital during the three and six months ended June 30 follow.

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
IDR payments	$	2,136	$	2,106	$	4,264	$	4,206

EARNINGS PER UNIT (Tables)

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Schedule of earnings per limited partner unit

The following table details the components of EPU.

	Three months ended June 30,					Six months ended June 30,
	2018			2017		2018			2017
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net (loss) income among limited partner interests:
Net (loss) income attributable to limited partners	$	(51,111	)	$	8,806	$	(57,099	)	$	6,029
Less net income attributable to Series A Preferred Units		7,125			—		14,250			—
Net (loss) income attributable to common limited partners	$	(58,236	)	$	8,806	$	(71,349	)	$	6,029

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		73,356			72,532		73,245			72,341
Effect of nonvested phantom units		—			310		—			367
Weighted-average common units outstanding – diluted		73,356			72,842		73,245			72,708

(Loss) earnings per limited partner unit:
Common unit – basic	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08
Common unit – diluted	$	(0.79	)	$	0.12	$	(0.97	)	$	0.08

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		1			—		3			—

RELATED-PARTY TRANSACTIONS (Tables)

6 Months Ended

Jun. 30, 2018

Schedule of related party transactions

Expenses incurred by the General Partner and reimbursed by us under our Partnership Agreement were as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Operation and maintenance expense	$	7,114	$	6,731	$	14,737	$	13,612
General and administrative expense		7,481		7,895		15,598		16,190

COMMITMENTS AND CONTINGENCIES (Tables)

6 Months Ended

Jun. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of total rent expense related to operating leases

Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(In thousands)
Rent expense	$	956	$	923	$	1,978	$	1,802

Schedule of accrued environmental remediation

A rollforward of the aggregate accrued environmental remediation liabilities follows.

	Total
	(In thousands)
Accrued environmental remediation, January 1, 2018	$	5,344
Payments made		(1,805	)
Additional accruals		1,600
Accrued environmental remediation, June 30, 2018	$	5,139

CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Tables)

6 Months Ended

Jun. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Condensed Balance Sheet

Balance sheets as of June 30, 2018 and December 31, 2017 follow.

	June 30, 2018
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	2,616	$	92	$	4,998	$	506	$	—		$	8,212
Accounts receivable		22		—		68,274		11,870		—			80,166
Other current assets		1,057		—		1,273		264		—			2,594
Due from affiliate		—		—		507,396		51,852		(559,248	)		—
Total current assets		3,695		92		581,941		64,492		(559,248	)		90,972
Property, plant and equipment, net		4,314		—		1,537,468		345,977		—			1,887,759
Intangible assets, net		—		—		263,416		23,249		—			286,665
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		670,827		—			670,827
Other noncurrent assets		3,288		9,762		94		—		—			13,144
Investment in subsidiaries		2,062,096		3,396,771		—		—		(5,458,867	)		—
Total assets	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

Liabilities and Partners' Capital
Trade accounts payable	$	141	$	—	$	18,280	$	3,277	$	—		$	21,698
Accrued expenses		803		—		17,030		1,600		—			19,433
Due to affiliate		374,074		185,265		—		—		(559,248	)		91
Deferred revenue		—		—		10,341		437		—			10,778
Ad valorem taxes payable		9		—		6,224		271		—			6,504
Accrued interest		—		12,259		—		—		—			12,259
Accrued environmental remediation		—		—		—		3,522		—			3,522
Other current liabilities		3,477		—		3,573		362		—			7,412
Total current liabilities		378,504		197,524		55,448		9,469		(559,248	)		81,697
Long-term debt		—		1,147,005		—		—		—			1,147,005
Deferred Purchase Price Obligation		453,922		—		—		—		—			453,922
Noncurrent deferred revenue		—		—		36,416		1,703		—			38,119
Noncurrent accrued environmental remediation		—		—		—		1,617		—			1,617
Other noncurrent liabilities		4,159		—		2,197		54		—			6,410
Total liabilities		836,585		1,344,529		94,061		12,843		(559,248	)		1,728,770

Total partners' capital		1,236,808		2,062,096		2,305,069		1,091,702		(5,458,867	)		1,236,808
Total liabilities and partners' capital	$	2,073,393	$	3,406,625	$	2,399,130	$	1,104,545	$	(6,018,115	)	$	2,965,578

	December 31, 2017
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	126	$	75	$	1,138	$	91	$	—		$	1,430
Accounts receivable		22		—		65,842		6,437		—			72,301
Other current assets		1,188		—		2,697		442		—			4,327
Due from affiliate		—		—		493,067		22,030		(515,097	)		—
Total current assets		1,336		75		562,744		29,000		(515,097	)		78,058
Property, plant and equipment, net		4,206		—		1,442,333		348,590		—			1,795,129
Intangible assets, net		—		—		278,958		22,387		—			301,345
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		690,485		—			690,485
Other noncurrent assets		2,547		10,913		105		—		—			13,565
Investment in subsidiaries		2,019,700		3,324,464		—		—		(5,344,164	)		—
Total assets	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Liabilities and Partners' Capital
Trade accounts payable	$	209	$	—	$	11,283	$	4,883	$	—		$	16,375
Accrued expenses		928		—		10,592		979		—			12,499
Due to affiliate		263,935		252,250		—		—		(515,097	)		1,088
Deferred revenue		—		—		4,000		—		—			4,000
Ad valorem taxes payable		—		—		7,809		520		—			8,329
Accrued interest		—		12,310		—		—		—			12,310
Accrued environmental remediation		—		—		—		3,130		—			3,130
Other current liabilities		6,395		—		4,385		478		—			11,258
Total current liabilities		271,467		264,560		38,069		9,990		(515,097	)		68,989
Long-term debt		—		1,051,192		—		—		—			1,051,192
Deferred Purchase Price Obligation		362,959		—		—		—		—			362,959
Deferred revenue		—		—		12,707		—		—			12,707
Noncurrent accrued environmental remediation		—		—		—		2,214		—			2,214
Other noncurrent liabilities		3,694		—		3,293		76		—			7,063
Total liabilities		638,120		1,315,752		54,069		12,280		(515,097	)		1,505,124

Total partners' capital		1,389,669		2,019,700		2,246,282		1,078,182		(5,344,164	)		1,389,669
Total liabilities and partners' capital	$	2,027,789	$	3,335,452	$	2,300,351	$	1,090,462	$	(5,859,261	)	$	2,894,793

Condensed Statement of Operations

Statements of operations for the three and six months ended June 30, 2018 and 2017 follow.

	Three months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	73,891		$	15,694		$	—		$	89,585
Natural gas, NGLs and condensate sales		—			—			31,891			—			—			31,891
Other revenues		—			—			6,509			198			—			6,707
Total revenues		—			—			112,291			15,892			—			128,183
Costs and expenses:
Cost of natural gas and NGLs		—			—			24,384			—			—			24,384
Operation and maintenance		—			—			22,221			2,245			—			24,466
General and administrative		—			—			11,907			1,577			—			13,484
Depreciation and amortization		444			—			22,776			3,564			—			26,784
(Gain) loss on asset sales, net		—			—			(1	)		63			—			62
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		444			—			81,874			7,449			—			89,767
Other income		27			—			—			—			—			27
Interest expense		—			(14,837	)		—			—			—			(14,837	)
Deferred Purchase Price Obligation		(69,305	)		—			—			—			—			(69,305	)
(Loss) income before income taxes and loss from equity method investees		(69,722	)		(14,837	)		30,417			8,443			—			(45,699	)
Income tax expense		(294	)		—			—			—			—			(294	)
Loss from equity method investees		—			—			—			(3,920	)		—			(3,920	)
Equity in earnings of consolidated subsidiaries		20,103			34,940			—			—			(55,043	)		—
Net (loss) income	$	(49,913	)	$	20,103		$	30,417		$	4,523		$	(55,043	)	$	(49,913	)

	Three months ended June 30, 2017
	SMLP		Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—	$	—		$	67,679	$	17,122		$	—		$	84,801
Natural gas, NGLs and condensate sales		—		—			10,595		—			—			10,595
Other revenues		—		—			6,332		64			—			6,396
Total revenues		—		—			84,606		17,186			—			101,792
Costs and expenses:
Cost of natural gas and NGLs		—		—			9,099		—			—			9,099
Operation and maintenance		—		—			22,169		1,847			—			24,016
General and administrative		—		—			10,886		2,063			—			12,949
Depreciation and amortization		154		—			25,282		3,252			—			28,688
Transaction costs		119		—			—		—			—			119
Loss on asset sales, net		—		—			67		—			—			67
Long-lived asset impairment		—		—			2		1			—			3
Total costs and expenses		273		—			67,505		7,163			—			74,941
Other income		64		—			—		—			—			64
Interest expense		—		(17,553	)		—		—			—			(17,553	)
Deferred Purchase Price Obligation		5,058		—			—		—			—			5,058
Income (loss) before income taxes and loss from equity method investees		4,849		(17,553	)		17,101		10,023			—			14,420
Income tax benefit		211		—			—		—			—			211
Loss from equity method investees		—		—			—		(3,385	)		—			(3,385	)
Equity in earnings of consolidated subsidiaries		6,186		23,739			—		—			(29,925	)		—
Net income	$	11,246	$	6,186		$	17,101	$	6,638		$	(29,925	)	$	11,246

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	143,357		$	30,589		$	—		$	173,946
Natural gas, NGLs and condensate sales		—			—			58,008			—			—			58,008
Other revenues		—			—			13,236			313			—			13,549
Total revenues		—			—			214,601			30,902			—			245,503
Costs and expenses:
Cost of natural gas and NGLs		—			—			44,670			—			—			44,670
Operation and maintenance		—			—			44,374			4,696			—			49,070
General and administrative		—			—			24,244			3,682			—			27,926
Depreciation and amortization		876			—			45,637			6,948			—			53,461
(Gain) loss on asset sales, net		—			—			(75	)		63			—			(12	)
Long-lived asset impairment		—			—			587			—			—			587
Total costs and expenses		876			—			159,437			15,389			—			175,702
Other income		20			—			—			—			—			20
Interest expense		—			(29,959	)		—			—			—			(29,959	)
Deferred Purchase Price Obligation		(90,963	)		—			—			—			—			(90,963	)
(Loss) income before income taxes and loss from equity method investees		(91,819	)		(29,959	)		55,164			15,513			—			(51,101	)
Income tax expense		(123	)		—			—			—			—			(123	)
Loss from equity method investees		—			—			—			(2,534	)		—			(2,534	)
Equity in earnings of consolidated subsidiaries		38,184			68,143			—			—			(106,327	)		—
Net (loss) income	$	(53,758	)	$	38,184		$	55,164		$	12,979		$	(106,327	)	$	(53,758	)

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	170,192	$	32,622		$	—		$	202,814
Natural gas, NGLs and condensate sales		—			—			21,715		—			—			21,715
Other revenues		—			—			12,932		136			—			13,068
Total revenues		—			—			204,839		32,758			—			237,597
Costs and expenses:
Cost of natural gas and NGLs		—			—			18,151		—			—			18,151
Operation and maintenance		—			—			44,189		3,519			—			47,708
General and administrative		—			—			23,371		3,710			—			27,081
Depreciation and amortization		308			—			50,524		6,425			—			57,257
Transaction costs		119			—			—		—			—			119
Loss on asset sales, net		—			—			70		—			—			70
Long-lived asset impairment		—			—			2		285			—			287
Total costs and expenses		427			—			136,307		13,939			—			150,673
Other income		135			—			—		—			—			135
Interest expense		—			(34,269	)		—		—			—			(34,269	)
Early extinguishment of debt		—			(22,020	)		—		—			—			(22,020	)
Deferred Purchase Price Obligation		(15,825	)		—			—		—			—			(15,825	)
(Loss) income before income taxes and loss from equity method investees		(16,117	)		(56,289	)		68,532		18,819			—			14,945
Income tax expense		(241	)		—			—		—			—			(241	)
Loss from equity method investees		—			—			—		(4,041	)		—			(4,041	)
Equity in earnings of consolidated subsidiaries		27,021			83,310			—		—			(110,331	)		—
Net income	$	10,663		$	27,021		$	68,532	$	14,778		$	(110,331	)	$	10,663

Condensed Cash Flow Statement

Statements of cash flows for the six months ended June 30, 2018 and 2017 follow.

	Six months ended June 30, 2018
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$	(68	)	$	(27,877	)	$	103,965		$	34,029		$	—		$	110,049

Cash flows from investing activities:
Capital expenditures		(972	)		—			(85,187	)		(4,235	)		—			(90,394	)
Proceeds from asset sales		—			—			—			496			—			496
Other, net		(306	)		—			—			—			—			(306	)
Advances to affiliates		—			(66,985	)		(14,329	)		(29,820	)		111,134			—
Net cash used in investing activities		(1,278	)		(66,985	)		(99,516	)		(33,559	)		111,134			(90,204	)

Cash flows from financing activities:
Distributions to common unitholders		(90,269	)		—			—			—			—			(90,269	)
Distributions to Series A Preferred unitholders		(14,250	)		—			—			—			—			(14,250	)
Borrowings under Revolving Credit Facility		—			148,000			—			—			—			148,000
Repayments under Revolving Credit Facility		—			(53,000	)		—			—			—			(53,000	)
Debt issuance costs		—			(121	)		—			—			—			(121	)
Other, net		(2,779	)		—			(589	)		(55	)		—			(3,423	)
Advances from affiliates		111,134			—			—			—			(111,134	)		—
Net cash provided by (used in) financing activities		3,836			94,879			(589	)		(55	)		(111,134	)		(13,063	)
Net change in cash and cash equivalents		2,490			17			3,860			415			—			6,782
Cash and cash equivalents, beginning of period		126			75			1,138			91			—			1,430
Cash and cash equivalents, end of period	$	2,616		$	92		$	4,998		$	506		$	—		$	8,212

	Six months ended June 30, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$	424		$	(32,466	)	$	114,863		$	38,520		$	—		$	121,341

Cash flows from investing activities:
Capital expenditures		(6,412	)		—			(23,578	)		(15,922	)		—			(45,912	)
Contributions to equity method investees		—			—			—			(15,649	)		—			(15,649	)
Other, net		(521	)		—			—			—			—			(521	)
Advances to affiliates		7,635			23,020			(95,361	)		—			64,706			—
Net cash provided by (used in) investing activities		702			23,020			(118,939	)		(31,571	)		64,706			(62,082	)

Cash flows from financing activities:
Distributions to unitholders		(89,029	)		—			—			—			—			(89,029	)
Borrowings under Revolving Credit Facility		—			112,500			—			—			—			112,500
Repayments under Revolving Credit Facility		—			(269,500	)		—			—			—			(269,500	)
Debt issuance costs		—			(15,613	)		—			—			—			(15,613	)
Payment of redemption and call premiums on senior notes		—			(17,913	)		—			—			—			(17,913	)
Proceeds from ATM Program issuances, net of costs		17,259			—			—			—			—			17,259
Contribution from General Partner		465			—			—			—			—			465
Issuance of senior notes		—			500,000			—			—			—			500,000
Tender and redemption of senior notes		—			(300,000	)		—			—			—			(300,000	)
Other, net		(2,632	)		—			372			(8	)		—			(2,268	)
Advances from affiliates		72,341			—			—			(7,635	)		(64,706	)		—
Net cash (used in) provided by financing activities		(1,596	)		9,474			372			(7,643	)		(64,706	)		(64,099	)
Net change in cash and cash equivalents		(470	)		28			(3,704	)		(694	)		—			(4,840	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	228		$	79		$	2,064		$	217		$	—		$	2,588

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION - Narrative (Details) - shares	1 Months Ended	6 Months Ended
	Feb. 29, 2016	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2016
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		73,355,775	73,085,996
Summit Midstream Niobrara, LLC \| Summit Midstream Partners, LP
Organization And Business Operations [Line Items]
Cost method ownership interest			1.00%
SMP Holdings \| Contributed Entities
Organization And Business Operations [Line Items]
Percent of issued and outstanding membership interest	100.00%
SMP Holdings \| OGC
Organization And Business Operations [Line Items]
Percentage of ownership interest	40.00%
Subsidiary of Summit Investments
Organization And Business Operations [Line Items]
Ownership interest				1.00%
Grand River Gathering, LLC
Organization And Business Operations [Line Items]
Percentage of ownership interest			100.00%
Common units \| SMP Holdings
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		25,854,581
Common units \| Energy Capital Partners
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)		5,915,827
General Partner
Organization And Business Operations [Line Items]
General partner interest		2.00%
General Partner \| Summit Investments \| Summit Midstream Partners, LP
Organization And Business Operations [Line Items]
General partner interest		2.00%

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Acceleration totaled, recognition of revenue	$ 16.7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606 (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Property, plant and equipment, net	$ 1,887,759	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	10,778	10,088	4,000
Deferred revenue, noncurrent	38,119	35,528	12,707
Partners' Capital	932,236	1,088,644
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Property, plant and equipment, net	1,852,126		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	11,133		12,707
Partners' Capital	921,898		$ 1,084,430
Accounting Standard Update 2014-09 \| Adjustments Due to Topic 606
Assets
Property, plant and equipment, net	35,633	33,123
Liabilities
Deferred revenue	6,707	6,088
Deferred revenue, noncurrent	26,986	22,821
Partners' Capital	$ 10,338	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable	$ 80,166
Property, plant and equipment, net	1,887,759	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	10,778	10,088	4,000
Deferred revenue, noncurrent	38,119	35,528	12,707
Partners' Capital	932,236	1,088,644
Balances Without Adoption of Topic 606
Assets
Accounts receivable	71,768
Property, plant and equipment, net	1,852,126		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	11,133		12,707
Partners' Capital	921,898		$ 1,084,430
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Assets
Accounts receivable	8,398
Property, plant and equipment, net	35,633	33,123
Liabilities
Deferred revenue	6,707	6,088
Deferred revenue, noncurrent	26,986	22,821
Partners' Capital	$ 10,338	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Gathering services and related fees	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,784	$ 28,688	$ 53,461	$ 57,257
Balances Without Adoption of Topic 606
Costs and expenses:
Cost of natural gas and NGLs	$ 27,559
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,472
Gathering Services and Related Fees
Revenues:
Gathering services and related fees	89,585	$ 84,801	$ 173,946	$ 202,814
Gathering Services and Related Fees \| Balances Without Adoption of Topic 606
Revenues:
Gathering services and related fees	86,892
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Costs and expenses:
Cost of natural gas and NGLs	$ (3,175)
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 312
Accounting Standard Update 2014-09 \| Gathering Services and Related Fees \| Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees	$ 2,693

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (49,913)	$ 11,246	$ (53,758)	$ 10,663
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization	$ 26,634	$ 28,537	53,160	56,955
Changes in operating assets and liabilities:
Accounts receivable			(6,265)	41,527
Deferred revenue, net			3,281	$ (39,671)
Balances Without Adoption of Topic 606
Cash flows from operating activities:
Net loss			(61,385)
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization			52,545
Changes in operating assets and liabilities:
Accounts receivable			2,133
Deferred revenue, net			3,125
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Cash flows from operating activities:
Net loss			7,627
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization			615
Changes in operating assets and liabilities:
Accounts receivable			(8,398)
Deferred revenue, net			$ 156

REVENUE - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
REVENUE - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue From Contract With Customer [Line Items]
Revenue, performance obligation, description of payment terms		Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components.
Receivables with customers	$ 67,500	$ 67,500
Contract assets included in accounts receivable	9,333	9,333	$ 0
Gathering Services and Related Fees
Revenue From Contract With Customer [Line Items]
Contract liabilities, revenue	$ 3,900	$ 5,000
Maximum
Revenue From Contract With Customer [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	25 years	25 years

REVENUE - Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	10,422		20,463
Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	33,416		61,840
Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	42,992		84,579
Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	16,155		32,325
Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	8,050		15,875
Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	111,035		215,082
All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	17,148		30,421
Gathering Services and Related Fees
Disaggregation Of Revenue [Line Items]
Total revenues	89,585		173,946
Gathering Services and Related Fees \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	10,422		20,463
Gathering Services and Related Fees \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	23,106		40,772
Gathering Services and Related Fees \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	36,170		71,464
Gathering Services and Related Fees \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	14,080		27,717
Gathering Services and Related Fees \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	8,050		15,875
Gathering Services and Related Fees \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	91,828		176,291
Gathering Services and Related Fees \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	(2,243)		(2,345)
Natural Gas, NGLs and Condensate Sales
Disaggregation Of Revenue [Line Items]
Total revenues	31,891		58,008
Natural Gas, NGLs and Condensate Sales \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Natural Gas, NGLs and Condensate Sales \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	7,350		15,196
Natural Gas, NGLs and Condensate Sales \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	4,675		9,000
Natural Gas, NGLs and Condensate Sales \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	381		926
Natural Gas, NGLs and Condensate Sales \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Natural Gas, NGLs and Condensate Sales \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	12,406		25,122
Natural Gas, NGLs and Condensate Sales \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	19,485		32,886
Other Revenues
Disaggregation Of Revenue [Line Items]
Total revenues	6,707		13,549
Other Revenues \| Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Other Revenues \| Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	2,960		5,872
Other Revenues \| Piceance/DJ Basins
Disaggregation Of Revenue [Line Items]
Total revenues	2,147		4,115
Other Revenues \| Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	1,694		3,682
Other Revenues \| Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0		0
Other Revenues \| Total Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	6,801		13,669
Other Revenues \| All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	$ (94)		$ (120)

REVENUE - Schedule of Information about Contract Assets from Contracts with Customers (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Contract With Customer Asset And Liability [Abstract]
Contract assets, December 31, 2017	$ 0
Net impact of Topic 606 day 1 adoption	3,514
Additions	11,523
Transfers out	(5,704)
Contract assets, June 30, 2018	9,333
Current	9,333
Noncurrent	$ 0

SEGMENT INFORMATION - Assets, Revenues, Depreciation and Amortization, and Capital Expenditures by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 2,965,578		$ 2,965,578		$ 2,894,793
Revenues	128,183	$ 101,792	245,503	$ 237,597
Depreciation and amortization	26,634	28,537	53,160	56,955
Capital expenditures			90,394	45,912
Corporate
Segment Reporting Information [Line Items]
Capital expenditures			1,000
Reportable Segments
Segment Reporting Information [Line Items]
Assets	2,823,286		2,823,286		2,815,046
Revenues	111,035	101,602	215,082	236,737
Depreciation and amortization	26,138	28,383	52,184	56,647
Capital expenditures			38,533	39,500
Reportable Segments \| Utica Shale
Segment Reporting Information [Line Items]
Assets	212,326		212,326		212,311
Revenues	10,422	10,456	20,463	19,252
Depreciation and amortization	2,033	1,748	3,886	3,395
Capital expenditures			1,846	16,473
Reportable Segments \| Ohio Gathering
Segment Reporting Information [Line Items]
Assets	670,827		670,827		690,485
Reportable Segments \| Williston Basin
Segment Reporting Information [Line Items]
Assets	520,799		520,799		512,860
Revenues	33,416	29,114	61,840	95,999
Depreciation and amortization	5,622	8,385	11,231	16,766
Capital expenditures			10,966	11,085
Reportable Segments \| Piceance/DJ Basins
Segment Reporting Information [Line Items]
Assets	828,999		828,999		798,722
Revenues	42,992	33,763	84,579	68,571
Depreciation and amortization	12,450	12,225	25,005	24,436
Capital expenditures			24,827	11,934
Reportable Segments \| Barnett Shale
Segment Reporting Information [Line Items]
Assets	376,969		376,969		383,306
Revenues	16,155	20,904	32,325	38,646
Depreciation and amortization	3,759	3,762	7,516	7,524
Capital expenditures			349	(399)
Reportable Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Assets	213,366		213,366		217,362
Revenues	8,050	7,365	15,875	14,269
Depreciation and amortization	2,274	2,263	4,546	4,526
Capital expenditures			545	407
Corporate and Other
Segment Reporting Information [Line Items]
Assets	143,450		143,450		79,996
Revenues	19,422	1,362	33,598	3,148
Depreciation and amortization	496	154	976	308
Capital expenditures			51,861	6,412
Eliminations
Segment Reporting Information [Line Items]
Assets	(1,158)		(1,158)		$ (249)
Revenues	$ (2,274)	$ (1,172)	$ (3,177)	$ (2,288)

SEGMENT INFORMATION - Concentration Risk (Details) - Revenue - Customer concentration	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Counterparty A - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	10.00%	0.00%	11.00%	0.00%
Counterparty B - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	10.00%	0.00%	0.00%
Counterparty C - Barnett Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	11.00%	11.00%	0.00%	0.00%
Counterparty D - Utica Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	10.00%	0.00%	0.00%
Counterparty E - Williston Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	0.00%	0.00%	21.00%

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 82,497	$ 82,012	$ 163,429	$ 163,515
Utica Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	9,223	9,533	17,938	17,445
Ohio Gathering
Segment Reporting Information [Line Items]
Adjusted EBITDA	8,935	9,606	19,412	18,679
Williston Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	19,030	17,155	35,000	34,964
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Adjusted EBITDA	27,673	27,274	56,908	56,248
Barnett Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	11,093	12,998	20,952	25,086
Marcellus Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 6,543	$ 5,446	$ 13,219	$ 11,093

SEGMENT INFORMATION - Reconciliation of Net Income to Adjusted EBITDA (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting [Abstract]
(Loss) income before income taxes and loss from equity method investees	$ (45,699,000)	$ 14,420,000	$ (51,101,000)	$ 14,945,000
Add:
Corporate and Other	9,002,000	9,435,000	19,625,000	19,528,000
Interest expense	14,837,000	17,553,000	29,959,000	34,269,000
Early extinguishment of debt	0	0	0	22,020,000
Deferred Purchase Price Obligation	69,305,000	(5,058,000)	90,963,000	15,825,000
Depreciation and amortization	26,634,000	28,537,000	53,160,000	56,955,000
Proportional adjusted EBITDA for equity method investees	8,935,000	9,606,000	19,412,000	18,679,000
Adjustments related to MVC shortfall payments	(3,542,000)	5,578,000	(3,542,000)	(23,062,000)
Adjustments related to capital reimbursement activity	115,000	0	155,000	0
Unit-based and noncash compensation	2,261,000	1,871,000	4,223,000	3,999,000
Loss (gain) on asset sales, net	62,000	67,000	(12,000)	70,000
Long-lived asset impairment	587,000	3,000	587,000	287,000
Total of reportable segments' measures of profit	$ 82,497,000	$ 82,012,000	$ 163,429,000	$ 163,515,000

SEGMENT INFORMATION - Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	$ 0	$ (1,186,000)	$ 0	$ (39,671,000)
Expected MVC shortfall adjustments	(3,542,000)	6,764,000	(3,542,000)	16,609,000
Total adjustments related to MVC shortfall payments	(3,542,000)	5,578,000	(3,542,000)	(23,062,000)
Williston Basin
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	0	0	(37,693,000)
Expected MVC shortfall adjustments	(3,386,000)	1,982,000	(3,386,000)	3,964,000
Total adjustments related to MVC shortfall payments	(3,386,000)	1,982,000	(3,386,000)	(33,729,000)
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	(1,186,000)	0	(1,978,000)
Expected MVC shortfall adjustments	(93,000)	6,522,000	(93,000)	13,067,000
Total adjustments related to MVC shortfall payments	(93,000)	5,336,000	(93,000)	11,089,000
Barnett Shale
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	0	0	0
Expected MVC shortfall adjustments	(63,000)	(1,740,000)	(63,000)	(422,000)
Total adjustments related to MVC shortfall payments	$ (63,000)	$ (1,740,000)	$ (63,000)	$ (422,000)

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Property, Plant, and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,237,608		$ 2,104,569
Less accumulated depreciation	349,849		309,440
Property, plant and equipment, net	1,887,759	$ 1,828,252	1,795,129
Gathering and processing systems and related equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,023,832		1,973,722
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	160,239		78,850
Land and line fill
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,735		11,735
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 41,802		$ 40,262

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Depreciation Expense and Capitalized Interest (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 18,657	$ 18,607	$ 37,214	$ 37,098
Capitalized interest	$ 1,863	$ 450	$ 3,085	$ 918

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 464,982	$ 462,629
Accumulated amortization	(178,317)	(161,284)
Net	286,665	301,345
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(13,128)	(12,350)
Net	11,067	11,845
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(130,891)	(117,821)
Net	147,557	160,627
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	162,339	159,986
Accumulated amortization	(34,298)	(31,113)
Net	128,041	128,873
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	(10,153)	(9,074)
Net	$ 809	$ 1,888

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Recognized Amortization Expense in Other Revenues and Cost and Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Revenue \| Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ (388)	$ (388)	$ (777)	$ (777)
Other Revenue \| Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Amortization expense	538	539	1,078	1,079
Costs And Expenses \| Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization expense	6,535	8,551	13,070	17,102
Costs And Expenses \| Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 1,592	$ 1,530	$ 3,177	$ 3,057

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Intangible assets
2018	$ 17,053
2019	33,217
2020	33,041
2021	29,349
2022	26,282
Unfavorable gas gathering contract
2018	809
2019	0
2020	0
2021	0
2022	$ 0

GOODWILL - Narrative (Details)	6 Months Ended
GOODWILL - Narrative (Details)	Jun. 30, 2018 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill impairment	$ 0

EQUITY METHOD INVESTMENTS - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Equity Method Investments [Line Items]
Contributions to acquire equity method investments			$ 0	$ 15,649,000
OGC
Schedule Of Equity Method Investments [Line Items]
Contributions to acquire equity method investments	$ 0		0
OGC and OCC
Schedule Of Equity Method Investments [Line Items]
Asset impairment	$ 12,300,000	$ 8,700,000	$ 12,300,000	$ 8,700,000
Principal Owner \| OGC
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest	40.00%		40.00%

EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	$ 670,827	$ 670,827
June cash distributions		17,124	$ 18,003
Investment in equity method investees, net of basis difference, May 31, 2018		690,485
OGC \| Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	670,827	$ 670,827
June cash distributions	2,580
Basis difference	(123,508)
Investment in equity method investees, net of basis difference, May 31, 2018	$ 549,899

EQUITY METHOD INVESTMENTS - Statements of Operations Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2018	May 31, 2017
OGC
Schedule Of Equity Method Investments [Line Items]
Total revenues	$ 34,123	$ 31,083	$ 69,083	$ 68,158
Total operating expenses	35,518	33,221	62,293	60,326
Net (loss) income	(1,396)	(139)	6,784	7,834
OCC
Schedule Of Equity Method Investments [Line Items]
Total revenues	2,070	2,004	4,559	4,057
Total operating expenses	1,958	1,836	4,099	4,309
Net (loss) income	$ (59)	$ 23	$ 121	$ (1,192)

DEFERRED REVENUE - Rollforward of deferred revenue (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 4,000
Current deferred revenue, Additions	12,926
Current deferred revenue, Less revenue recognized	12,236
Current deferred revenue, ending balance	10,778
Noncurrent deferred revenue, beginning balance	12,707
Noncurrent deferred revenue, Additions	8,405
Noncurrent deferred revenue, Less reclassification to current deferred revenue	5,814
Noncurrent deferred revenue, ending balance	38,119
Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	10,088
Noncurrent deferred revenue, beginning balance	35,528
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	6,088
Noncurrent deferred revenue, beginning balance	22,821
Utica Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	9
Current deferred revenue, Less revenue recognized	9
Current deferred revenue, ending balance	18
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	9
Noncurrent deferred revenue, ending balance	30
Utica Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Noncurrent deferred revenue, beginning balance	39
Utica Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Noncurrent deferred revenue, beginning balance	39
Williston Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	1,088
Current deferred revenue, Less revenue recognized	643
Current deferred revenue, ending balance	1,462
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	1,622
Noncurrent deferred revenue, Less reclassification to current deferred revenue	1,017
Noncurrent deferred revenue, ending balance	4,820
Williston Basin \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Noncurrent deferred revenue, beginning balance	4,215
Williston Basin \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Noncurrent deferred revenue, beginning balance	4,215
Piceance/DJ Basins
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	4,000
Current deferred revenue, Additions	10,996
Current deferred revenue, Less revenue recognized	10,762
Current deferred revenue, ending balance	7,630
Noncurrent deferred revenue, beginning balance	12,707
Noncurrent deferred revenue, Additions	5,200
Noncurrent deferred revenue, Less reclassification to current deferred revenue	3,955
Noncurrent deferred revenue, ending balance	23,969
Piceance/DJ Basins \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	7,396
Noncurrent deferred revenue, beginning balance	22,724
Piceance/DJ Basins \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	3,396
Noncurrent deferred revenue, beginning balance	10,017
Barnett Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	814
Current deferred revenue, Less revenue recognized	803
Current deferred revenue, ending balance	1,630
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	1,583
Noncurrent deferred revenue, Less reclassification to current deferred revenue	814
Noncurrent deferred revenue, ending balance	8,986
Barnett Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Noncurrent deferred revenue, beginning balance	8,217
Barnett Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Noncurrent deferred revenue, beginning balance	8,217
Marcellus Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0
Current deferred revenue, Additions	19
Current deferred revenue, Less revenue recognized	19
Current deferred revenue, ending balance	38
Noncurrent deferred revenue, beginning balance	0
Noncurrent deferred revenue, Additions	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	19
Noncurrent deferred revenue, ending balance	314
Marcellus Shale \| Accounting Standard Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Noncurrent deferred revenue, beginning balance	333
Marcellus Shale \| Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Noncurrent deferred revenue, beginning balance	$ 333

DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,147,005	$ 1,051,192
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,147,005	1,051,192
SMP Holdings \| Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	356,000	261,000
SMP Holdings \| Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(2,588)	(2,910)
SMP Holdings \| Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	500,000
Less unamortized debt issuance costs	$ (6,407)	$ (6,898)

DEBT - Components of Long-Term Debt (Parenthetical) (Details) - SMP Holdings	Jun. 30, 2018	Dec. 31, 2017
Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate	4.35%	4.07%
Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Stated interest rate	5.50%	5.50%
Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Stated interest rate	5.75%	5.75%

DEBT - Revolving Credit Facility (Narrative) (Details) - Revolving credit facility	6 Months Ended
	Jun. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Borrowing capacity	$ 1,250,000,000
Revolving credit facility maturity date	2022-05
Accordion feature	$ 250,000,000.0
Commitment fee on unused portion of the facility	0.375%
Interest rate at period end	4.35%
Unused portion under the facility	$ 894,000,000.0
Debt issuance costs	9,800,000
Debt defaults	$ 0
LIBOR
Line of Credit Facility [Line Items]
Applicable margin	2.25%
Minimum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility	0.30%
Minimum \| ABR
Line of Credit Facility [Line Items]
Applicable margin	0.75%
Minimum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin	1.75%
Maximum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility	0.50%
Maximum \| ABR
Line of Credit Facility [Line Items]
Applicable margin	1.75%
Maximum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin	2.75%

DEBT - Senior Notes (Narrative) (Details) - USD ($)		1 Months Ended
DEBT - Senior Notes (Narrative) (Details) - USD ($)	Jul. 31, 2014	Jul. 31, 2014	Feb. 08, 2017
5.75% Senior Notes
Debt Instrument [Line Items]
Stated interest rate	5.75%	5.75%	5.75%
Debt face amount			$ 500,000,000.0
Summit Holdings and Finance Corporation \| Senior Notes Due August 2022
Debt Instrument [Line Items]
Senior unsecured notes	$ 300,000,000.0	$ 300,000,000.0
Stated interest rate	5.50%	5.50%
Maturity date	Aug. 15, 2022
Finance Corp \| Summit Holdings
Debt Instrument [Line Items]
Cumulative percentage ownership in subsidiary		100.00%

FINANCIAL INSTRUMENTS - Concentration Risk - Narrative (Details) - Customer	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
CONCENTRATIONS OF RISK
Number of top customers	5
Five Largest Customers \| Accounts receivable \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	46.00%	44.00%

FINANCIAL INSTRUMENTS - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 liability, beginning of period	$ 362,959
Change in fair value	90,963
Level 3 liability, end of period	$ 453,922

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying value \| 5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 297,412	$ 297,090
Carrying value \| Senior Notes 5.75%
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	493,593	493,102
Fair Value \| 5.5% Senior Notes \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	293,875	301,750
Fair Value \| Senior Notes 5.75% \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 474,375	$ 501,667

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Parenthetical) (Details) - Summit Holdings - USD ($)	Jun. 30, 2018	Dec. 31, 2017
5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate	5.50%	5.50%
Debt face amount	$ 300,000,000.0	$ 300,000,000.0
Senior Notes 5.75%
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate	5.75%
Debt face amount	$ 500,000,000.0

PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details)	6 Months Ended
	Jun. 30, 2018 shares
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	300,000
Units, ending balance (in shares)	300,000
Net units issued under the SMLP LTIP (in shares)	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	73,085,996
Units, ending balance (in shares)	73,355,775
Net units issued under the SMLP LTIP (in shares)	269,779
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,490,999
Units, ending balance (in shares)	1,490,999
Net units issued under the SMLP LTIP (in shares)	0

PARTNERS' CAPITAL - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 26, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 $ / shares	Jun. 30, 2018 USD ($) Transaction $ / shares shares	Jun. 30, 2017 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares
Partners Capital [Line Items]
Preferred units issued \| shares		300,000		300,000		300,000
Per-unit annual distributions to unitholders (in dollars per unit)		$ 0.575	$ 0.575	$ 1.150	$ 1.150
Surplus in excess (in dollars per share)				$ 0.46
Subsequent Event
Partners Capital [Line Items]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.575
Cash paid to unit holders \| $	$ 45.2
Cash distribution date	Aug. 14, 2018
Cash distribution date of record	Aug. 07, 2018
Series A Preferred Units
Partners Capital [Line Items]
Preferred units issued \| shares						300,000
Interest in partnership per unit						$ 1,000
Common \| Public Offering
Partners Capital [Line Items]
Aggregate offering price \| $						$ 150.0
Amount of stock sold (in shares) \| shares		0		0
Aggregate offering price \| $		$ 132.3		$ 132.3
Transactions under ATM Program \| Transaction				0
General Partner
Partners Capital [Line Items]
Percentage interest in distributions (maximum)				50.00%

PARTNERS' CAPITAL - Cash Distributions Paid and Declared (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.575	$ 0.575	$ 1.150	$ 1.150

PARTNERS' CAPITAL - Incentive Distribution Rights (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
IDR payments	$ 2,136	$ 2,106	$ 4,264	$ 4,206

EARNINGS PER UNIT (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ (51,111)	$ 8,806	$ (57,099)	$ 6,029
Phantom Units
(Loss) earnings per limited partner unit:
Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU (in shares)	1	0	3	0
Series A Preferred Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ 7,125	$ 0	$ 14,250	$ 0
Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners	$ (58,236)	$ 8,806	$ (71,349)	$ 6,029
Weighted-average units outstanding (in shares)	73,356	72,532	73,245	72,341
Effect of nonvested phantom units (in shares)	0	310	0	367
Weighted-average units outstanding – diluted (shares)	73,356	72,842	73,245	72,708
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08
Diluted (in dollars per share)	$ (0.79)	$ 0.12	$ (0.97)	$ 0.08

UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) - SMLP LTIP - $ / shares	1 Months Ended
	Mar. 31, 2018	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Units remaining available (in shares)		3,200,000
Phantom Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted units (in shares)	515,358
Grant date fair value (in dollars per share)	$ 15.25
Vesting period (years)	3 years
Units vested (in shares)	328,388

RELATED-PARTY TRANSACTIONS - Expenses Incurred by the General Partner and Reimbursed (Details) - General Partner - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operation and maintenance expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 7,114	$ 6,731	$ 14,737	$ 13,612
General and administrative expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 7,481	$ 7,895	$ 15,598	$ 16,190

COMMITMENTS AND CONTINGENCIES - Rent Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 956	$ 923	$ 1,978	$ 1,802

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2017	Jun. 30, 2018	Dec. 31, 2015
Maximum \| Meadowlark Midstream Gathering System \| Principal Owner
Loss Contingencies [Line Items]
Coverage from pollution liability insurance policy		$ 25,000,000.0	$ 25,000,000.0
Coverage from property and business interruption insurance policy		$ 200,000,000.0
Business Interruption
Loss Contingencies [Line Items]
Recoveries recognized	$ 2,600,000
Property Recoveries
Loss Contingencies [Line Items]
Recoveries recognized	$ 400,000

COMMITMENTS AND CONTINGENCIES - Environmental Matters (Details) - Meadowlark Midstream Gathering System $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 5,344
Payments made	(1,805)
Additional accruals	1,600
Accrued environmental remediation, ending balance	$ 5,139

ACQUISITIONS AND DROP DOWN TRANSACTIONS - 2016 Drop Down (Details) - USD ($) $ in Thousands		1 Months Ended
	Mar. 03, 2016	Jun. 30, 2016	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Deferred purchase price obligation			$ 453,922	$ 362,959
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Initial payment	$ 360,000
Working capital adjustment		$ 600
Estimated future payment obligation			538,400
Deferred purchase price obligation			$ 453,900
Discount rate			10.25%

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 8,212		$ 1,430	$ 2,588	$ 7,428
Accounts receivable	80,166		72,301
Other current assets	2,594		4,327
Due from affiliate	0		0
Total current assets	90,972		78,058
Property, plant and equipment, net	1,887,759	$ 1,828,252	1,795,129
Intangible assets, net	286,665		301,345
Goodwill	16,211		16,211
Investment in equity method investees	670,827		690,485
Other noncurrent assets	13,144		13,565
Investment in subsidiaries	0		0
Total assets	2,965,578		2,894,793
Liabilities and Partners' Capital
Trade accounts payable	21,698		16,375
Accrued expenses	19,433		12,499
Due to affiliate	91		1,088
Deferred revenue	10,778	10,088	4,000
Ad valorem taxes payable	6,504		8,329
Accrued interest	12,259		12,310
Accrued environmental remediation	3,522		3,130
Other current liabilities	7,412		11,258
Total current liabilities	81,697		68,989
Long-term debt	1,147,005		1,051,192
Deferred Purchase Price Obligation	453,922		362,959
Noncurrent deferred revenue	38,119	$ 35,528	12,707
Noncurrent accrued environmental remediation	1,617		2,214
Other noncurrent liabilities	6,410		7,063
Total liabilities	1,728,770		1,505,124
Total partners' capital	1,236,808		1,389,669	1,110,899	1,169,673
Total liabilities and partners' capital	2,965,578		2,894,793
Consolidating adjustments
Assets
Cash and cash equivalents	0		0	0	0
Accounts receivable	0		0
Other current assets	0		0
Due from affiliate	(559,248)		(515,097)
Total current assets	(559,248)		(515,097)
Property, plant and equipment, net	0		0
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	0		0
Investment in subsidiaries	(5,458,867)		(5,344,164)
Total assets	(6,018,115)		(5,859,261)
Liabilities and Partners' Capital
Trade accounts payable	0		0
Accrued expenses	0		0
Due to affiliate	(559,248)		(515,097)
Deferred revenue	0		0
Ad valorem taxes payable	0		0
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	0		0
Total current liabilities	(559,248)		(515,097)
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	0		0
Total liabilities	(559,248)		(515,097)
Total partners' capital	(5,458,867)		(5,344,164)
Total liabilities and partners' capital	(6,018,115)		(5,859,261)
Summit Midstream Partners, LP \| Reportable Legal Entities
Assets
Cash and cash equivalents	2,616		126	228	698
Accounts receivable	22		22
Other current assets	1,057		1,188
Due from affiliate	0		0
Total current assets	3,695		1,336
Property, plant and equipment, net	4,314		4,206
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	3,288		2,547
Investment in subsidiaries	2,062,096		2,019,700
Total assets	2,073,393		2,027,789
Liabilities and Partners' Capital
Trade accounts payable	141		209
Accrued expenses	803		928
Due to affiliate	374,074		263,935
Deferred revenue	0		0
Ad valorem taxes payable	9		0
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	3,477		6,395
Total current liabilities	378,504		271,467
Long-term debt	0		0
Deferred Purchase Price Obligation	453,922		362,959
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	4,159		3,694
Total liabilities	836,585		638,120
Total partners' capital	1,236,808		1,389,669
Total liabilities and partners' capital	2,073,393		2,027,789
Co-Issuers \| Reportable Legal Entities
Assets
Cash and cash equivalents	92		75	79	51
Accounts receivable	0		0
Other current assets	0		0
Due from affiliate	0		0
Total current assets	92		75
Property, plant and equipment, net	0		0
Intangible assets, net	0		0
Goodwill	0		0
Investment in equity method investees	0		0
Other noncurrent assets	9,762		10,913
Investment in subsidiaries	3,396,771		3,324,464
Total assets	3,406,625		3,335,452
Liabilities and Partners' Capital
Trade accounts payable	0		0
Accrued expenses	0		0
Due to affiliate	185,265		252,250
Deferred revenue	0		0
Ad valorem taxes payable	0		0
Accrued interest	12,259		12,310
Accrued environmental remediation	0		0
Other current liabilities	0		0
Total current liabilities	197,524		264,560
Long-term debt	1,147,005		1,051,192
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	0		0
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	0		0
Total liabilities	1,344,529		1,315,752
Total partners' capital	2,062,096		2,019,700
Total liabilities and partners' capital	3,406,625		3,335,452
Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	4,998		1,138	2,064	5,768
Accounts receivable	68,274		65,842
Other current assets	1,273		2,697
Due from affiliate	507,396		493,067
Total current assets	581,941		562,744
Property, plant and equipment, net	1,537,468		1,442,333
Intangible assets, net	263,416		278,958
Goodwill	16,211		16,211
Investment in equity method investees	0		0
Other noncurrent assets	94		105
Investment in subsidiaries	0		0
Total assets	2,399,130		2,300,351
Liabilities and Partners' Capital
Trade accounts payable	18,280		11,283
Accrued expenses	17,030		10,592
Due to affiliate	0		0
Deferred revenue	10,341		4,000
Ad valorem taxes payable	6,224		7,809
Accrued interest	0		0
Accrued environmental remediation	0		0
Other current liabilities	3,573		4,385
Total current liabilities	55,448		38,069
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	36,416		12,707
Noncurrent accrued environmental remediation	0		0
Other noncurrent liabilities	2,197		3,293
Total liabilities	94,061		54,069
Total partners' capital	2,305,069		2,246,282
Total liabilities and partners' capital	2,399,130		2,300,351
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	506		91	$ 217	$ 911
Accounts receivable	11,870		6,437
Other current assets	264		442
Due from affiliate	51,852		22,030
Total current assets	64,492		29,000
Property, plant and equipment, net	345,977		348,590
Intangible assets, net	23,249		22,387
Goodwill	0		0
Investment in equity method investees	670,827		690,485
Other noncurrent assets	0		0
Investment in subsidiaries	0		0
Total assets	1,104,545		1,090,462
Liabilities and Partners' Capital
Trade accounts payable	3,277		4,883
Accrued expenses	1,600		979
Due to affiliate	0		0
Deferred revenue	437		0
Ad valorem taxes payable	271		520
Accrued interest	0		0
Accrued environmental remediation	3,522		3,130
Other current liabilities	362		478
Total current liabilities	9,469		9,990
Long-term debt	0		0
Deferred Purchase Price Obligation	0		0
Noncurrent deferred revenue	1,703		0
Noncurrent accrued environmental remediation	1,617		2,214
Other noncurrent liabilities	54		76
Total liabilities	12,843		12,280
Total partners' capital	1,091,702		1,078,182
Total liabilities and partners' capital	$ 1,104,545		$ 1,090,462

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 24,466	$ 24,016	$ 49,070	$ 47,708
General and administrative	13,484	12,949	27,926	27,081
Depreciation and amortization	26,784	28,688	53,461	57,257
Transaction costs	0	119	0	119
Loss (gain) on asset sales, net	62	67	(12)	70
Long-lived asset impairment	587	3	587	287
Total costs and expenses	89,767	74,941	175,702	150,673
Other income	27	64	20	135
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt	0	0	0	(22,020)
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(45,699)	14,420	(51,101)	14,945
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	0	0	0	0
Transaction costs		0		0
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	0	0	0	0
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	(55,043)	(29,925)	(106,327)	(110,331)
Net (loss) income	(55,043)	(29,925)	(106,327)	(110,331)
Gathering Services and Related Fees
Revenues:
Total revenues	89,585	84,801	173,946	202,814
Gathering Services and Related Fees \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Natural Gas, NGLs and Condensate Sales \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Other Revenues
Revenues:
Total revenues	6,707	6,396	13,549	13,068
Other Revenues \| Consolidating adjustments
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	444	154	876	308
Transaction costs		119		119
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	444	273	876	427
Other income	27	64	20	135
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	(69,305)	5,058	(90,963)	(15,825)
(Loss) income before income taxes and loss from equity method investees	(69,722)	4,849	(91,819)	(16,117)
Income tax (expense) benefit	(294)	211	(123)	(241)
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	20,103	6,186	38,184	27,021
Net (loss) income	(49,913)	11,246	(53,758)	10,663
Summit Midstream Partners, LP \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Summit Midstream Partners, LP \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 0	$ 0	$ 0	$ 0
General and administrative	0	0	0	0
Depreciation and amortization	0	0	0	0
Transaction costs		0		0
Loss (gain) on asset sales, net	0	0	0	0
Long-lived asset impairment	0	0	0	0
Total costs and expenses	0	0	0	0
Other income	0	0	0	0
Interest expense	(14,837)	(17,553)	(29,959)	(34,269)
Early extinguishment of debt				(22,020)
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	(14,837)	(17,553)	(29,959)	(56,289)
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	34,940	23,739	68,143	83,310
Net (loss) income	20,103	6,186	38,184	27,021
Co-Issuers \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Co-Issuers \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Total revenues	112,291	84,606	214,601	204,839
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 22,221	$ 22,169	$ 44,374	$ 44,189
General and administrative	11,907	10,886	24,244	23,371
Depreciation and amortization	22,776	25,282	45,637	50,524
Transaction costs		0		0
Loss (gain) on asset sales, net	(1)	67	(75)	70
Long-lived asset impairment	587	2	587	2
Total costs and expenses	81,874	67,505	159,437	136,307
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	30,417	17,101	55,164	68,532
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	0	0	0	0
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	30,417	17,101	55,164	68,532
Guarantor Subsidiaries \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	73,891	67,679	143,357	170,192
Guarantor Subsidiaries \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	31,891	10,595	58,008	21,715
Guarantor Subsidiaries \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	6,509	6,332	13,236	12,932
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Total revenues	15,892	17,186	30,902	32,758
Costs and expenses:
Cost of natural gas and NGLs	$ 0	$ 0	$ 0	$ 0
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 2,245	$ 1,847	$ 4,696	$ 3,519
General and administrative	1,577	2,063	3,682	3,710
Depreciation and amortization	3,564	3,252	6,948	6,425
Transaction costs		0		0
Loss (gain) on asset sales, net	63	0	63	0
Long-lived asset impairment	0	1	0	285
Total costs and expenses	7,449	7,163	15,389	13,939
Other income	0	0	0	0
Interest expense	0	0	0	0
Early extinguishment of debt				0
Deferred Purchase Price Obligation	0	0	0	0
(Loss) income before income taxes and loss from equity method investees	8,443	10,023	15,513	18,819
Income tax (expense) benefit	0	0	0	0
Loss from equity method investees	(3,920)	(3,385)	(2,534)	(4,041)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Net (loss) income	4,523	6,638	12,979	14,778
Non-Guarantor Subsidiaries \| Gathering Services and Related Fees \| Reportable Legal Entities
Revenues:
Total revenues	15,694	17,122	30,589	32,622
Non-Guarantor Subsidiaries \| Natural Gas, NGLs and Condensate Sales \| Reportable Legal Entities
Revenues:
Total revenues	0	0	0	0
Non-Guarantor Subsidiaries \| Other Revenues \| Reportable Legal Entities
Revenues:
Total revenues	$ 198	$ 64	$ 313	$ 136

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Cash Flow Statement (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net cash (used in) provided by operating activities	$ 110,049	$ 121,341
Cash flows from investing activities:
Capital expenditures	(90,394)	(45,912)
Contributions to equity method investees	0	(15,649)
Proceeds from asset sale	496	0
Other, net	(306)	(521)
Advances to affiliates	0	0
Net cash provided by (used in) investing activities	(90,204)	(62,082)
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes	0	(17,913)
Proceeds from ATM Program common unit issuances, net of costs	0	17,259
Contribution from General Partner	0	465
Issuance of senior notes	0	500,000
Tender and redemption of senior notes	0	(300,000)
Other, net	(3,423)	(2,268)
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	(13,063)	(64,099)
Net change in cash and cash equivalents	6,782	(4,840)
Cash and cash equivalents, beginning of period	1,430	7,428
Cash and cash equivalents, end of period	8,212	2,588
Consolidating adjustments
Cash flows from operating activities:
Net cash (used in) provided by operating activities	0	0
Cash flows from investing activities:
Capital expenditures	0	0
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	111,134	64,706
Net cash provided by (used in) investing activities	111,134	64,706
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	0	0
Advances from affiliates	(111,134)	(64,706)
Net cash provided by (used in) financing activities	(111,134)	(64,706)
Net change in cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0	0
Common units
Cash flows from financing activities:
Distributions to unitholders	(90,269)	(89,029)
Common units \| Consolidating adjustments
Cash flows from financing activities:
Distributions to unitholders	0
Series A Preferred Units
Cash flows from financing activities:
Distributions to unitholders	(14,250)	0
Series A Preferred Units \| Consolidating adjustments
Cash flows from financing activities:
Distributions to unitholders	0
Summit Midstream Partners, LP \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(68)	424
Cash flows from investing activities:
Capital expenditures	(972)	(6,412)
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	(306)	(521)
Advances to affiliates	0	7,635
Net cash provided by (used in) investing activities	(1,278)	702
Cash flows from financing activities:
Distributions to unitholders		(89,029)
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		17,259
Contribution from General Partner		465
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(2,779)	(2,632)
Advances from affiliates	111,134	72,341
Net cash provided by (used in) financing activities	3,836	(1,596)
Net change in cash and cash equivalents	2,490	(470)
Cash and cash equivalents, beginning of period	126	698
Cash and cash equivalents, end of period	2,616	228
Summit Midstream Partners, LP \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	(90,269)
Summit Midstream Partners, LP \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	(14,250)
Co-Issuers \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	(27,877)	(32,466)
Cash flows from investing activities:
Capital expenditures	0	0
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	(66,985)	23,020
Net cash provided by (used in) investing activities	(66,985)	23,020
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	148,000	112,500
Repayments under Revolving Credit Facility	(53,000)	(269,500)
Debt issuance costs	(121)	(15,613)
Payment of redemption and call premiums on senior notes		(17,913)
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		500,000
Tender and redemption of senior notes		(300,000)
Other, net	0	0
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	94,879	9,474
Net change in cash and cash equivalents	17	28
Cash and cash equivalents, beginning of period	75	51
Cash and cash equivalents, end of period	92	79
Co-Issuers \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Co-Issuers \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Guarantor Subsidiaries \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	103,965	114,863
Cash flows from investing activities:
Capital expenditures	(85,187)	(23,578)
Contributions to equity method investees		0
Proceeds from asset sale	0
Other, net	0	0
Advances to affiliates	(14,329)	(95,361)
Net cash provided by (used in) investing activities	(99,516)	(118,939)
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(589)	372
Advances from affiliates	0	0
Net cash provided by (used in) financing activities	(589)	372
Net change in cash and cash equivalents	3,860	(3,704)
Cash and cash equivalents, beginning of period	1,138	5,768
Cash and cash equivalents, end of period	4,998	2,064
Guarantor Subsidiaries \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Guarantor Subsidiaries \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Cash flows from operating activities:
Net cash (used in) provided by operating activities	34,029	38,520
Cash flows from investing activities:
Capital expenditures	(4,235)	(15,922)
Contributions to equity method investees		(15,649)
Proceeds from asset sale	496
Other, net	0	0
Advances to affiliates	(29,820)	0
Net cash provided by (used in) investing activities	(33,559)	(31,571)
Cash flows from financing activities:
Distributions to unitholders		0
Borrowings under Revolving Credit Facility	0	0
Repayments under Revolving Credit Facility	0	0
Debt issuance costs	0	0
Payment of redemption and call premiums on senior notes		0
Proceeds from ATM Program common unit issuances, net of costs		0
Contribution from General Partner		0
Issuance of senior notes		0
Tender and redemption of senior notes		0
Other, net	(55)	(8)
Advances from affiliates	0	(7,635)
Net cash provided by (used in) financing activities	(55)	(7,643)
Net change in cash and cash equivalents	415	(694)
Cash and cash equivalents, beginning of period	91	911
Cash and cash equivalents, end of period	506	$ 217
Non-Guarantor Subsidiaries \| Common units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	0
Non-Guarantor Subsidiaries \| Series A Preferred Units \| Reportable Legal Entities
Cash flows from financing activities:
Distributions to unitholders	$ 0

REVENUE - Schedule of Estimated Revenue Expected to be Recognized and MVC Shortfall Payments (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	21 years
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 97,523
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	133,743
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	122,429
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	102,777
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	83,648
Gathering Services and Related Fees \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 174,227