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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 16, 2018	Jun. 30, 2017
Entity Information [Line Items]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	0001549922
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 928,653,216
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	SMLP
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Common units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		73,085,996
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,490,999

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 1,430	$ 7,428
Accounts receivable	72,301	97,364
Other current assets	4,327	4,309
Total current assets	78,058	109,101
Property, plant and equipment, net	1,795,129	1,853,671
Intangible assets, net	301,345	421,452
Goodwill	16,211	16,211
Investment in equity method investees	690,485	707,415
Other noncurrent assets	13,565	7,329
Total assets	2,894,793	3,115,179
Current liabilities:
Trade accounts payable	16,375	16,251
Accrued expenses	12,499	11,389
Due to affiliate	1,088	258
Deferred revenue	4,000	0
Ad valorem taxes payable	8,329	10,588
Accrued interest	12,310	17,483
Accrued environmental remediation	3,130	4,301
Other current liabilities	11,258	11,471
Total current liabilities	68,989	71,741
Long-term debt	1,051,192	1,240,301
Deferred Purchase Price Obligation	362,959	563,281
Deferred revenue	12,707	57,465
Noncurrent accrued environmental remediation	2,214	5,152
Other noncurrent liabilities	7,063	7,566
Total liabilities	1,505,124	1,945,506
Commitments and contingencies (Note 15)
Series A Preferred Units (300 units issued and outstanding at December 31, 2017)	294,426
Common limited partner capital (73,086 units issued and outstanding at December 31, 2017 and 72,111 units issued and outstanding at December 31, 2016)	1,056,510	1,129,132
General Partner interests (1,491 units issued and outstanding at December 31, 2017 and 1,471 units issued and outstanding at December 31, 2016)	27,920	29,294
Noncontrolling interest	10,813	11,247
Total partners' capital	1,389,669	1,169,673
Total liabilities and partners' capital	$ 2,894,793	$ 3,115,179

CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares shares in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Series A preferred unitholders, issued	300
Series A preferred unitholders, outstanding	300
Common limited partner capital (in shares), issued	73,086	72,111
Common limited partner capital (in shares), outstanding	73,086	72,111
General partner interests (in shares), issued	1,491	1,471
General partner interests (in shares), outstanding	1,491	1,471

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Gathering services and related fees	$ 394,427,000	$ 345,961,000	$ 337,819,000
Natural gas, NGLs and condensate sales	68,459,000	35,833,000	42,079,000
Other revenues	25,855,000	20,568,000	20,659,000
Total revenues	488,741,000	402,362,000	400,557,000
Costs and expenses:
Cost of natural gas and NGLs	57,237,000	27,421,000	31,398,000
Operation and maintenance	93,882,000	95,334,000	94,986,000
General and administrative	54,681,000	52,410,000	45,108,000
Depreciation and amortization	115,475,000	112,239,000	105,117,000
Transaction costs	73,000	1,321,000	1,342,000
Environmental remediation	0	0	21,800,000
Loss (gain) on asset sales, net	527,000	93,000	(172,000)
Long-lived asset impairment	188,702,000	1,764,000	9,305,000
Goodwill impairment	0	0	248,851,000
Total costs and expenses	510,577,000	290,582,000	557,735,000
Other income	298,000	116,000	2,000
Interest expense	(68,131,000)	(63,810,000)	(59,092,000)
Early extinguishment of debt	(22,039,000)	0	0
Deferred Purchase Price Obligation	200,322,000	(55,854,000)	0
Income (loss) before income taxes and loss from equity method investees	88,614,000	(7,768,000)	(216,268,000)
Income tax (expense) benefit	(341,000)	(75,000)	603,000
Loss from equity method investees	(2,223,000)	(30,344,000)	(6,563,000)
Net income (loss)	86,050,000	(38,187,000)	(222,228,000)
Net income (loss) attributable to Summit Investments	0	2,745,000	(30,016,000)
Net income (loss) attributable to noncontrolling interest	363,000	(14,000)	0
Net income (loss) attributable to SMLP	85,687,000	(40,918,000)	(192,212,000)
Less net income (loss) attributable to General Partner, including IDRs	10,202,000	7,261,000	3,398,000
Net income (loss) attributable to limited partners	75,485,000	(48,179,000)	(195,610,000)
Common units
Costs and expenses:
Net income (loss) attributable to limited partners	$ 71,922,000	$ (48,179,000)	$ (195,610,000)
Earnings (loss) per limited partner unit:
Basic (in dollars per share)	$ 0.99	$ (0.71)	$ (3.20)
Diluted (in dollars per share)	$ 0.98	$ (0.71)	$ (3.20)
Weighted-average limited partner units outstanding:
Basic (shares)	72,705	68,264	39,217
Diluted (shares)	73,047	68,264	39,217
Series A Preferred Units
Costs and expenses:
Net income (loss) attributable to limited partners	$ 3,563,000	$ 0	$ 0
Subordinated Units
Costs and expenses:
Net income (loss) attributable to limited partners	$ 0	$ 0	$ (70,173,000)
Earnings (loss) per limited partner unit:
Basic (in dollars per share)	$ 0	$ 0	$ (2.88)
Diluted (in dollars per share)	$ 0	$ 0	$ (2.88)
Weighted-average limited partner units outstanding:
Basic (shares)	0	0	24,410
Diluted (shares)	0	0	24,410

CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) $ in Thousands	Total	Class B membership interest	Noncontrolling interest	Noncontrolling interest Class B membership interest	Summit Investments' equity in contributed subsidiaries	Summit Investments' equity in contributed subsidiaries Class B membership interest	Common	Common Class B membership interest	Limited partners, Subordinated	Limited partners, Subordinated Class B membership interest	Limited partners, Series A Preferred Units	General Partner	General Partner Class B membership interest	At Market Program	At Market Program Noncontrolling interest	At Market Program Common	At Market Program Limited partners, Series A Preferred Units	At Market Program General Partner
Beginning balance at Dec. 31, 2014	$ 1,830,678				$ 863,789		$ 649,060		$ 293,153			$ 24,676
Net (loss) income	(222,228)				(30,016)		(123,817)		(71,793)			3,398
Distributions to unitholders	(152,074)						(86,880)		(55,410)			(9,784)
Unit-based compensation	6,174	$ 843				$ 843	6,174
Tax withholdings on vested SMLP LTIP awards	(1,616)						(1,616)
Issuance of common/preferred units, net of offering costs	221,977						221,977
Contribution from General Partner	4,737											4,737
Purchase of contributed Subsidiaries	(285,677)				(285,677)
Excess of acquired carrying value over consideration paid					(130,367)		80,079		47,681			2,607
Cash advance from Summit Investments to contributed subsidiaries, net	320,527				320,527
Expenses paid by Summit Investments on behalf of contributed subsidiaries	22,879				22,879
Capitalized interest allocated to contributed subsidiaries from Summit Investments	1,079				1,079
Ending balance at Dec. 31, 2015	1,747,299		$ 0		763,057		744,977		213,631			25,634
Net (loss) income	(38,187)		(14)		2,745		(49,219)		1,040			7,261
Distributions to unitholders	(167,504)		0		0		(142,214)		(14,034)			(11,256)
Unit-based compensation	7,550	$ 435	0	$ 0	0	$ 130	7,550	$ 305	0	$ 0		0	$ 0
Tax withholdings on vested SMLP LTIP awards	(1,181)		0		0		(1,181)		0			0
Issuance of common/preferred units, net of offering costs	125,233		0		0		125,233		0			0
Contribution from General Partner	2,702		0		0		0		0			2,702
Subordinated units conversion	0		0		0		200,637		(200,637)			0
Purchase of contributed Subsidiaries	(866,858)		0		(866,858)		0		0			0
Establishment of noncontrolling interest	0		11,261		(11,261)		0		0			0
Distribution of debt related to Carve-Out Financial Statements of Summit Investments	342,926		0		342,926		0		0			0
Excess of acquired carrying value over consideration paid	0		0		(247,997)		243,044		0			4,953
Cash advance from Summit Investments to contributed subsidiaries, net	12,214		0		12,214		0		0			0
Expenses paid by Summit Investments on behalf of contributed subsidiaries	4,821		0		4,821		0		0			0
Capitalized interest allocated to contributed subsidiaries from Summit Investments	223		0		223		0		0			0
Ending balance at Dec. 31, 2016	1,169,673		11,247		$ 0		1,129,132		$ 0		$ 0	29,294
Net (loss) income	86,050		363				71,922				3,563	10,202
Distributions to unitholders	(181,478)		0				(167,062)				(2,375)	(12,041)
Unit-based compensation	7,878		0				7,878				0	0
Tax withholdings on vested SMLP LTIP awards	(2,236)		0				(2,236)				0	0
Issuance of common/preferred units, net of offering costs	293,238		0				0				293,238	0		$ 17,078	$ 0	$ 17,078	$ 0	$ 0
Contribution from General Partner	465		0				0				0	465
Purchase of noncontrolling interest	(797)		(797)				0				0	0
Other	(202)		0				(202)				0	0
Ending balance at Dec. 31, 2017	$ 1,389,669		$ 10,813				$ 1,056,510				$ 294,426	$ 27,920

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net (loss) income	$ 86,050,000	$ (38,187,000)	$ (222,228,000)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	114,872,000	112,661,000	105,903,000
Amortization of debt issuance costs	4,158,000	3,976,000	4,309,000
Deferred Purchase Price Obligation	(200,322,000)	55,854,000	0
Unit-based and noncash compensation	7,951,000	7,985,000	7,017,000
Loss from equity method investees	2,223,000	30,344,000	6,563,000
Distributions from equity method investees	40,220,000	44,991,000	34,641,000
Loss (gain) on asset sales, net	527,000	93,000	(172,000)
Long-lived asset impairment	188,702,000	1,764,000	9,305,000
Goodwill impairment	0	0	248,851,000
Early extinguishment of debt	22,039,000	0	0
Write-off of debt issuance costs	302,000	0	727,000
Changes in operating assets and liabilities:
Accounts receivable	25,063,000	(7,783,000)	3,328,000
Insurance receivable	0	0	25,000,000
Trade accounts payable	(3,246,000)	2,001,000	(1,450,000)
Accrued expenses	1,110,000	4,613,000	(1,967,000)
Due from (to) affiliate	830,000	(891,000)	1,377,000
Deferred revenue, net	(40,758,000)	11,302,000	(11,453,000)
Ad valorem taxes payable	(2,259,000)	317,000	1,092,000
Accrued interest	(5,173,000)	0	(1,375,000)
Accrued environmental remediation, net	(4,109,000)	(4,211,000)	(16,336,000)
Other, net	(348,000)	5,666,000	(1,757,000)
Net cash provided by operating activities	237,832,000	230,495,000	191,375,000
Cash flows from investing activities:
Capital expenditures	(124,215,000)	(142,719,000)	(272,225,000)
Proceeds from asset sale	2,300,000	0	0
Contributions to equity method investees	(25,513,000)	(31,582,000)	(86,200,000)
Acquisitions of gathering systems from affiliate, net of acquired cash	0	(359,431,000)	(288,618,000)
Purchase of noncontrolling interest	(797,000)	0	0
Other, net	(458,000)	(394,000)	323,000
Net cash provided by (used in) investing activities	(148,683,000)	(534,126,000)	(646,720,000)
Cash flows from financing activities:
Distributions to unitholders	(181,478,000)	(167,504,000)	(152,074,000)
Borrowings under Revolving Credit Facility	247,500,000	520,300,000	367,000,000
Repayments under Revolving Credit Facility	(634,500,000)	(204,300,000)	(151,000,000)
Repayments under term loan	0	0	(182,500,000)
Debt issuance costs	(16,390,000)	(3,032,000)	(412,000)
Payment of redemption and call premiums on senior notes	(17,932,000)	0	0
Proceeds from ATM Program common unit issuances, net of costs	17,078,000	0	0
Proceeds from underwritten issuance of common units, net of costs	0	125,233,000	221,977,000
Proceeds from issuance of Series A Preferred Units, net of costs	293,238,000	0	0
Contribution from General Partner	465,000	2,702,000	4,737,000
Cash advance from Summit Investments to contributed subsidiaries, net	0	12,214,000	320,527,000
Expenses paid by Summit Investments on behalf of contributed subsidiaries	0	4,821,000	22,879,000
Issuance of senior notes	500,000,000	0	0
Tender and redemption of senior notes	(300,000,000)	0	0
Other, net	(3,128,000)	(1,168,000)	(1,807,000)
Net cash (used in) provided by financing activities	(95,147,000)	289,266,000	449,327,000
Net change in cash and cash equivalents	(5,998,000)	(14,365,000)	(6,018,000)
Cash and cash equivalents, beginning of period	7,428,000	21,793,000	27,811,000
Cash and cash equivalents, end of period	1,430,000	7,428,000	21,793,000
Supplemental cash flow disclosures:
Cash interest paid	71,488,000	63,000,000	59,302,000
Less capitalized interest	2,579,000	3,709,000	3,372,000
Interest paid (net of capitalized interest)	68,909,000	59,291,000	55,930,000
Cash paid for taxes	0	0	0
Noncash investing and financing activities
Capital expenditures in trade accounts payable (period-end accruals)	11,792,000	8,422,000	34,977,000
Issuance of Deferred Purchase Price Obligation to affiliate to partially fund the 2016 Drop Down	0	507,427,000	0
Excess of acquired carrying value over consideration paid and recognized for 2016 Drop Down Assets	0	247,997,000	0
Distribution of debt related to Carve-Out Financial Statements of Summit Investments	0	342,926,000	0
Excess of acquired carrying value over consideration paid for Polar and Divide	0	0	130,367,000
Capitalized interest allocated to contributed subsidiaries from Summit Investments	$ 0	$ 223,000	$ 1,079,000

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION

12 Months Ended

Dec. 31, 2017

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 in respect of SMLP), Summit Investments has indirect ownership interests in our common units. As of December 31, 2017, 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.

In May 2015, the Partnership acquired all of the membership interests of Polar Midstream and Epping from a subsidiary of Summit Investments (the "Polar and Divide Drop Down"). As such, the Polar and Divide Drop Down was determined to be a transaction among entities under common control. Polar Midstream's net assets were carved out of Meadowlark Midstream immediately prior to the Polar and Divide Drop Down. Concurrent with the closing of the Polar and Divide Drop Down, Epping became a wholly owned subsidiary of Polar Midstream and SMLP contributed Polar Midstream (including Epping) to Bison Midstream. Common control began in (i) February 2013 for Polar Midstream and (ii) April 2014 for Epping.

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 and collectively with Summit Utica and Meadowlark Midstream, (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.

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 under development in the northern Delaware Basin in southeastern New Mexico.

Summit Marketing (formerly known as Summit Midstream OpCo GP, LLC), 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 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.

The consolidated financial statements include the assets, liabilities and results of operations of SMLP and its subsidiaries. All intercompany transactions among the consolidated entities have been eliminated in consolidation. Comprehensive income or loss is the same as net income or loss for all periods presented. The financial position, results of operations and cash flows of acquired drop down assets, liabilities, expenses or entities that were carved out of entities held by Summit Investments and included herein have been derived from the accounting records of the respective Summit Investments' subsidiary on a carve-out basis.

SMLP recognized its drop down acquisitions at Summit Investments' historical cost because the acquisitions were executed by entities under common control. The excess of Summit Investments' net investment over the consideration paid and recognized for a contributed subsidiary is recognized as an addition to partners' capital, while the excess of purchase price paid and recognized over net investment is recognized as a reduction to partners' capital. Due to the common control aspect, we account for drop down transactions on an “as-if pooled” basis for the periods during which common control existed.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Cash and Cash Equivalents. We consider all highly liquid investments with an original maturity of three months or less to be cash equivalents.

Accounts Receivable. Accounts receivable relate to gathering and other services provided to our customers and other counterparties. We evaluate the collectability of accounts receivable and the need for an allowance for doubtful accounts based on customer-specific facts and circumstances. To the extent we doubt the collectability of a specific customer or counterparty receivable, we recognize an allowance for doubtful accounts. Uncollectible receivables are written off when a settlement is reached for an amount that is less than the outstanding historical balance or a receivable amount is deemed otherwise unrealizable.

Other Current Assets. Other current assets primarily consist of the current portion of prepaid expenses that are charged to expense over the period of benefit or the life of the related contract.

Property, Plant and Equipment. We record property, plant and equipment at historical cost of construction or fair value of the assets at acquisition. We capitalize expenditures that extend the useful life of an asset or enhance its productivity or efficiency from its original design over the expected remaining period of use. For maintenance and repairs that do not add capacity or extend the useful life of an asset, we recognize expenditures as an expense as incurred. We capitalize project costs incurred during construction, including interest on funds borrowed to finance the construction of facilities, as construction in progress. To the extent that Summit Investments incurred interest expense related to capital projects of assets that have been acquired by the Partnership, the associated interest expense is allocated to the drop down assets as a noncash equity contribution and capitalized into the basis of the asset.

We record depreciation on a straight-line basis over an asset’s estimated useful life. We base our estimates for useful life on various factors including age (in the case of acquired assets), manufacturing specifications, technological advances and historical data concerning useful lives of similar assets. Estimates of useful lives follow.

Useful lives

(In years)

Gathering and processing systems and related equipment

12-30

Other

4-15

Construction in progress is depreciated consistent with its applicable asset class once it is placed in service. Land and line fill are not depreciated.

We base an asset’s carrying value on estimates, assumptions and judgments for useful life and salvage value. Upon sale, retirement or other disposal, we remove the carrying value of an asset and its accumulated depreciation from our balance sheet and recognize the related gain or loss, if any.

Accrued capital expenditures are reflected in trade accounts payable.

Asset Retirement Obligations. We record a liability for asset retirement obligations only if and when a future asset retirement obligation with a determinable life is identified. For identified asset retirement obligations, we then evaluate whether the expected date and related costs of retirement can be estimated. We have concluded that our gathering and processing assets have an indeterminate life because they are owned and will operate for an indeterminate period when properly maintained. Because we did not have sufficient information to reasonably estimate the amount or timing of such obligations and we have no current plan to discontinue use of any significant assets, we did not provide for any asset retirement obligations as of December 31, 2017 or 2016.

Amortizing Intangibles. Upon the acquisition of DFW Midstream, certain of its gas gathering contracts were deemed to have above-market pricing structures while another was deemed to have pricing that was below market. We have recognized the above-market contracts as favorable gas gathering contracts. We have recognized the below-market contract as the unfavorable gas gathering contract and included it in other noncurrent liabilities. We amortize these contracts using a straight-line method over the contract’s estimated useful life. We define useful life as the period over which the contract is expected to contribute to our future cash flows. These contracts have original terms ranging from 10 years to 20 years. We recognize the amortization expense associated with these contracts in other revenues.

We amortize all other gas gathering contracts, or contract intangibles, over the period of economic benefit based upon expected revenues over the life of the contract. The useful life of these contracts ranges from 3 years to 25 years. We recognize the amortization expense associated with these contracts in depreciation and amortization expense.

We have rights-of-way associated with city easements and easements granted within existing rights-of-way. We amortize these intangible assets over the shorter of the contractual term of the rights-of-way or the estimated useful life of the gathering system. The contractual terms of the rights-of-way range from 20 years to 30 years. We recognize the amortization expense associated with rights-of-way assets in depreciation and amortization expense.

Goodwill. Goodwill represents consideration paid in excess of the fair value of the net identifiable assets acquired in a business combination. 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 amount.

We test goodwill for impairment using a quantitative test. We compare the fair value of the reporting unit to its carrying value, including goodwill. To estimate the fair value of the reporting units, we utilize two valuation methodologies: the market approach and the income approach. Both of these approaches incorporate significant estimates and assumptions to calculate enterprise fair value for a reporting unit. The most significant estimates and assumptions inherent within these two valuation methodologies are: (i) determination of the weighted-average cost of capital; (ii) the selection of guideline public companies; (iii) market multiples; (iv) weighting of the income and market approaches; (v) growth rates; (vi) commodity prices; and (vi) the expected levels of throughput volume gathered. Changes in these and other assumptions could materially affect the estimated amount of fair value for any of our reporting units.

If the reporting unit’s fair value exceeds its carrying amount, we conclude that the goodwill of the reporting unit has not been impaired and no further work is performed.

If we determine that the reporting unit’s carrying value exceeds its fair value, we recognize the excess of the carrying value over the fair value as an impairment loss.

Equity Method Investments. We account for investments in which we exercise significant influence using the equity method so long as we (i) do not control the investee and (ii) are not the primary beneficiary. We recognize these investments in investment in equity method investees in the accompanying consolidated balance sheets. We recognize our proportionate share of earnings or loss in net income on a one-month lag based on the financial information available to us during the reporting period.

We recognize an other-than-temporary impairment for losses in the value of equity method investees when evidence indicates that the carrying amount is no longer supportable. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the equity method investee to sustain an earnings capacity that would justify the carrying amount of the investment. A current fair value of an investment that is less than its carrying amount may indicate a loss in value of the investment. We evaluate our equity method investments whenever evidence exists that would indicate a need to assess the investment for potential impairment.

Other Noncurrent Assets. Other noncurrent assets primarily consist of external costs incurred in connection with the closing of our Revolving Credit Facility and related amendments. We capitalize and then amortize these debt issuance costs on a straight-line basis, which approximates the effect of the effective interest rate method, over the life of the respective debt instrument. We recognize amortization of Revolving Credit Facility debt issuance costs in interest expense.

Debt Issuance Costs. Debt issuance costs, other than those associated with our Revolving Credit Facility, are reflected in the carrying value of the Senior Notes as an adjustment to the principal amount and amortized on a straight-line basis, which approximates the effect of the effective interest rate method, over the life of the respective debt instrument. We recognize Senior Notes debt issuance costs in interest expense.

Deferred Purchase Price Obligation. We recognize a liability for the Deferred Purchase Price Obligation to reflect the expected value of the Remaining Consideration to be paid in 2020 for the acquisition of the 2016 Drop Down Assets. We estimate Remaining Consideration by summing the calculations of (i) actual capital expenditures incurred and Business Adjusted EBITDA (as defined later) recognized from the 2016 Drop Down Assets during the period since closing the 2016 Drop Down to the current balance sheet date and (ii) estimates of projected capital expenditures and Business Adjusted EBITDA related to the 2016 Drop Down Assets for periods subsequent to the respective balance sheet date until December 31, 2019. We discount the Remaining Consideration using a commensurate risk-adjusted discount rate and recognize the change in present value of the Remaining Consideration in earnings in the period of change. Our recognition of the change in present value of the Remaining Consideration in the consolidated statements of operations represents the change in present value, which comprises a time value of money concept, as well as (i) actual results from the 2016 Drop Down Assets and (ii) adjustments to projections and the expected value of the Remaining Consideration (see Note 16).

Impairment of Long-Lived Assets. We test assets for impairment when events or circumstances indicate that the carrying value of a long-lived asset may not be recoverable. The carrying value of a long-lived asset (except goodwill) is not recoverable if it exceeds the sum of the undiscounted cash flows expected to result from its use and eventual disposition. If we conclude that an asset's carrying value will not be recovered through future cash flows, we recognize an impairment loss on the long-lived asset equal to the amount by which the carrying value exceeds its fair value. We determine fair value using either a market-based approach or an income-based approach. We discuss our policy for goodwill impairment above.

Derivative Contracts. We have commodity price exposure related to our sale of the physical natural gas we retain from certain DFW Midstream system customers and our procurement of electricity to operate the DFW Midstream system's electric-drive compression assets. Our gas gathering agreements with certain DFW Midstream customers permit us to retain a certain quantity of natural gas that we gather to offset the power costs we incur to operate these electric-drive compression assets. We manage this direct exposure to natural gas and power prices through the use of forward power purchase contracts with wholesale power providers that require us to purchase a fixed quantity of power at a fixed heat rate based on prevailing natural gas prices based on the Waha Hub Index. Because we sell our retainage gas from these customers at prices that are based on the Waha Hub Index, we have effectively fixed the relationship between a portion of our compression electricity expense and natural gas retainage sales.

Accounting standards related to derivative instruments and hedging activities allow for normal purchase or sale elections and hedge accounting designations, which generally eliminate or defer the requirement for mark-to-market recognition in net income and thus reduce the volatility of net income that can result from fluctuations in fair values. We have designated these contracts as normal under the normal purchase and sale exception under the accounting standards for derivatives. We do not enter into risk management contracts for speculative purposes.

Fair Value of Financial Instruments. The fair-value-measurement standard under GAAP defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The standard characterizes inputs used in determining fair value according to a hierarchy that prioritizes those inputs based upon the degree to which the inputs are observable. The three levels of the fair value hierarchy are as follows:

•

Level 1. Inputs represent quoted prices in active markets for identical assets or liabilities;

•

Level 2. Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly (for example, quoted market prices for similar assets or liabilities in active markets or quoted market prices for identical assets or liabilities in markets not considered to be active, inputs other than quoted prices that are observable for the asset or liability, or market-corroborated inputs); and

•

Level 3. Inputs that are not observable from objective sources, such as management’s internally developed assumptions used in pricing an asset or liability (for example, an internally developed present value of future cash flows model that underlies management's fair value measurement).

Commitments and Contingencies. We record accruals for loss contingencies when we determine that it is probable that a liability has been incurred and that such economic loss can be reasonably estimated. Such determinations are subject to interpretations of current facts and circumstances, forecasts of future events and estimates of the financial impacts of such events. We recognize gain contingencies when their realization is assured beyond a reasonable doubt.

Noncontrolling Interest. Noncontrolling interest represents the ownership interests of third-party entities in the net assets of our consolidated subsidiaries. For financial reporting purposes, we consolidate OpCo and its wholly owned subsidiaries with our wholly owned subsidiaries and the 1% ownership interest in OpCo is reflected as noncontrolling interest in partners' capital. We reflect changes in our ownership of OpCo as adjustments to noncontrolling interest.

Revenue Recognition. We generate the majority of our revenue from the gathering, treating and processing services that we provide to our customers. We also generate revenue from our marketing of natural gas, NGLs and condensate. We realize revenues by receiving fees from our customers or by selling the residue natural gas, NGLs and condensate.

We recognize revenue earned from fee-based gathering, treating and processing services in gathering services and related fees revenue. We also earn revenue from the sale of physical natural gas purchased from our customers under percentage-of-proceeds arrangements. These revenues are recognized in natural gas, NGLs and condensate sales with corresponding expense recognition for the producer's share of the proceeds in cost of natural gas and NGLs. We sell substantially all of the 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 retainage 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.

We recognize revenue when all of the following criteria are met: (i) persuasive evidence of an exchange arrangement exists, (ii) delivery has occurred or services have been rendered, (iii) the price is fixed or determinable and (iv) collectability is reasonably assured.

We provide gathering and/or processing services principally under contracts that contain one or more of the following arrangements:

•

Fee-based arrangements. Under fee-based arrangements, we receive a fee or fees for one or more of the following services (i) natural gas gathering, treating and/or processing and (ii) crude oil and/or produced water gathering.

•

Percent-of-proceeds arrangements. Under percent-of-proceeds arrangements, we generally purchase natural gas from producers at the wellhead, or other receipt points, gather the wellhead natural gas through our gathering system, treat the natural gas, process the natural gas and/or sell the natural gas to a third party for processing. We then remit to our producers an agreed-upon percentage of the actual proceeds received from sales of the residue natural gas and NGLs. Certain of these arrangements may also result in returning all or a portion of the residue natural gas and/or the NGLs to the producer, in lieu of returning sales proceeds. The margins earned are directly related to the volume of natural gas that flows through the system and the price at which we are able to sell the residue natural gas and NGLs.

Certain of our gathering and processing agreements provide for a monthly, quarterly or annual MVC. 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 billings for obligations under their MVCs as revenue when the obligations are billable under the contract and the customer does not have the right to utilize shortfall payments to offset gathering or processing fees in excess of its MVCs in subsequent periods.

We record customer billings for obligations under their MVCs as deferred revenue when the customer has the right to utilize shortfall payments to offset gathering or processing fees in subsequent periods. We recognize deferred revenue under these arrangements in revenue once all contingencies or potential performance obligations associated with the related volumes have either (i) been satisfied through the gathering or processing of future excess volume throughput, or (ii) expired (or lapsed) through the passage of time pursuant to the terms of the applicable gathering or processing agreement. We also recognize deferred revenue in revenues when it is determined that a given amount of MVC shortfall payments cannot be recovered by offsetting gathering or processing fees in subsequent contracted measurement periods. In making this determination, we consider both quantitative and qualitative facts and circumstances, including, but not limited to, contract terms, capacity of the associated pipeline or receipt point and/or expectations regarding future investment, drilling and production.

We classify deferred revenue as a current liability for arrangements where the expiration of a customer's right to utilize shortfall payments is 12 months or less. We classify deferred revenue as noncurrent for arrangements where the expiration of the right to utilize shortfall payments and our estimate of its potential utilization is more than 12 months.

Unit-Based Compensation. For awards of unit-based compensation, we determine a grant date fair value and recognize the related compensation expense in the statements of operations over the vesting period of the respective awards.

Income Taxes. As a partnership, we are generally not subject to federal and state income taxes, except as noted below. However, our unitholders are individually responsible for paying federal and state income taxes on their share of our taxable income. Net income or loss for GAAP purposes may differ significantly from taxable income reportable to our unitholders as a result of differences between the tax basis and the GAAP basis of assets and liabilities and the taxable income allocation requirements under our Partnership Agreement.

In general, legal entities that are chartered, organized or conducting business in the state of Texas are subject to a franchise tax (the "Texas Margin Tax"). The Texas Margin Tax has the characteristics of an income tax because it is determined by applying a tax rate to a tax base that considers both revenues and expenses. Our financial statements reflect provisions for these tax obligations.

Earnings or Loss Per Unit. We determine basic EPU by dividing the net income or loss that is attributed, in accordance with the net income and loss allocation provisions of our Partnership Agreement, to common limited partners under the two-class method, after deducting (i) the 1% noncontrolling interest in OpCo (for periods subsequent to the 2016 Drop Down), (ii) any net income or loss of contributed subsidiaries that is attributable to Summit Investments, (iii) the General Partner's approximate 2% interest in net income or loss and (iv) any payment of IDRs, by the weighted-average number of limited partner units outstanding. Diluted EPU reflects the potential dilution that could occur if securities or other agreements to issue common units, such as unit-based compensation, were exercised, settled or converted into common units and included in the weighted-average number of units outstanding. When it is determined that potential common units resulting from an award subject to performance or market conditions should be included in the diluted EPU calculation, the impact is reflected by applying the treasury stock method.

Comprehensive Income or Loss. Comprehensive income or loss is the same as net income or loss for all periods presented.

Environmental Matters. We are subject to various federal, state and local laws and regulations relating to the protection of the environment. Liabilities for loss contingencies, including environmental remediation costs, arising from claims, assessments, litigation, fines and penalties and other sources are charged to expense when it is probable that a liability has been incurred and the amount of the assessment and/or remediation can be reasonably estimated. We accrue for losses associated with environmental remediation obligations when such losses are probable and reasonably estimable. Such accruals are adjusted as further information develops or circumstances change. Recoveries of environmental remediation costs from other parties or insurers are recorded as assets when their realization is assured beyond a reasonable doubt.

Carve-Out Entities, Assets, Liabilities and Expenses. For drop down transactions involving entities that were carved out of other entities, the majority of the assets and liabilities allocated to the carve-out entity are specifically identified based on the original entity's existing divisional organization. Goodwill is allocated to the carve-out entity based on initial purchase accounting estimates. Revenues and depreciation and amortization are specifically identified based on the relationship of the carve-out entity to the original entity's existing divisional structure. Operation and maintenance and general and administrative expenses are allocated to the carve-out entity based on volume throughput.

For drop down transactions involving assets, liabilities and expenses that were carved out of other entities, the majority of the assets and liabilities allocated to the carve-out are specifically identified based on the original entity's existing divisional organization. Depreciation and amortization are specifically identified based on the relationship of the carve-out entity to the original entity's existing divisional structure. General and administrative expenses are allocated to the carve-out entity based on an allocation of Summit Investments' consolidated expenses.

Allocation of Certain Liabilities in Drop Downs. For drop down transactions involving assets for which their development was funded with debt incurred by Summit Investments or a subsidiary thereof, which was allocated to but not ultimately assumed by the Partnership and later replaced with bank borrowings or debt capital at the Partnership, we allocate a portion of that debt, net of debt issuance costs, to the drop down assets during the common control period. Interest expense is allocated and recognized during the common control period. Any outstanding debt balance or principal is included in the calculation of the excess or deficit of acquired carrying value relative to consideration paid and recognized.

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. 2016-09 Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ("ASU 2016-09"). ASU 2016-09 simplifies several aspects for share-based payment award transactions, including income tax consequences, the liability or equity classification of awards and classification on the statements of cash flows. ASU 2016-09 is effective for public companies for fiscal years beginning after December 15, 2016. It does not specify a single transition approach, rather it specifies retrospective, modified retrospective and/or prospective transition approaches based on the aspect being applied. We adopted the provisions of ASU 2016-09 effective January 1, 2017. 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 December 31, 2017:

•

ASU No. 2014-09 Revenue from Contracts with Customers (Topic 606) ("ASU 2014-09"). Under ASU 2014-09, revenue will be recognized under a five-step model: (i) identify the contract with the customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to performance obligations; and (v) recognize revenue when (or as) the performance obligation is satisfied. ASU 2014-09 is effective for fiscal years and interim periods within those years, beginning after December 15, 2017 and allows for early adoption. We will adopt the provisions of ASU 2014-09 effective January 1, 2018 using the modified retrospective method.

We have completed the review of our existing contract portfolio in place as of December 31, 2017 under ASU 2014-09 and have quantified the January 1, 2018 impact of adoption. For contracts where we perform gathering services and earn a per-unit fee which is recognized at a point in time, revenue will be recognized over time as the service is performed and will result in revenue recognition materially consistent with current GAAP. In addition, our contracts generally contain forms of what will be considered 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 current GAAP. Furthermore, additional disclosures will be required to describe the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts.

Prior to adoption, contributions in aid of construction received were recognized as a reduction to our cost basis of property, plant and equipment. Upon adoption of the new guidance, the amounts previously received will be capitalized to property, plant and equipment, net of any accumulated depreciation, and depreciated over the remaining useful lives. The cumulative impact upon adoption for contributions in aid of construction is a net increase in property, plant and equipment of $34.8 million, a net increase in partners’ capital of $8.7 million and an increase in deferred revenue of $26.1 million. Going forward, any contributions in aid of construction will be recognized as revenue over the remaining term of the respective contract in accordance with ASU 2014-09. Additionally, the cumulative impact upon adoption for facility fees will be a decrease in partners’ capital of $16.2 million and an increase in deferred revenue of $16.2 million because amounts that were previously recognized as revenue when the cash was received will now 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. Under Topic 606, we have concluded that these contracts are not within the scope of Topic 606 and thus, such amounts which were previously presented gross in gathering services and related fees will now be presented net within cost of natural gas and NGLs. However, this change will not have any impact on our net income (loss), cash flows, or the amount we present as segment adjusted EBITDA.

For contracts where we previously deferred revenue under MVC arrangements with banking mechanisms, under ASU 2014-9, the recognition of revenue will be accelerated and the impact upon adoption is a decrease in deferred revenue and a corresponding increase in retained earnings of $16.7 million.

•

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. We are currently evaluating the impact of this guidance on lessor accounting but have made no determinations at this time. 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 expect to adopt its provisions effective January 1, 2019.

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ASU No. 2016-08 Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) ("ASU 2016-08"). ASU 2016-08 does not change the core principle of Topic 606, rather it clarifies the implementation guidance on principal versus agent considerations. We expect to adopt the provisions of ASU 2016-08 effective January 1, 2018. Our position regarding the impact of and transition method for this update is the same as for ASU 2014-09.

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ASU No. 2016-10 Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing ("ASU 2016-10"). ASU 2016-10 clarifies the following two aspects of Topic 606: (i) identifying performance obligations; and (ii) the licensing implementation guidance, while retaining the related principles for those areas. We expect to adopt the provisions of ASU 2016-10 effective January 1, 2018. Our position regarding the impact of and transition method for this update is the same as for ASU 2014-09.

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ASU No. 2016-12 Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients ("ASU 2016-12"). ASU 2016-12 does not change the core principle of the guidance in Topic 606. Rather, the amendments therein affect only the narrow aspects of Topic 606 including assessing the collectability criterion and issues related to contract modification at transition and completed contracts at transition. We expect to adopt the provisions of ASU 2016-12 effective January 1, 2018. Our position regarding the impact of and transition method for this update is the same as for ASU 2014-09.

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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. We plan to early adopt the provisions of ASU 2017-04 effective January 1, 2018. The adoption of this standard will have no impact on our consolidated financial statements.

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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 under Topic 842 existing or expired land easements that were not previously accounting for as leases under the current lease guidance in Topic 840. 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.

SEGMENT INFORMATION

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

SEGMENT INFORMATION

3. SEGMENT INFORMATION

We evaluate our business operations each reporting period to determine whether any of our gathering system operating segments in which we internally report financial information are considered significant and would require us to separately disclose certain segment financial information in our external reporting. As a result of our evaluation during the second quarter of 2017, we determined that both the Summit Utica natural gas gathering system and the Ohio Gathering natural gas gathering system, each previously reported within the Utica Shale reportable segment, were and are expected to continue to be individually significant operating segments. As such, we modified our current segments in the second quarter of 2017 such that the Utica Shale reportable segment includes the Summit Utica gathering system and the Ohio Gathering reportable segment includes our ownership interest in OGC and OCC. For the year ended December 31, 2017, we have disclosed the required segment information for Summit Utica and Ohio Gathering and the periods prior to January 1, 2017 have been recast to reflect this change.

As of December 31, 2017, our reportable segments are:

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the Utica Shale, which is served by Summit Utica;

•

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

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

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

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

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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 7). 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 7).

Corporate and other represents those results that are: (i) not specifically attributable to a reportable segment; (ii) not individually reportable; or (iii) that have not been 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.

	December 31,
	2017			2016			2015
	(In thousands)
Assets:
Utica Shale	$	212,311		$	199,392		$	135,056
Ohio Gathering		690,485			707,415			751,168
Williston Basin		512,860			724,084			740,361
Piceance/DJ Basins		798,722			843,440			866,095
Barnett Shale		383,306			404,314			416,586
Marcellus Shale		217,362			224,709			233,116
Total reportable segment assets		2,815,046			3,103,354			3,142,382
Corporate and other		79,996			12,294			22,290
Eliminations		(249	)		(469	)		—
Total assets	$	2,894,793		$	3,115,179		$	3,164,672

Revenues by reportable segment follow.

	Year ended December 31,
	2017			2016			2015
	(In thousands)
Revenues (1):
Utica Shale	$	38,907		$	24,263		$	4,700
Williston Basin		161,503			122,174			98,929
Piceance/DJ Basins		166,753			149,903			180,418
Barnett Shale		71,667			79,956			88,042
Marcellus Shale		30,394			26,111			28,468
Total reportable segments revenue		469,224			402,407			400,557
Corporate and other		26,446			412			—
Eliminations		(6,929	)		(457	)		—
Total revenues	$	488,741		$	402,362		$	400,557

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

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

	Year ended December 31,
	2017			2016			2015
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins	*				14	%		16	%
Counterparty B - Williston Basin		13	%	*			*
Counterparty C - Piceance/DJ Basins	*			*				14	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments (see Note 8).

(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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	7,009	$	4,331	$	1,417
Williston Basin		33,772		33,676		31,376
Piceance/DJ Basins		48,925		49,140		47,433
Barnett Shale (2)		15,001		16,093		16,392
Marcellus Shale		9,047		8,841		8,682
Total reportable segment depreciation and amortization		113,754		112,081		105,300
Corporate and other		1,118		580		603
Total depreciation and amortization	$	114,872	$	112,661	$	105,903

(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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	22,921	$	78,708	$	94,994
Williston Basin		17,309		31,541		147,477
Piceance/DJ Basins		23,714		25,719		21,144
Barnett Shale		569		3,910		6,875
Marcellus Shale		641		1,173		1,306
Total reportable segment capital expenditures		65,154		141,051		271,796
Corporate and other		59,061		1,668		429
Total cash paid for capital expenditures	$	124,215	$	142,719	$	272,225

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

During the year ended December 31, 2017, Corporate included cash paid of $3.0 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, (iii) depreciation and amortization, (iv) adjustments related to MVC shortfall payments, (v) unit-based and noncash compensation, (vi) change in the Deferred Purchase Price Obligation fair value, (vii) early extinguishment of debt expense, (viii) impairments and (ix) 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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	34,011	$	21,035	$	2,206
Ohio Gathering		41,246		45,602		33,667
Williston Basin		66,413		79,475		34,008
Piceance/DJ Basins		117,737		109,241		110,222
Barnett Shale		46,232		54,634		59,526
Marcellus Shale		23,888		19,203		23,214
Total of reportable segments' measures of profit or loss	$	329,527	$	329,190	$	262,843

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.

	Year ended December 31,
	2017			2016			2015
	(In thousands)
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:
Income (loss) before income taxes and income (loss) from equity method investees	$	88,614		$	(7,768	)	$	(216,268	)
Add:
Corporate and other		39,140			37,589			27,352
Interest expense		68,131			63,810			59,092
Early extinguishment of debt		22,039			—			—
Deferred Purchase Price Obligation		(200,322	)		55,854			—
Depreciation and amortization		114,872			112,661			105,903
Proportional adjusted EBITDA for equity method investees		41,246			45,602			33,667
Adjustments related to MVC shortfall payments		(41,373	)		11,600			(11,902	)
Unit-based and noncash compensation		7,951			7,985			7,017
Loss (gain) on asset sales, net		527			93			(172	)
Long-lived asset impairment		188,702			1,764			9,305
Goodwill impairment		—			—			248,851
Less:
Interest income		—			—			2
Total of reportable segments' measures of profit or loss	$	329,527		$	329,190		$	262,843

We include 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 MVC shortfall payments. With respect to the impact of a net change in deferred revenue for MVC shortfall payments, we treat increases in deferred revenue balances as a favorable adjustment to segment adjusted EBITDA, while decreases in deferred revenue balances are treated as an unfavorable adjustment to segment adjusted EBITDA. We also include a proportional amount of any historical and expected MVC shortfall payments in each quarter prior to the quarter in which we actually recognize the shortfall payment. The expected MVC shortfall payment adjustments have not been billed to our customers and are not recognized in our consolidated financial statements.

Adjustments related to MVC shortfall payments by reportable segment follow.

	Year ended December 31, 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	)	$	(3,065	)	$	—		$	(40,758	)
Expected MVC shortfall adjustments		—			(3	)		(612	)		(615	)
Total adjustments related to MVC shortfall payments	$	(37,693	)	$	(3,068	)	$	(612	)	$	(41,373	)

	Year ended December 31, 2016
	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	$	8,691	$	3,288		$	(677	)	$	11,302
Expected MVC shortfall adjustments		—		(317	)		615			298
Total adjustments related to MVC shortfall payments	$	8,691	$	2,971		$	(62	)	$	11,600

	Year Ended December 31, 2015
	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	$	11,870	$	(21,623	)	$	(1,700	)	$	(11,453	)
Expected MVC shortfall adjustments		—		33			(482	)		(449	)
Total adjustments related to MVC shortfall payments	$	11,870	$	(21,590	)	$	(2,182	)	$	(11,902	)

PROPERTY, PLANT, AND EQUIPMENT, NET

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

PROPERTY, PLANT, AND EQUIPMENT, NET

4. PROPERTY, PLANT AND EQUIPMENT, NET

Details on property, plant and equipment follow.

	December 31,
	2017		2016
	(In thousands)
Gathering and processing systems and related equipment	$	1,973,722	$	2,026,363
Construction in progress		78,850		39,954
Land and line fill		11,735		11,442
Other		40,262		35,227
Total		2,104,569		2,112,986
Less accumulated depreciation		309,440		259,315
Property, plant and equipment, net	$	1,795,129	$	1,853,671

During 2017, 2016 and 2015, we identified certain events, facts and circumstances which indicated that certain of our property, plant and equipment could be impaired. As such, we reviewed the assets that had been identified as potentially impaired and estimated the fair value of the identified property, plant and equipment using an income-based approach.

In December 2017, in connection with certain strategic initiatives, we performed a financial review of certain assets within the Williston Basin reporting segment. This resulted in a triggering event that required us to perform a recoverability test. Based on the results of the test, we concluded that the carrying value of certain long-lived assets relating to the Bison Midstream system within the Williston Basin were not fully recoverable. We recorded an impairment charge of $101.9 million related to these assets after comparing the fair value of the long-lived assets to their carrying values. See Note 5 for additional details.

During the fourth quarter of 2015, we identified a need to evaluate the goodwill associated with certain of our gathering systems (see Note 6). In connection with these evaluations, we also evaluated the related property, plant and equipment associated therewith for impairment and concluded that no impairment was necessary.

During 2017, 2016 and 2015, we recognized the following long-lived asset impairments, by segment.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Long-lived asset impairment:
Williston Basin	$	101,961	$	569	$	7,554
Piceance/DJ Basins		697		—		1,220
Barnett Shale		—		1,195		531
Utica Shale		878		—		—

Our impairment determinations, in the context of these reviews, involved significant assumptions and judgments. 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 estimates 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.

Depreciation expense and capitalized interest follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Depreciation expense	$	75,120	$	70,770	$	63,915
Capitalized interest		2,579		3,709		3,372

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

5. 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.

	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

	December 31, 2016
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(10,795	)	$	13,400
Contract intangibles		426,464		(146,468	)		279,996
Rights-of-way		153,015		(24,959	)		128,056
Total intangible assets	$	603,674	$	(182,222	)	$	421,452

Unfavorable gas gathering contract	$	10,962	$	(6,916	)	$	4,046

In December 2017, in connection with certain strategic initiatives, we evaluated certain long-lived assets relating to the Bison Midstream system within the Williston Basin reporting segment (see Note 4). In connection with this evaluation, we evaluated the related intangible assets associated therewith for impairment consisting of contract intangible assets and rights-of-way intangible assets. We concluded the contract intangible assets were also impaired and, as a result, we recorded an impairment charge of $85.2 million.

During the fourth quarter of 2015, we identified a need to evaluate the goodwill associated with certain of our gathering systems (see Note 6). In connection with these evaluations, we also evaluated the related intangible assets associated therewith for impairment and concluded that no impairment was necessary.

We recognized amortization expense in other revenues as follows:

	Year ended December 31,
	2017			2016			2015
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(1,555	)	$	(1,261	)	$	(1,478	)
Amortization expense – unfavorable gas gathering contract		2,158			839			692

We recognized amortization expense in costs and expenses as follows:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Amortization expense – contract intangibles	$	34,202	$	35,416	$	35,339
Amortization expense – rights-of-way		6,153		6,053		5,863

The estimated aggregate annual amortization expected to be recognized for as of December 31, 2017 for each of the five succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	34,026	$	1,888
2019		33,139		—
2020		32,963		—
2021		29,271		—
2022		26,204		—

GOODWILL

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

GOODWILL

6. GOODWILL

Goodwill for the periods presented is related to the acquisition of the Mountaineer Midstream system in 2013. As of December 31, 2017 and 2016, goodwill in the Marcellus Shale reporting segment, which is served by the Mountaineer Midstream system, amounted to $16.2 million.

Accumulated goodwill impairments by reportable segment for those reporting units that have previously recognized goodwill follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Accumulated goodwill impairment:
Piceance/DJ Basins	$	45,478	$	45,478	$	45,478
Williston Basin		257,572		257,572		257,572
Total accumulated goodwill impairment	$	303,050	$	303,050	$	303,050

As discussed in Note 2, we evaluate goodwill for impairment annually on September 30 and 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.

We performed our annual goodwill impairment testing for the Mountaineer Midstream reporting unit as of September 30, 2017 using a combination of the income and market approaches. We determined that its fair value substantially exceeded its carrying value, including goodwill.

We had no impairments of goodwill for the years ended December 31, 2017 and 2016.

2015 Annual Impairment Evaluation. We performed our 2015 annual goodwill impairment testing and determined that the fair value of the Grand River, Mountaineer Midstream and Polar and Divide reporting units did not exceed their carrying value, including goodwill. As a result, we recognized goodwill impairments of $45.5 million for Grand River and $203.4 million for Polar and Divide for the year ended December 31, 2015.

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. Differing assumptions regarding any of these inputs could have a significant effect on the 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

12 Months Ended

Dec. 31, 2017

Equity Method Investments And Joint Ventures [Abstract]

EQUITY METHOD INVESTMENTS

7. 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 Play in southeastern Ohio. Ohio Gathering provides gathering services pursuant to primarily long-term, fee-based gathering agreements, which include acreage dedications.

In January 2014, Summit Investments acquired a 1% ownership interest in Ohio Gathering from Blackhawk Midstream, LLC ("Blackhawk") for $190.0 million. Concurrent with this acquisition, Summit Investments made an $8.4 million capital contribution to Ohio Gathering to maintain its 1% ownership interest.

The ownership interest Summit Investments acquired from Blackhawk included an option to increase the holder's ownership interest in Ohio Gathering to 40% (the "Option"). In May 2014, Summit Investments exercised the Option to increase its ownership to 40% (the "Option Exercise") and made the following payments (i) $326.6 million of capital contribution true-ups, (ii) $50.4 million of additional capital contributions to maintain its 40% ownership interest and (iii) $5.4 million of management fee payments that were recognized as capital contributions in its Ohio Gathering capital accounts. Concurrent with and subsequent to the Option Exercise, the non-affiliated owners have retained their respective 60% ownership interest in Ohio Gathering (the "Non-affiliated Owners").

Summit Investments accounted for its initial ownership interests in Ohio Gathering under the cost method due to its ownership percentage and because it determined that it was not the primary beneficiary. Subsequent to the Option Exercise, Summit Investments accounted for its ownership interests in Ohio Gathering as equity method investments because it had joint control with the Non-affiliated Owners, which gave it significant influence. This shift from the cost method to the equity method required that Summit Investments retrospectively reflect its investment in Ohio Gathering and the associated results of operations as if it had been utilizing the equity method since the inception of its investment.

Summit Investments recognized the $190.0 million that it paid to Blackhawk as an investment in Ohio Gathering at inception. In addition, Ohio Gathering had assigned a value of $7.5 million to the Option, recognized it initially as an asset and concurrently attributed the value of the Option to Blackhawk's capital account. Upon acquiring Blackhawk's interest, the Option was reclassified from Blackhawk's capital account to Summit Investments' capital account in Ohio Gathering's records. Neither of these transactions involved a flow of funds to or from Ohio Gathering. As such, they created a basis difference between its recorded investment in equity method investees and that recognized and attributed to Summit Investments by Ohio Gathering. In accordance with the retrospective recognition triggered by the Option Exercise, in February 2014, Summit Investments began amortizing these basis differences over the weighted-average remaining life of the contracts underlying Ohio Gathering's operations. The impact of amortizing these two basis differences resulted in a net decrease to Summit Investments' investment in equity method investees. Subsequent to the Option Exercise, Summit Investments continued to make capital contributions to Ohio Gathering along with receiving distributions such that it maintained its 40% ownership interest through the 2016 Drop Down. Subsequent to the 2016 Drop Down, SMLP began making contributions and receiving distributions and will also continue amortizing the two basis differences, as noted above.

In December 2017, an asset impairment was recognized by Ohio Gathering. Although we recognize activity for Ohio Gathering on a one-month lag, we recorded the asset impairment in our results of operations for the years ending December 31, 2017 because the information was available to us. We recorded our 40% share of the asset impairment, or $1.4 million in 2017 in loss from equity method investees in the consolidated statements of operations.

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

	2017			2016
	(In thousands)
Investment in equity method investees, December 31	$	690,485		$	707,415
December cash distributions		4,032			3,172
December cash contributions		(3,932	)		(5,318	)
Impairment loss		1,383			—
Basis difference		(130,184	)		(143,536	)
Investment in equity method investees, net of basis difference, November 30	$	561,784		$	561,733

Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information).

	November 30, 2017				November 30, 2016
	OGC		OCC		OGC		OCC
	(In thousands)
Current assets	$	34,383	$	3,650	$	43,797	$	2,546
Noncurrent assets		1,319,448		29,156		1,330,199		31,195
Total assets	$	1,353,831	$	32,806	$	1,373,996	$	33,741

Current liabilities	$	10,882	$	3,382	$	22,067	$	3,448
Noncurrent liabilities		3,272		11,715		8,396		13,111
Total liabilities	$	14,154	$	15,097	$	30,463	$	16,559

Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information).

	Twelve months ended November 30, 2017					Twelve months ended November 30, 2016					Twelve months ended November 30, 2015
	OGC		OCC			OGC		OCC			OGC		OCC
	(In thousands)
Total revenues	$	140,679	$	8,607		$	148,662	$	15,791		$	120,623	$	9,467
Total operating expenses		111,897		8,298			96,647		111,528			96,948		15,633
Net income (loss)		28,785		(907	)		52,009		(94,230	)		23,655		(6,852	)

DEFERRED REVENUE

12 Months Ended

Dec. 31, 2017

Deferred Revenue Disclosure [Abstract]

Deferred Revenue

8. DEFERRED REVENUE

Certain of our gas gathering agreements provide for a monthly, quarterly or annual MVC from our customers. The amount of the shortfall payment is based on the difference between the actual throughput volume shipped or processed for the applicable period and the MVC for the applicable period, multiplied by the applicable gathering or processing fee.

Many of our gas gathering agreements contain provisions that can reduce or delay the cash flows that we expect to receive from our MVCs to the extent that a customer's actual throughput volumes are above or below its MVC for the applicable contracted measurement period. These provisions include the following:

•

To the extent that a customer's throughput volumes are less than its MVC for the applicable period and the customer makes a shortfall payment, it may be entitled to an offset in one or more subsequent periods to the extent that its throughput volumes in subsequent periods exceed its MVC for those periods. In such a situation, we would not receive gathering fees on throughput in excess of that customer's MVC (depending on the terms of the specific gas gathering agreement) to the extent that the customer had made a shortfall payment with respect to one or more preceding measurement periods (as applicable).

•

To the extent that a customer's throughput volumes exceed its MVC in the applicable contracted measurement period, it may be entitled to apply the excess throughput against its aggregate MVC, thereby reducing the period for which its annual MVC applies. As a result of this mechanism, the weighted-average remaining period for which our MVCs apply will be less than the weighted-average of the original stated contract terms of our MVCs.

•

To the extent that certain of our customers' throughput volumes exceed its MVC for the applicable period, there is a crediting mechanism that allows the customer to build a bank of credits that it can utilize in the future to reduce shortfall payments owed in subsequent periods, subject to expiration if there is no shortfall in subsequent periods. The period over which this credit bank can be applied to future shortfall payments varies, depending on the particular gas gathering agreement.

A rollforward of current deferred revenue follows.

	Williston Basin		Piceance/DJ Basins		Barnett Shale		Total current
	(In thousands)
Current deferred revenue, January 1, 2016	$	—	$	—	$	677	$	677
Additions		—		11,672		—		11,672
Less revenue recognized		—		11,672		677		12,349
Current deferred revenue, December 31, 2016		—		—		—		—
Additions		—		18,294		—		18,294
Less revenue recognized		—		14,294		—		14,294
Current deferred revenue, December 31, 2017	$	—	$	4,000	$	—	$	4,000

A rollforward of noncurrent deferred revenue follows.

	Williston Basin		Piceance/DJ Basins		Barnett Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, January 1, 2016	$	29,002	$	16,484	$	—	$	45,486
Additions		8,691		3,700		—		12,391
Less revenue recognized		—		412		—		412
Noncurrent deferred revenue, December 31, 2016		37,693		19,772		—		57,465
Less revenue recognized		37,693		3,065		—		40,758
Less reclassification to current deferred revenue		—		4,000		—		4,000
Noncurrent deferred revenue, December 31, 2017	$	—	$	12,707	$	—	$	12,707

As of December 31, 2017, accounts receivable included $18.4 million of total shortfall payment billings, of which none related to MVC arrangements that can be utilized to offset gathering fees in subsequent periods.

During the first quarter of 2017, we amended an agreement with one of our key customers in the Williston Basin segment. Based on our review of the amendment and original contract, we determined this was not a material modification to the contract and that we had no further performance obligations in regards to the previously-made MVC payments. As a result, we recognized previously deferred revenue of $37.7 million as gathering services and related fees during the first quarter of 2017.

DEBT

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

DEBT

9. DEBT

Debt consisted of the following:

	December 31,
	2017			2016
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.07% at December 31, 2017 and 3.27% at December 31, 2016) due May 2022	$	261,000		$	648,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,910	)		(3,516	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			—
Less unamortized debt issuance costs (1)		(6,898	)		—
Summit Holdings 7.5% senior unsecured notes redeemed March 2017 (2)		—			300,000
Less unamortized debt issuance costs (1)(2)		—			(4,183	)
Total long-term debt	$	1,051,192		$	1,240,301

__________

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

(2) Debt was extinguished following the 5.75% Senior Notes offering in February 2017. In conjunction with the early debt extinguishment, the remaining unamortized debt issuance costs were written off.

The aggregate amount of debt maturing during each of the years after December 31, 2017 are as follow (in thousands):

2018	$	—
2019		—
2020		—
2021		—
2022		561,000
Thereafter		500,000
Total long-term debt	$	1,061,000

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

In May 2017, Summit Holdings amended and restated its revolving credit facility with a third amended and restated credit agreement which: (i) maintained the revolving credit facility commitments of $1.25 billion, (ii) extended the maturity from November 2018 to May 2022, (iii) includes a $250.0 million accordion feature, (iv) maintained the same leverage-based pricing and commitment fee grid, (v) increased the maximum permitted total leverage ratio, as defined in the credit agreement, from 5.00 to 1.00 to 5.50 to 1.00 and (vi) includes a maximum permitted senior secured leverage ratio, as defined in the credit agreement, of 3.75 to 1.00.

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 December 31, 2017, the applicable margin under LIBOR borrowings was 2.50%, the interest rate was 4.07% and the unused portion of the Revolving Credit Facility totaled $989.0 million (subject to a commitment fee of 0.50%).

The revolving credit facility is secured by the membership interests of Summit Holdings and the membership interests of all the subsidiaries of Summit Holdings and by substantially all of the assets of Summit Holdings and its subsidiaries (subject to exclusions set forth in the credit agreement). It is guaranteed by SMLP and all of the subsidiaries of Summit Holdings other than the Specified Subsidiaries (as defined in the credit agreement). The credit agreement contains affirmative and negative covenants customary for credit facilities of its size and nature that, among other things, limit or restrict the ability (i) to incur additional debt; (ii) to make investments; (iii) to engage in certain mergers, consolidations, acquisitions or sales of assets; (iv) to enter into swap agreements and power purchase agreements; (v) to enter into leases that would cumulatively obligate payments in excess of $50.0 million over any 12 -month period; and (vi) of Summit Holdings to make distributions, with certain exceptions, including the distribution of Available Cash (as defined in the SMLP partnership agreement) if no default or event of default then exists or would result therefrom and Summit Holdings is in pro forma compliance with its financial covenants. The credit agreement also contains an affirmative covenant that could require our Non-Guarantor Subsidiaries (OpCo, Summit Utica, Meadowlark Midstream and Tioga Midstream) to become guarantor subsidiaries in certain circumstances. In addition, the revolving credit facility requires Summit Holdings to maintain (i) a ratio of consolidated trailing 12 -month earnings before interest, income taxes, depreciation and amortization ("EBITDA") to net interest expense of not less than 2.5 to 1.0 as defined in the credit agreement, (ii) a ratio of total net indebtedness to consolidated trailing 12 -month EBITDA of not more than 5.50 to 1.00 and, (iii) a ratio of first lien net indebtedness to consolidated trailing 12 -month EBITDA of not more than 3.75 to 1.00.

As a result of the amendment, SMLP incurred approximately $8.1 million of debt issuance costs. As of December 31, 2017, we had $10.9 million of debt issuance costs attributable to our Revolving Credit Facility and related amendments which are included in noncurrent assets on the consolidated balance sheet.

As of December 31, 2017, we were in compliance with the Revolving Credit Facility's covenants. There were no defaults or events of default during the year ended December 31, 2017.

Senior Notes. In June 2013, Summit Holdings and its 100% owned finance subsidiary, Finance Corp. (together with Summit Holdings, the "Co-Issuers") co-issued $300.0 million of 7.5% senior unsecured notes (the "7.5% Senior Notes"). In July 2014, 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”).

On February 8, 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 below. Concurrent with the 5.75% Senior Notes offering, we made a tender offer to purchase all the outstanding 7.5% Senior Notes. The tender offer expired on February 14, 2017 and resulted in approximately $276.9 million of our 7.5% Senior Notes being validly tendered and retired. On February 16, 2017, we issued a notice of redemption for the remaining 7.5% Senior Notes. The remaining $23.1 million of 7.5% Senior Notes were redeemed on March 18, 2017 (the "redemption date"), with payment made on March 20, 2017. References to the “Senior Notes,” when used for dates or periods ended on or after the date of issuance of the 5.75% Senior Notes but before the redemption date, refer collectively to 5.5% Senior Notes, 7.5% Senior Notes and 5.75% Senior Notes. References to the "Senior Notes," when used for dates or periods ended on or prior to the date of issuance of the 5.75% Senior Notes, refer collectively to 5.5% Senior Notes and 7.5% Senior Notes. References to the "Senior Notes," when used for dates or periods that ended after the redemption date, refer collectively to the 5.5% Senior Notes and the 5.75% Senior Notes. In conjunction with the tender offer and mandatory redemption of the 7.5% Senior Notes, we paid redemption and call premiums totaling $17.9 million. These costs, as well as $4.1 million of unamortized debt issuance costs, are presented on our consolidated statement of operations as early extinguishment of debt.

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.

5.75% Senior Notes. In February 2017, the Co-Issuers completed a public offering of $500.0 million of 5.75% senior unsecured notes maturing April 15, 2025. We pay interest on the 5.75% Senior Notes semi-annually in cash in arrears on April 15 and October 15 of each year. The 5.75% Senior Notes are senior, unsecured obligations and rank equally in right of payment with all of our existing and future senior obligations. The 5.75% Senior Notes are effectively subordinated in right of payment to all of our secured indebtedness, to the extent of the collateral securing such indebtedness. We used the proceeds from the issuance of the 5.75% Senior Notes to (i) fund the repurchase of the outstanding $300.0 million principal 7.5% Senior Notes, (ii) pay redemption and call premiums on the 7.5% Senior Notes totaling $17.9 million and (iii) pay $172.0 million of the balance outstanding under our Revolving Credit Facility.

At any time prior to April 15, 2020, the Co-Issuers may redeem up to 35% of the aggregate principal amount of the 5.75% Senior Notes at a redemption price of 105.750% of the principal amount of the 5.75% Senior Notes, plus accrued and unpaid interest, if any, but not including, the redemption date, with an amount not greater than the net cash proceeds of certain equity offerings. On and after April 15, 2020, the Co-Issuers may redeem all or part of the 5.75% Senior Notes at a redemption price of 104.313% (with the redemption premium declining ratably each year to 100.000% on and after April 15, 2023), plus accrued and unpaid interest, if any, to, but not including, the redemption date. Debt issuance costs of $7.7 million are being amortized over the life of the senior notes.

The 5.75% Senior Notes' indenture restricts SMLP’s and the Co-Issuers’ ability and the ability of certain of their subsidiaries to: (i) incur additional debt or issue preferred stock; (ii) make distributions, repurchase equity or redeem subordinated debt; (iii) make payments on subordinated indebtedness; (iv) create liens or other encumbrances; (v) make investments, loans or other guarantees; (vi) sell or otherwise dispose of a portion of their assets; (vii) engage in transactions with affiliates; and (viii) make acquisitions or merge or consolidate with another entity. These covenants are subject to a number of important exceptions and qualifications. At any time when the senior notes are rated investment grade by each of Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Services and no default or event of default under the indenture has occurred and is continuing, many of these covenants will terminate.

The 5.75% Senior Notes' indenture provides that each of the following is an event of default: (i) default for 30 days in the payment when due of interest on the 5.75% Senior Notes; (ii) default in the payment when due of the principal of, or premium, if any, on the 5.75% Senior Notes; (iii) failure by the Co-Issuers or SMLP to comply with certain covenants relating to mergers and consolidations, change of control or asset sales; (iv) failure by SMLP for 180 days after notice to comply with certain covenants relating to the filing of reports with the SEC; (v) failure by the Co-Issuers or SMLP for 30 days after notice to comply with any of the other agreements in the indenture; (vi) specified defaults under any mortgage, indenture or instrument under which there may be issued or by which there may be secured or evidenced any indebtedness for money borrowed by SMLP or any of its restricted subsidiaries (or the payment of which is guaranteed by SMLP or any of its restricted subsidiaries); (vii) failure by SMLP or any of its restricted subsidiaries to pay certain final judgments aggregating in excess of $75.0 million ; (viii) except as permitted by the indenture, any guarantee of the senior notes shall cease for any reason to be in full force and effect or any guarantor, or any person acting on behalf of any guarantor, shall deny or disaffirm its obligations under its guarantee of the senior notes; and (ix) certain events of bankruptcy, insolvency or reorganization described in the indenture. In the case of an event of default as described in the foregoing clause (ix), all outstanding 5.75% Senior Notes will become due and payable immediately without further action or notice. If any other event of default occurs and is continuing, the trustee or the holders of at least 25% in principal amount of the then outstanding 5.75% Senior Notes may declare all the 5.75% Senior Notes to be due and payable immediately.

5.5% Senior Notes. We pay interest on the 5.5% Senior Notes semi-annually in cash in arrears on February 15 and August 15 of each year. The 5.5% Senior Notes are senior, unsecured obligations and rank equally in right of payment with all of our existing and future senior obligations. The 5.5% Senior Notes are effectively subordinated in right of payment to all of our secured indebtedness, to the extent of the collateral securing such indebtedness. We used the proceeds from the issuance of the 5.5% Senior Notes to repay a portion of the balance outstanding under our Revolving Credit Facility.

At any time prior to August 15, 2018, the Co-Issuers may redeem all or part of the 5.5% Senior Notes at a redemption price of 104.125% (with the redemption premium declining ratably each year to 100.000% on and after August 15, 2020), plus accrued and unpaid interest, if any. Debt issuance costs of $5.1 million are being amortized over the life of the senior notes.

The 5.5% Senior Notes' indenture restricts SMLP’s and the Co-Issuers’ ability and the ability of certain of their subsidiaries to: (i) incur additional debt or issue preferred stock; (ii) make distributions, repurchase equity or redeem subordinated debt; (iii) make payments on subordinated indebtedness; (iv) create liens or other encumbrances; (v) make investments, loans or other guarantees; (vi) sell or otherwise dispose of a portion of their assets; (vii) engage in transactions with affiliates; and (viii) make acquisitions or merge or consolidate with another entity. These covenants are subject to a number of important exceptions and qualifications. At any time when the senior notes are rated investment grade by each of Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Services and no default or event of default under the indenture has occurred and is continuing, many of these covenants will terminate.

The 5.5% Senior Notes' indenture provides that each of the following is an event of default: (i) default for 30 days in the payment when due of interest on the 5.5% Senior Notes; (ii) default in the payment when due of the principal of, or premium, if any, on the 5.5% Senior Notes; (iii) failure by the Co-Issuers or SMLP to comply with certain covenants relating to mergers and consolidations, change of control or asset sales; (iv) failure by SMLP for 180 days after notice to comply with certain covenants relating to the filing of reports with the SEC; (v) failure by the Co-Issuers or SMLP for 30 days after notice to comply with any of the other agreements in the indenture; (vi) specified defaults under any mortgage, indenture or instrument under which there may be issued or by which there may be secured or evidenced any indebtedness for money borrowed by SMLP or any of its restricted subsidiaries (or the payment of which is guaranteed by SMLP or any of its restricted subsidiaries); (vii) failure by SMLP or any of its restricted subsidiaries to pay certain final judgments aggregating in excess of $20.0 million; (viii) except as permitted by the indenture, any guarantee of the senior notes shall cease for any reason to be in full force and effect or any guarantor, or any person acting on behalf of any guarantor, shall deny or disaffirm its obligations under its guarantee of the senior notes; and (ix) certain events of bankruptcy, insolvency or reorganization described in the indenture. In the case of an event of default as described in the foregoing clause (ix), all outstanding 5.5% Senior Notes will become due and payable immediately without further action or notice. If any other event of default occurs and is continuing, the trustee or the holders of at least 25% in principal amount of the then outstanding 5.5% Senior Notes may declare all the 5.5% Senior Notes to be due and payable immediately.

As of and during the year ended December 31, 2017, we were in compliance with the covenants governing our Senior Notes. There were no defaults or events of default during the year ended December 31, 2017.

SMP Holdings Credit Facility. SMP Holdings had a $250.0 million revolving credit facility (the "SMP Revolving Credit Facility") and a $200.0 million term loan (the "Term Loan" and, collectively with the SMP Revolving Credit Facility, the "SMP Holdings Credit Facility"). Because funding from the SMP Holdings Credit Facility was used to support the development of the 2016 Drop Down Assets, Summit Investments allocated the SMP Holdings Credit Facility to the Partnership during the common control period. Borrowings under the SMP Holdings Credit Facility incurred interest at LIBOR or a base rate (as defined in the credit agreement) plus an applicable margin. In March, 2016, the remaining balances on the SMP Revolving Credit Facility and the Incremental Term Loan were repaid in full and the SMP Holdings Credit Facility was terminated concurrent with the closing of the 2016 Drop Down.

FINANCIAL INSTRUMENTS

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

FINANCIAL INSTRUMENTS

10. 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 44% of total accounts receivable at December 31, 2017, compared with 62% as of December 31, 2016.

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 non-recurring Level 3 measurement in the fair value hierarchy because our assumptions and judgments are not observable from objective sources (see Note 16).

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, 2016	$	—
Addition		507,427
Change in fair value		55,854
Level 3 liability, December 31, 2016		563,281
Change in fair value		(200,322	)
Level 3 liability, December 31, 2017	$	362,959

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

	December 31, 2017				December 31, 2016
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings Revolving Credit Facility	$	261,000	$	261,000	$	648,000	$	648,000
Summit Holdings 5.5% Senior Notes ($300.0 million principal)		297,090		301,750		296,484		294,500
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,102		501,667		—		—
Summit Holdings 7.5% Senior Notes ($300.0 million principal) (1)		—		—		295,817		316,000

(1)	Debt was extinguished following the 5.75% Senior Notes offering in February 2017. In conjunction with the early debt extinguishment, the remaining unamortized debt issuance costs were written off.

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 December 31, 2017 and December 31, 2016. 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

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

PARTNERS' CAPITAL

11. PARTNERS' CAPITAL

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

	Limited partners
	Series A Preferred Units		Common		Subordinated			General Partner
Units, January 1, 2015		—		34,426,513		24,409,850			1,200,651
Units issued in connection with the May 2015 Equity Offering		—		7,475,000		—			—
General Partner 2% contribution		—		—		—			152,551
Net units issued under SMLP LTIP		—		161,131		—			1,498
Units, December 31, 2015		—		42,062,644		24,409,850			1,354,700
Subordinated units conversion		—		24,409,850		(24,409,850	)		—
Units issued in connection with the September 2016 Equity Offering		—		5,500,000		—			—
General Partner 2% contribution		—		—		—			112,245
Net units issued under SMLP LTIP		—		138,627		—			4,242
Units, December 31, 2016		—		72,111,121		—			1,471,187
Units issued in connection with the November 2017 Equity Offering		300,000		—		—			—
Net units issued under SMLP LTIP		—		211,327		—			—
Units issued under ATM program		—		763,548		—			—
General Partner 2% contribution		—		—		—			19,812
Units, December 31, 2017		300,000		73,085,996		—			1,490,999

Unit Offerings. In May 2015, we completed an underwritten public offering of 6,500,000 common units at a price of $30.75 per unit pursuant to an effective shelf registration statement on Form S-3 previously filed with the SEC (the "May 2015 Equity Offering"). On May 22, 2015, the underwriters exercised in full their option to purchase an additional 975,000 common units from us at a price of $30.75 per unit. Concurrent with both transactions, our General Partner made a capital contribution to us to maintain its approximate 2% general partner interest.

In September 2016, we completed an underwritten public offering of 5,500,000 common units at a price of $23.20 per unit pursuant to an effective shelf registration statement on Form S-3 previously filed with the SEC (the "September 2016 Equity Offering"). Following the September 2016 Equity Offering, our General Partner made a capital contribution to us to maintain its approximate 2% general partner interest. We used the net proceeds from the September 2016 Equity Offering to pay down our Revolving Credit Facility.

In February 2017, we completed a secondary underwritten public offering of 4,000,000 SMLP common units held by a subsidiary of Summit Investments pursuant to the 2016 SRS. We did not receive any proceeds from this offering.

Subordination. The subordination period ended in conjunction with the February 2016 distribution payment in respect of the fourth quarter of 2015 and the then-outstanding subordinated units converted to common units on a one-for-one basis. Prior to the end of the subordination period, the principal difference between our common units and subordinated units was that holders of the subordinated units were not entitled to receive any distribution of available cash until the common units had received the minimum quarterly distribution ("MQD") plus any arrearages in the payment of the MQD from prior quarters.

At-the-market Program. In February 2017, we executed a new equity distribution agreement and filed a prospectus 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 year ended December 31, 2017, we sold 763,548 units under the ATM Program for aggregate gross proceeds of $17.7 million, and paid approximately $0.2 million as compensation to the sales agents pursuant to the terms of the equity distribution agreement. After taking into account the aggregate sales price of common units sold under the ATM Program through December 31, 2017, 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 November 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. We used the net proceeds of $293.2 million (after deducting underwriting discounts and offering expenses) to repay outstanding borrowings under our revolving credit facility.

The Series A Preferred Units rank senior to (i) common units and incentive distribution rights, each representing limited partner interests in the Partnership and (ii) each other class or series of limited partner interests or other equity securities in the Partnership that may be established in the future that expressly ranks junior to the Series A Preferred Units as to the payment of distributions and amounts payable upon a liquidation event (the “Junior Securities”). The Series A Preferred Units rank equal in all respects with each class or series of limited partner interests or other equity securities in the Partnership that may be established in the future that is not expressly made senior or subordinated to the Series A Preferred Units as to the payment of distributions and amounts payable on a liquidation event (the “Parity Securities”). The Series A Preferred Units rank junior to (i) all of the Partnership’s existing and future indebtedness and other liabilities with respect to assets available to satisfy claims against the Partnership and (ii) each other class or series of limited partner interests or other equity securities in the Partnership established in the future that is expressly made senior to the Series A Preferred Units as to the payment of distributions and amounts payable upon a liquidation event. Income is allocated to the Series A Preferred Units in an amount equal to the earned distributions for the respective reporting period.

Distributions on the Series A Preferred Units are cumulative and compounding and are payable semi-annually in arrears on the 15th day of each June and December through and including December 15, 2022, and, thereafter, quarterly in arrears on the 15th day of March, June, September and December of each year (each, a “Distribution Payment Date”) to holders of record as of the close of business on the first business day of the month of the applicable Distribution Payment Date, in each case, when, as, and if declared by the General Partner out of legally available funds for such purpose.

The initial distribution rate for the Series A Preferred Units is 9.50% per annum of the $1,000 liquidation preference per Series A Preferred Unit. On and after December 15, 2022, distributions on the Series A Preferred Units will accumulate for each distribution period at a percentage of the liquidation preference equal to the three-month LIBOR plus a spread of 7.43%. A pro-rated initial distribution on the Series A Preferred Units was paid on December 15, 2017 in an amount equal to approximately $7.9167 per Series A Preferred Unit which totaled $2.4 million.

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

Summit Investments' Equity in Contributed Subsidiaries. Summit Investments' equity in contributed subsidiaries represents its position in the net assets of the 2016 Drop Down Assets and Polar and Divide that have been acquired by SMLP. The balance also reflects net income attributable to Summit Investments for the 2016 Drop Down Assets and Polar and Divide for the periods beginning on their respective acquisition dates by Summit Investments and ending on the dates they were acquired by the Partnership. Net income or loss was attributed to Summit Investments for:

•

the 2016 Drop Down Assets for the period from January 1, 2014 to March 3, 2016; and

•

Polar and Divide for the period from January 1, 2014 to May 18, 2015.

Although included in partners' capital, any net income or loss attributable to Summit Investments is excluded from the calculation of EPU.

2016 Drop Down. On March 3, 2016, we acquired the 2016 Drop Down Assets from a subsidiary of Summit Investments. We paid cash consideration of $360.0 million and recognized a Deferred Purchase Price Obligation of $507.4 million in exchange for Summit Investments' $1.11 billion net investment in the 2016 Drop Down Assets (see Note 16). In June 2016, we received a working capital adjustment of $0.6 million from a subsidiary of Summit Investments. We recognized a capital contribution from Summit Investments for the difference between (i) the net cash consideration paid and the Deferred Purchase Price Obligation and (ii) Summit Investments' net investment in the 2016 Drop Down Assets.

The calculation of the capital contribution and its allocation to partners' capital follows (in thousands).

Summit Investments' net investment in the 2016 Drop Down Assets	$	771,929
SMP Holdings borrowings allocated to 2016 Drop Down Assets and retained by Summit Investments	342,926
Acquired carrying value of 2016 Drop Down Assets				$	1,114,855

Deferred Purchase Price Obligation	$	507,427
Borrowings under Revolving Credit Facility	360,000
Working capital adjustment received from a subsidiary of Summit Investments	(569		)
Total consideration paid and recognized by SMLP				866,858
Excess of acquired carrying value over consideration paid and recognized				$	247,997

Allocation of capital contribution:
General Partner interest	$	4,953
Common limited partner interest	243,044
Partners' capital contribution – excess of acquired carrying value over consideration paid and recognized				$	247,997

Polar and Divide Drop Down. On May 18, 2015, we acquired 100% of the membership interests in Polar Midstream and Epping from a subsidiary of Summit Investments. We paid total net cash consideration of $285.7 million in exchange for Summit Investments' $416.0 million net investment in Polar Midstream and Epping, including customary working capital and capital expenditures adjustments (see Note 16 for additional information). We recognized a capital contribution from Summit Investments for the difference between cash consideration paid and Summit Investments' net investment in Polar Midstream and Epping.

The calculation of the capital contribution and its allocation to partners' capital follow (in thousands).

Summit Investments' net investment in Polar Midstream and Epping			$	416,044
Total net cash consideration paid to a subsidiary of Summit Investments			285,677
Excess of acquired carrying value over consideration paid			$	130,367

Allocation of capital contribution:
General Partner interest	$	2,607
Common limited partner interest	80,079
Subordinated limited partner interest	47,681
Partners' capital contribution – excess of acquired carrying value over consideration paid			$	130,367

Cash Distribution Policy

Our cash distribution policy, as expressed in our Partnership Agreement, may not be modified or repealed without amending our Partnership Agreement. Our Partnership Agreement requires that we distribute all of our available cash (as defined below) within 45 days after the end of each quarter to unitholders of record on the applicable record date. Our policy is to distribute to our unitholders an amount of cash each quarter that is equal to or greater than the MQD stated in our Partnership Agreement.

General Partner Interest. Our General Partner is entitled to an equivalent percentage of all distributions that we make prior to our liquidation based on its respective general partner interest, up to a maximum of 2%. Our General Partner has the right, but not the obligation, to contribute a proportionate amount of capital to us to maintain its current general partner interest. Our General Partner's interest in our distributions will be reduced if we issue additional units in the future and our General Partner does not contribute a proportionate amount of capital to us to maintain its general partner interest immediately prior to the unit issuance.

Minimum Quarterly Distribution. Our Partnership Agreement generally requires that we make a minimum quarterly distribution to the holders of our common units of $0.40 per unit, or $1.60 on an annualized basis, to the extent we have sufficient cash from our operations after the establishment of cash reserves and the payment of costs and expenses, including reimbursements of expenses to our General Partner. The amount of distributions paid under our policy is subject to fluctuations based on the amount of cash we generate from our business and the decision to make any distribution is determined by our General Partner, taking into consideration the terms of our Partnership Agreement.

Definition of Available Cash. Available cash generally means, for any quarter, all cash on hand at the end of that quarter:

•

less the amount of cash reserves established by our General Partner at the date of determination of available cash for that quarter to:

•

provide for the proper conduct of our business (including reserves for our future capital expenditures and anticipated future debt service requirements);

•

comply with applicable law, any of our debt instruments or other agreements; or

•

provide funds for distributions to our unitholders and to our General Partner for any one or more of the next four quarters (provided that our General Partner may not establish cash reserves for distributions unless it determines that the establishment of reserves will not prevent us from distributing the minimum quarterly distribution on all common units and any cumulative arrearages on such common units for the current quarter);

•

plus, if our General Partner so determines, all or any portion of the cash on hand on the date of determination of available cash for the quarter resulting from working capital borrowings made subsequent to the end of such quarter.

Cash Distributions Paid and Declared. We paid the following per-unit distributions during the years ended December 31:

	Year ended December 31,
	2017		2016		2015
Per-unit distributions to unitholders	$	2.300	$	2.300	$	2.270

On January 25, 2018, the Board of Directors of our General Partner declared a distribution of $0.575 per unit for the quarterly period ended December 31, 2017. This distribution, which totaled $45.1 million, was paid on February 14, 2018 to unitholders of record at the close of business on February 7, 2018.

We allocated the February 2018 distribution in accordance with the third target distribution level (see "Incentive Distribution Rights—Percentage Allocations of Available Cash" below for additional information.)

Incentive Distribution Rights.

Our General Partner also currently holds IDRs that entitle it to receive increasing percentage allocations of the cash we distribute from operating surplus (as set forth in the chart below). The maximum distribution includes distributions paid to our General Partner on an assumed 2% general partner interest. The maximum distribution does not include any distributions that our General Partner may receive on any common units that it owns.

Percentage Allocations of Available Cash. The following table illustrates the percentage allocations of available cash between the unitholders and our General Partner based on the specified target distribution levels. The amounts set forth in the column Marginal Percentage Interest in Distributions are the percentage interests of our General Partner and the unitholders in any available cash we distribute up to and including the corresponding amount in the column Total Quarterly Distribution Per Unit Target Amount. The percentage interests shown for our unitholders and our General Partner for the minimum quarterly distribution are also applicable to quarterly distribution amounts that are less than the MQD. The percentage interests set forth below for our General Partner assume (i) a 2% general partner interest, (ii) that our General Partner has not transferred its IDRs and (iii) that there are no arrearages on common units.

		Marginal percentage interest in distributions
	Total quarterly distribution per unit target amount	Unitholders	General Partner
Minimum quarterly distribution	$0.40	98%	2%
First target distribution	$0.40 up to $0.46	98%	2%
Second target distribution	above $0.46 up to $0.50	85%	15%
Third target distribution	above $0.50 up to $0.60	75%	25%
Thereafter	above $0.60	50%	50%

We reached the second target distribution in connection with the distribution declared in respect of the fourth quarter of 2013. We reached the third target distribution in connection with the distribution declared in respect of the second quarter of 2014.

Our payment of IDRs as reported in distributions to unitholders – General Partner in the statements of partners' capital during the years ended December 31 follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
IDR payments	$	8,460	$	7,912	$	6,743

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

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

EARNINGS PER UNIT

12. EARNINGS PER UNIT

The following table details the components of EPU.

	Year ended December 31,
	2017		2016			2015
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net income (loss) among limited partner interests:
Net income (loss) attributable to limited partners	$	75,485	$	(48,179	)	$	(125,437	)
Net loss attributable to subordinated units (1)		—		—			(70,173	)
Less net income attributable to Series A Preferred Units		3,563		—			—
Net income (loss) attributable to common limited partners	$	71,922	$	(48,179	)	$	(195,610	)

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		72,705		68,264			39,217
Effect of nonvested phantom units		342		—			—
Weighted-average common units outstanding – diluted		73,047		68,264			39,217

Weighted-average subordinated units outstanding – basic and diluted							24,410

Earnings (loss) per limited partner unit:
Common unit – basic	$	0.99	$	(0.71	)	$	(3.20	)
Common unit – diluted	$	0.98	$	(0.71	)	$	(3.20	)
Subordinated unit – basic and diluted (1)						$	(2.88	)

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		42		125			109

__________

(1) The subordination period ended on February 16, 2016 and all 24,409,850 subordinated units converted to common units on a one-for-one basis (see Note 11).

UNIT-BASED AND NONCASH COMPENSATION

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

UNIT-BASED AND NONCASH COMPENSATION

13. 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, thereby linking the recipients' compensation directly to SMLP’s performance. The SMLP LTIP is administered by our General Partner's Board of Directors, though such administration function may be delegated to a committee appointed by the board. A total of 5.0 million common units was reserved for issuance pursuant to and in accordance with the SMLP LTIP. As of December 31, 2017, approximately 3.6 million common units remained available for future issuance.

The SMLP LTIP provides for the granting, from time to time, of unit-based awards, including common units, restricted units, phantom units, unit options, unit appreciation rights, distribution equivalent rights, profits interest units and other unit-based awards. Grants are made at the discretion of the Board of Directors or Compensation Committee of our General Partner. The administrator of the SMLP LTIP may make grants under the SMLP LTIP that contain such terms, consistent with the SMLP LTIP, as the administrator may determine are appropriate, including vesting conditions. The administrator of the SMLP LTIP may, in its discretion, base vesting on the grantee's completion of a period of service or upon the achievement of specified financial objectives or other criteria or upon a change of control (as defined in the SMLP LTIP) or as otherwise described in an award agreement. Termination of employment prior to vesting will result in forfeiture of the awards, except in limited circumstances as described in the plan documents. Units that are canceled or forfeited will be available for delivery pursuant to other awards.

The following table presents phantom and restricted unit activity:

	Units			Weighted-average grant date fair value
Nonvested phantom units, January 1, 2015		336,202		$	30.61
Phantom units granted		289,735			29.21
Phantom units vested		(229,497	)		27.66
Phantom units forfeited		(16,529	)		35.09
Nonvested phantom units, December 31, 2015		379,911			31.13
Phantom units granted		495,535			14.91
Phantom units vested		(178,953	)		33.80
Phantom units forfeited		(4,538	)		16.89
Nonvested phantom units, December 31, 2016		691,955			19.59
Phantom units granted		371,972			22.50
Phantom units vested		(293,222	)		24.76
Phantom units forfeited		(21,431	)		20.07
Nonvested phantom units, December 31, 2017		749,274		$	20.07

A phantom unit is a notional unit that entitles the grantee to receive a common unit upon the vesting of the phantom unit or on a deferred basis upon specified future dates or events or, in the discretion of the administrator, cash equal to the fair market value of a common unit. Distribution equivalent rights for each phantom unit provide for a lump sum cash amount equal to the accrued distributions from the grant date to be paid in cash upon the vesting date. A restricted unit is a common limited partner unit that is subject to a restricted period during which the unit remains subject to forfeiture.

The phantom units granted in connection with the IPO vested on the third anniversary of the IPO. All other phantom units granted to date vest ratably over a three-year period. Grant date fair value is determined based on the closing price of our common units on the date of grant multiplied by the number of phantom units awarded to the grantee. Holders of all phantom units granted to date are entitled to receive distribution equivalent rights for each phantom unit, providing for a lump sum cash amount equal to the accrued distributions from the grant date of the phantom units to be paid in cash upon the vesting date. Upon vesting, phantom unit awards may be settled, at our discretion, in cash and/or common units, but the current intention is to settle all phantom unit awards with common units. The restricted units granted in 2013 maintained the vesting provisions of the share-based compensation awards they replaced, each of which had an original vesting period of four years.

The intrinsic value of phantom and restricted units that vested during the years ended December 31, follows.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Intrinsic value of vested LTIP awards	$	6,657	$	2,957	$	5,362

As of December 31, 2017, the unrecognized unit-based compensation related to the SMLP LTIP was $5.7 million. Incremental unit-based compensation will be recorded over the remaining vesting period of approximately 2.2 years. Due to the limited and insignificant forfeiture history associated with the grants under the SMLP LTIP, no forfeitures were assumed in the determination of estimated compensation expense.

Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP follows.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
SMLP LTIP unit-based compensation	$	7,951	$	7,550	$	6,174

RELATED-PARTY TRANSACTIONS

12 Months Ended

Dec. 31, 2017

RELATED-PARTY TRANSACTIONS

14. RELATED-PARTY TRANSACTIONS

Acquisitions. See Notes 1, 11 and 16 for disclosure of the 2016 Drop Down and the Polar and Divide Drop Down and the funding of those transactions.

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:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Operation and maintenance expense	$	27,450	$	26,485	$	25,050
General and administrative expense		30,899		31,947		26,193

Expenses Incurred by Summit Investments. Prior to the 2016 Drop Down and the Polar and Divide Drop Down, Summit Investments incurred:

•

certain support expenses and capital expenditures on behalf of the contributed subsidiaries. These transactions were settled periodically through membership interests prior to the respective drop down;

•

interest expense that was related to capital projects for the contributed subsidiaries. As such, the associated interest expense was allocated to the respective contributed subsidiary's capital projects as a noncash contribution and capitalized into the basis of the asset; and

•

noncash compensation expense for the SMP net profits interests, which were accounted for as compensatory awards. As such, the annual expense associated with the SMP net profits was allocated to the respective contributed subsidiary and is reflected in general and administrative expenses in the statements of operations.

Subsequent to any drop down, these expenses are retrospectively included in the reimbursement of General Partner expenses disclosed above due to common control.

In February 2017, SMP Holdings sold 4,000,000 common units representing limited partner interests in SMLP at a price to the public of $24.00 per common unit. Consistent with our obligations under the Partnership Agreement, we paid all costs and expenses of the secondary offering (other than underwriting discounts and fees and expenses of counsel and advisors to SMP Holdings in the sale). We did not receive any of the proceeds from the secondary offering.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

15. 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:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Rent expense	$	3,772	$	2,861	$	2,395

We lease office space and equipment under agreements that expire in various years through 2027. Future minimum lease payments due under noncancelable operating leases at December 31, 2017, were as follows (in thousands):

2018	$	3,373
2019		3,181
2020		824
2021		389
2022		349
Thereafter		731
Total future minimum lease payments	$	8,847

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.

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 2016 Annual Report, in January 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, 2016	$	13,664
Payments made, including those by affiliates		(4,211	)
Accrued environmental remediation, December 31, 2016	$	9,453
Payments made		(4,109	)
Accrued environmental remediation, December 31, 2017	$	5,344

As of December 31, 2017, 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 December 31, 2018. 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.

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

12 Months Ended

Dec. 31, 2017

Business Combinations [Abstract]

ACQUISITIONS AND DROP DOWN TRANSACTIONS

16. ACQUISITIONS AND DROP DOWN TRANSACTIONS

2016 Drop Down. On March 3, 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 Deferred Purchase Price Obligation will be equal to:

•

six-and-one-half (6.5) multiplied by the average Business Adjusted EBITDA, as defined below and in the Contribution Agreement, of the 2016 Drop Down Assets for 2018 and 2019, less the G&A Adjuster, as defined in the Contribution Agreement;

•

less the Initial Payment;

•

less all capital expenditures incurred for the 2016 Drop Down Assets between the March 3, 2016 and December 31, 2019;

•

plus all Business Adjusted EBITDA from the 2016 Drop Down Assets between March 3, 2016 and December 31, 2019, less the Cumulative G&A Adjuster, as defined in the Contribution Agreement.

Business Adjusted EBITDA is defined as the net income or loss of the 2016 Drop Down Assets for such period:

•

plus interest expense, income tax expense and depreciation and amortization of the 2016 Drop Down Assets for such period;

•

plus any adjustments related to MVC shortfall payments, impairments and other noncash expenses or losses with respect to the 2016 Drop Down Assets for such period;

•

plus any Special Liability Expenses, as defined below and in the Contribution Agreement, for such period;

•

less interest income and income tax benefit of the 2016 Drop Down Assets for such period;

•

less adjustments related to any other noncash income or gains with respect to the 2016 Drop Down Assets for such period.

Business Adjusted EBITDA shall exclude the effect of any Partnership expenses allocated by or to SMLP or its affiliates in respect of the 2016 Drop Down Assets, such as general and administrative expenses (including compensation-related expenses and professional services fees), transaction costs, allocated interest expense and allocated income tax expense.

Special Liability Expenses are defined as any and all expenses incurred by SMLP with respect to the Special Liabilities, as defined in the Contribution Agreement, including fines, legal fees, consulting fees and remediation costs.

The present value of the Deferred Purchase Price Obligation will be reflected as a liability on our balance sheet until paid. As of the acquisition date, the estimated future payment obligation (based on management’s estimate of the Partnership’s share of forecasted Business Adjusted EBITDA and capital expenditures for the 2016 Drop Down Assets) was estimated to be $860.3 million and had a net present value of $507.4 million, using a discount rate of 13.0%. As of December 31, 2017, Remaining Consideration was estimated to be $454.4 million and the net present value, as recognized on the consolidated balance sheet, was $363.0 million, using a discount rate of 10.50%. 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.

At the discretion of the Board of Directors of our General Partner, the Deferred Purchase Price Obligation can be paid in cash, SMLP common units or a combination thereof. 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.

Because of the common control aspects in a drop down transaction, the 2016 Drop Down was deemed a transaction between entities under common control. As such, the 2016 Drop Down has been accounted for on an “as-if pooled” basis for all periods in which common control existed and the Partnership’s financial results retrospectively include the combined financial results of the 2016 Drop Down Assets for all common-control periods.

Ohio Gathering. For information on the acquisition and initial recognition of Ohio Gathering, see Note 7.

Meadowlark Midstream. At the time of the 2016 Drop Down, Meadowlark Midstream owned Niobrara G&P and certain crude oil and produced water gathering pipelines located in Williams County, North Dakota. Summit Investments accounted for its purchase of Meadowlark Midstream under the acquisition method of accounting, whereby the various gathering systems' identifiable tangible and intangible assets acquired and liabilities assumed were recorded based on their fair values as of initial acquisition on February 15, 2013. Both Bison Midstream and Polar Midstream have previously been carved out of Meadowlark Midstream. Their fair values were determined based upon assumptions related to future cash flows, discount rates, asset lives and projected capital expenditures to complete the system. We recognized the 2016 acquisition of Meadowlark Midstream at Summit Investments' historical cost of construction and fair value of assets and liabilities at acquisition, which reflected its fair value accounting for the initial acquisition of Meadowlark Midstream in 2013, due to common control.

The fair values of the assets acquired and liabilities assumed as of February 15, 2013, were as follows (in thousands):

Purchase price assigned to Meadowlark Midstream			$	25,376
Current assets	$	2,227
Property, plant and equipment		18,795
Other noncurrent assets		4,354
Total assets acquired		25,376
Total liabilities assumed	$	—
Net identifiable assets acquired			$	25,376

From a financial position and operational standpoint, the crude oil and produced water gathering pipelines held by Meadowlark Midstream and acquired in connection with the 2016 Drop Down are recognized as part of the Polar and Divide system.

Polar and Divide. On May 18, 2015, SMLP acquired the Polar and Divide system, a crude oil and produced water gathering system, including under-development transmission pipelines, located in North Dakota from a subsidiary of Summit Investments, subject to customary working capital and capital expenditures adjustments. We funded the initial combined purchase price of $290.0 million with (i) $92.0 million of borrowings under SMLP’s Revolving Credit Facility and (ii) the issuance of $193.4 million of SMLP common units and $4.1 million of general partner interests to SMLP’s General Partner in connection with the May 2015 Equity Offering. In July 2015, we received $4.3 million of cash from a subsidiary of Summit Investments as payment in full for working capital and capital expenditure adjustments.

Summit Investments accounted for its purchase of Meadowlark Midstream, the entity that Polar Midstream was carved out of, under the acquisition method of accounting, whereby the various gathering systems' identifiable tangible and intangible assets acquired and liabilities assumed were recorded based on their fair values as of initial acquisition on February 15, 2013. Their fair values were determined based upon assumptions related to future cash flows, discount rates, asset lives and projected capital expenditures to complete the system. We recognized the acquisition of Polar Midstream at Summit Investments' historical cost of construction and fair value of assets and liabilities at acquisition, which reflected its fair value accounting for the acquisition of Meadowlark Midstream, due to common control.

Supplemental Disclosures – As-If Pooled Basis. As a result of accounting for our drop down transactions similar to a pooling of interests, our historical financial statements and those of the acquired drop down assets have been combined to reflect the historical operations, financial position and cash flows of the acquired drop down assets from the date common control began. Revenues and net income for the previously separate entities and the combined amounts, as presented in these consolidated financial statements follow.

	Year ended December 31,
	2016			2015
	(In thousands)
SMLP revenues	$	393,495		$	358,046
2016 Drop Down Assets revenues (1)		8,867			29,238
Polar and Divide revenues (1)		—			13,273
Combined revenues	$	402,362		$	400,557

SMLP net loss	$	(40,932	)	$	(192,212	)
2016 Drop Down Assets net income (loss) (1)		2,745			(35,419	)
Polar and Divide net income (1)		—			5,403
Combined net loss	$	(38,187	)	$	(222,228	)

_______

(1) Results are fully reflected in SMLP's results of operations subsequent to closing the respective drop down.

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

CONDENSED CONSOLIDATING FINANCIAL INFORMATION

17. 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 9).

In December 2017, the Niobrara associated natural gas gathering and processing assets held by Meadowlark Midstream (See Note 16) were contributed to Summit Niobrara, a newly formed entity in 2017. Concurrent with this contribution (i) a subsidiary of SMLP purchased the remaining 1% ownership interest in Summit Niobrara held by Summit Epping, LLC for approximately $0.8 million; 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. As a result, Summit Niobrara became a guarantor subsidiary on the Senior Notes and Revolving Credit Facility and, as such, all prior periods presented have been recast to reflect this change.

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; and

•

the balances and results of operations associated with the assets, liabilities and expenses that were carved out of Summit Investments and allocated to SMLP in connection with the 2016 Drop Down have been attributed to SMLP during the common control period.

Condensed Consolidating Balance Sheets. Balance sheets as of December 31, 2017 and 2016 follow.

	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

	December 31, 2016
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	698	$	51	$	5,768	$	911	$	—		$	7,428
Accounts receivable		53		—		91,152		6,159		—			97,364
Other current assets		1,526		—		2,428		355		—			4,309
Due from affiliate		11,768		38,013		366,867		—		(416,648	)		—
Total current assets		14,045		38,064		466,215		7,425		(416,648	)		109,101
Property, plant and equipment, net		2,266		—		1,508,824		342,581		—			1,853,671
Intangible assets, net		—		—		398,992		22,460		—			421,452
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		707,415		—			707,415
Other noncurrent assets		1,993		5,198		138		—		—			7,329
Investment in subsidiaries		2,132,757		3,347,393		—		—		(5,480,150	)		—
Total assets	$	2,151,061	$	3,390,655	$	2,390,380	$	1,079,881	$	(5,896,798	)	$	3,115,179

Liabilities and Partners' Capital
Trade accounts payable	$	978	$	—	$	10,640	$	4,633	$	—		$	16,251
Accrued expenses		2,399		114		6,284		2,592		—			11,389
Due to affiliate		405,138		—		—		11,768		(416,648	)		258
Ad valorem taxes payable		16		—		9,847		725		—			10,588
Accrued interest		—		17,483		—		—		—			17,483
Accrued environmental remediation		—		—		—		4,301		—			4,301
Other current liabilities		6,717		—		4,047		707		—			11,471
Total current liabilities		415,248		17,597		30,818		24,726		(416,648	)		71,741
Long-term debt		—		1,240,301		—		—		—			1,240,301
Deferred Purchase Price Obligation		563,281		—		—		—		—			563,281
Deferred revenue		—		—		57,465		—		—			57,465
Noncurrent accrued environmental remediation		—		—		—		5,152		—			5,152
Other noncurrent liabilities		2,859		—		4,602		105		—			7,566
Total liabilities		981,388		1,257,898		92,885		29,983		(416,648	)		1,945,506

Total partners' capital		1,169,673		2,132,757		2,297,495		1,049,898		(5,480,150	)		1,169,673
Total liabilities and partners' capital	$	2,151,061	$	3,390,655	$	2,390,380	$	1,079,881	$	(5,896,798	)	$	3,115,179

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 years ended December 31, 2017, 2016 and 2015 follow.

	Year ended December 31, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments		Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	334,625		$	59,802		$	—	$	394,427
Natural gas, NGLs and condensate sales		—			—			68,459			—			—		68,459
Other revenues		—			—			25,583			272			—		25,855
Total revenues		—			—			428,667			60,074			—		488,741
Costs and expenses:
Cost of natural gas and NGLs		—			—			57,237			—			—		57,237
Operation and maintenance		—			—			85,599			8,283			—		93,882
General and administrative		—			—			47,878			6,803			—		54,681
Depreciation and amortization		1,101			—			101,319			13,055			—		115,475
Transaction costs		73			—			—			—			—		73
(Gain) loss on asset sales, net		—			—			(15	)		542			—		527
Long-lived asset impairment		—			—			187,823			879			—		188,702
Total costs and expenses		1,174			—			479,841			29,562			—		510,577
Other income		298			—			—			—			—		298
Interest expense		—			(68,131	)		—			—			—		(68,131	)
Early extinguishment of debt		—			(22,039	)		—			—			—		(22,039	)
Deferred Purchase Price Obligation		200,322			—			—			—			—		200,322
Income (loss) before income taxes and loss from equity method investees		199,446			(90,170	)		(51,174	)		30,512			—		88,614
Income tax expense		(341	)		—			—			—			—		(341	)
Loss from equity method investees		—			—			—			(2,223	)		—		(2,223	)
Equity in loss of consolidated subsidiaries		(113,055	)		(22,885	)		—			—			135,940		—
Net income	$	86,050		$	(113,055	)	$	(51,174	)	$	28,289		$	135,940	$	86,050

	Year ended December 31, 2016
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	294,837	$	51,124		$	—		$	345,961
Natural gas, NGLs and condensate sales		—			—			35,833		—			—			35,833
Other revenues		—			—			20,227		341			—			20,568
Total revenues		—			—			350,897		51,465			—			402,362
Costs and expenses:
Cost of natural gas and NGLs		—			—			27,421		—			—			27,421
Operation and maintenance		—			—			89,031		6,303			—			95,334
General and administrative		—			—			44,922		7,488			—			52,410
Depreciation and amortization		580			—			101,415		10,244			—			112,239
Transaction costs		1,321			—			—		—			—			1,321
Loss (gain) on asset sales, net		—			—			97		(4	)		—			93
Long-lived asset impairment		—			—			1,235		529			—			1,764
Total costs and expenses		1,901			—			264,121		24,560			—			290,582
Other income		116			—			—		—			—			116
Interest expense		(1,441	)		(62,369	)		—		—			—			(63,810	)
Deferred Purchase Price Obligation		(55,854	)		—			—		—			—			(55,854	)
(Loss) income before income taxes and loss from equity method investees		(59,080	)		(62,369	)		86,776		26,905			—			(7,768	)
Income tax expense		(75	)		—			—		—			—			(75	)
Loss from equity method investees		—			—			—		(30,344	)		—			(30,344	)
Equity in earnings of consolidated subsidiaries		20,968			83,337			—		—			(104,305	)		—
Net (loss) income	$	(38,187	)	$	20,968		$	86,776	$	(3,439	)	$	(104,305	)	$	(38,187	)

	Year ended December 31, 2015
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments		Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	320,454		$	17,365		$	—	$	337,819
Natural gas, NGLs and condensate sales		—			—			42,079			—			—		42,079
Other revenues		—			—			20,283			376			—		20,659
Total revenues		—			—			382,816			17,741			—		400,557
Costs and expenses:
Cost of natural gas and NGLs		—			—			31,398			—			—		31,398
Operation and maintenance		—			—			91,763			3,223			—		94,986
General and administrative		—			—			40,134			4,974			—		45,108
Depreciation and amortization		603			—			98,002			6,512			—		105,117
Transaction costs		1,342			—			—			—			—		1,342
Environmental remediation		—			—			—			21,800			—		21,800
Gain on asset sales, net		—			—			(172	)		—			—		(172	)
Long-lived asset impairment		—			—			9,305			—			—		9,305
Goodwill impairment		—			—			248,851			—			—		248,851
Total costs and expenses		1,945			—			519,281			36,509			—		557,735
Other income		2			—			—			—			—		2
Interest expense		(10,494	)		(48,598	)		—			—			—		(59,092	)
Loss before income taxes and loss from equity method investees		(12,437	)		(48,598	)		(136,465	)		(18,768	)		—		(216,268	)
Income tax benefit		603			—			—			—			—		603
Loss from equity method investees		—			—			—			(6,563	)		—		(6,563	)
Equity in loss of consolidated subsidiaries		(210,394	)		(161,796	)		—			—			372,190		—
Net loss	$	(222,228	)	$	(210,394	)	$	(136,465	)	$	(25,331	)	$	372,190	$	(222,228	)

Condensed Consolidating Statements of Cash Flows. Statements of cash flows for the years ended December 31, 2017 and 2016 follow.

	Year ended December 31, 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	$	7,122		$	(68,915	)	$	221,115		$	78,510		$	—		$	237,832

Cash flows from investing activities:
Capital expenditures		(3,041	)		—			(98,926	)		(22,248	)		—			(124,215	)
Proceeds from asset sales		—			—			—			2,300			—			2,300
Contributions to equity method investees		—			—			—			(25,513	)		—			(25,513	)
Purchase of noncontrolling interest		(797	)		—			—			—			—			(797	)
Other, net		(458	)		—			—			—			—			(458	)
Advances to affiliates		(278,493	)		—			(126,198	)		(22,031	)		426,722			—
Net cash used in investing activities		(282,789	)		—			(225,124	)		(67,492	)		426,722			(148,683	)

Cash flows from financing activities:
Distributions to unitholders		(181,478	)		—			—			—			—			(181,478	)
Borrowings under Revolving Credit Facility		—			247,500			—			—			—			247,500
Repayments under Revolving Credit Facility		—			(634,500	)		—			—			—			(634,500	)
Debt issuance costs		—			(16,390	)		—			—			—			(16,390	)
Payment of redemption and call premiums on senior notes		—			(17,932	)		—			—			—			(17,932	)
Proceeds from ATM Program common unit issuances, net of costs		17,078			—			—			—			—			17,078
Proceeds from issuance of Series A Preferred Units, net of costs		293,238			—			—			—			—			293,238
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		(2,437	)		—			(621	)		(70	)		—			(3,128	)
Advances from affiliates		148,229			290,261			—			(11,768	)		(426,722	)		—
Net cash provided by (used in) financing activities		275,095			68,939			(621	)		(11,838	)		(426,722	)		(95,147	)
Net change in cash and cash equivalents		(572	)		24			(4,630	)		(820	)		—			(5,998	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	126		$	75		$	1,138		$	91		$	—		$	1,430

	Year ended December 31, 2016
	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	$	9,691		$	(58,254	)	$	201,516		$	77,542		$	—		$	230,495

Cash flows from investing activities:
Capital expenditures		(1,668	)		—			(55,185	)		(85,866	)		—			(142,719	)
Contributions to equity method investees		—			—			—			(31,582	)		—			(31,582	)
Acquisitions of gathering systems from affiliate		(359,431	)		—			—			—			—			(359,431	)
Other, net		(394	)		—			—			—			—			(394	)
Advances to affiliates		(15,697	)		(255,070	)		(150,775	)		—			421,542			—
Net cash used in investing activities		(377,190	)		(255,070	)		(205,960	)		(117,448	)		421,542			(534,126	)

Cash flows from financing activities:
Distributions to unitholders		(167,504	)		—			—			—			—			(167,504	)
Borrowings under Revolving Credit Facility		12,000			508,300			—			—			—			520,300
Repayments under Revolving Credit Facility		—			(204,300	)		—			—			—			(204,300	)
Debt issuance costs		—			(3,032	)		—			—			—			(3,032	)
Proceeds from underwritten issuance of common units, net of costs		125,233			—			—			—			—			125,233
Contribution from General Partner		2,702			—			—			—			—			2,702
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		(12,000	)		—			3,223			20,991			—			12,214
Expenses paid by Summit Investments on behalf of contributed subsidiaries		3,030			—			—			1,791			—			4,821
Other, net		(1,182	)		—			(121	)		135			—			(1,168	)
Advances from affiliates		405,845			—			—			15,697			(421,542	)		—
Net cash provided by (used in) financing activities		368,124			300,968			3,102			38,614			(421,542	)		289,266
Net change in cash and cash equivalents		625			(12,356	)		(1,342	)		(1,292	)		—			(14,365	)
Cash and cash equivalents, beginning of period		73			12,407			7,110			2,203			—			21,793
Cash and cash equivalents, end of period	$	698		$	51		$	5,768		$	911		$	—		$	7,428

	Year ended December 31, 2015
	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	$	409		$	(46,716	)	$	207,684		$	29,998		$	—		$	191,375

Cash flows from investing activities:
Capital expenditures		(429	)		—			(120,784	)		(151,012	)		—			(272,225	)
Contributions to equity method investees		—			—			—			(86,200	)		—			(86,200	)
Acquisitions of gathering systems from affiliate		(288,618	)		—			—			—			—			(288,618	)
Other, net		—			—			323			—			—			323
Advances to affiliates		(2,589	)		(88,221	)		(110,003	)		—			200,813			—
Net cash used in investing activities		(291,636	)		(88,221	)		(230,464	)		(237,212	)		200,813			(646,720	)

Cash flows from financing activities:
Distributions to unitholders		(152,074	)		—			—			—			—			(152,074	)
Borrowings under Revolving Credit Facility		180,000			187,000			—			—			—			367,000
Repayments under Revolving Credit Facility		(100,000	)		(51,000	)		—			—			—			(151,000	)
Repayments under term loan		(182,500	)		—			—			—			—			(182,500	)
Debt issuance costs		(135	)		(277	)		—			—			—			(412	)
Proceeds from underwritten issuance of common units, net of costs		221,977			—			—			—			—			221,977
Contribution from General Partner		4,737			—			—			—			—			4,737
Cash advance from Summit Investments to contributed subsidiaries, net		102,500			—			18,811			199,216			—			320,527
Expenses paid by Summit Investments on behalf of contributed subsidiaries		12,655			—			3,864			6,360			—			22,879
Other, net		(1,615	)		—			(192	)		—			—			(1,807	)
Advances from affiliates		198,224			—			—			2,589			(200,813	)		—
Net cash provided by financing activities		283,769			135,723			22,483			208,165			(200,813	)		449,327
Net change in cash and cash equivalents		(7,458	)		786			(297	)		951			—			(6,018	)
Cash and cash equivalents, beginning of period		7,531			11,621			7,407			1,252			—			27,811
Cash and cash equivalents, end of period	$	73		$	12,407		$	7,110		$	2,203		$	—		$	21,793

UNAUDITED QUARTERLY FINANCIAL DATA

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

UNAUDITED QUARTERLY FINANCIAL DATA

18. UNAUDITED QUARTERLY FINANCIAL DATA

Summarized information on the consolidated results of operations for each of the quarters during the two-year period ended December 31, 2017, follows.

	Quarter ended
	December 31, 2017			September 30, 2017		June 30, 2017		March 31, 2017
	(In thousands, except per-unit amounts)
Total revenues	$	126,199		$	124,945	$	101,792	$	135,805

Net (loss) income attributable to SMLP	$	(18,331	)	$	93,546	$	11,157	$	(685	)
Less net (loss) income and IDRs attributable to General Partner		1,760			3,999		2,351		2,092
Less net income attributable to Series A Preferred Units		3,563			—		—		—
Net (loss) income attributable to common limited partners	$	(23,654	)	$	89,547	$	8,806	$	(2,777	)

(Loss) earnings per limited partner unit:
Common unit - basic	$	(0.32	)	$	1.23	$	0.12	$	(0.04	)
Common unit - diluted	$	(0.32	)	$	1.22	$	0.12	$	(0.04	)

	Quarter ended
	December 31, 2016		September 30, 2016			June 30, 2016			March 31, 2016
	(In thousands, except per-unit amounts)
Total revenues	$	127,083	$	95,073		$	89,635		$	90,571

Net income (loss) attributable to SMLP	$	13,901	$	1,922		$	(50,287	)	$	(6,454	)
Less net income (loss) and IDRs attributable to General Partner		2,379		2,137			935			1,810
Net income (loss) attributable to common limited partners	$	11,522	$	(215	)	$	(51,222	)	$	(8,264	)

Earnings (loss) per limited partner unit:
Common unit - basic	$	0.16	$	0.00		$	(0.77	)	$	(0.12	)
Common unit - diluted	$	0.16	$	0.00		$	(0.77	)	$	(0.12	)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Cash and Cash Equivalents

Cash and Cash Equivalents. We consider all highly liquid investments with an original maturity of three months or less to be cash equivalents.

Accounts Receivable

Other Current Assets

Other Current Assets. Other current assets primarily consist of the current portion of prepaid expenses that are charged to expense over the period of benefit or the life of the related contract.

Property, Plant and Equipment

Useful lives

(In years)

Gathering and processing systems and related equipment

12-30

Other

4-15

Construction in progress is depreciated consistent with its applicable asset class once it is placed in service. Land and line fill are not depreciated.

Accrued capital expenditures are reflected in trade accounts payable.

Asset Retirement Obligations

Amortizing Intangibles

Goodwill

If the reporting unit’s fair value exceeds its carrying amount, we conclude that the goodwill of the reporting unit has not been impaired and no further work is performed.

If we determine that the reporting unit’s carrying value exceeds its fair value, we recognize the excess of the carrying value over the fair value as an impairment loss.

Equity Method Investments

Other Noncurrent Assets

Debt Issuance Costs

Deferred Purchase Price Obligation

Impairment of Long-Lived Assets

Derivative Contracts

Fair Value of Financial Instruments

•

Level 1. Inputs represent quoted prices in active markets for identical assets or liabilities;

•

Commitments and Contingencies

Noncontrolling Interest

Revenue Recognition

We provide gathering and/or processing services principally under contracts that contain one or more of the following arrangements:

•

Unit-Based Compensation

Income Taxes

Earnings or Loss Per Unit

Comprehensive Income or Loss

Comprehensive Income or Loss. Comprehensive income or loss is the same as net income or loss for all periods presented.

Environmental Matters

Carve-Out Entities, Assets, Liabilities and Expenses and Allocation of Certain Liabilities in Drop Downs

Accounting Pronouncements

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

•

Accounting Pronouncements Pending Adoption. We have not yet adopted the following accounting pronouncements as of December 31, 2017:

•

Basis of Presentation

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Useful lives of Property, Plant and Equipment

Estimates of useful lives follow.

Useful lives

(In years)

Gathering and processing systems and related equipment

12-30

Other

4-15

SEGMENT INFORMATION (Tables)

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Schedule of assets by reportable segment

Assets by reportable segment follow.

	December 31,
	2017			2016			2015
	(In thousands)
Assets:
Utica Shale	$	212,311		$	199,392		$	135,056
Ohio Gathering		690,485			707,415			751,168
Williston Basin		512,860			724,084			740,361
Piceance/DJ Basins		798,722			843,440			866,095
Barnett Shale		383,306			404,314			416,586
Marcellus Shale		217,362			224,709			233,116
Total reportable segment assets		2,815,046			3,103,354			3,142,382
Corporate and other		79,996			12,294			22,290
Eliminations		(249	)		(469	)		—
Total assets	$	2,894,793		$	3,115,179		$	3,164,672

Schedule of segment reporting information

Revenues by reportable segment follow.

	Year ended December 31,
	2017			2016			2015
	(In thousands)
Revenues (1):
Utica Shale	$	38,907		$	24,263		$	4,700
Williston Basin		161,503			122,174			98,929
Piceance/DJ Basins		166,753			149,903			180,418
Barnett Shale		71,667			79,956			88,042
Marcellus Shale		30,394			26,111			28,468
Total reportable segments revenue		469,224			402,407			400,557
Corporate and other		26,446			412			—
Eliminations		(6,929	)		(457	)		—
Total revenues	$	488,741		$	402,362		$	400,557

(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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Depreciation and amortization (1):
Utica Shale	$	7,009	$	4,331	$	1,417
Williston Basin		33,772		33,676		31,376
Piceance/DJ Basins		48,925		49,140		47,433
Barnett Shale (2)		15,001		16,093		16,392
Marcellus Shale		9,047		8,841		8,682
Total reportable segment depreciation and amortization		113,754		112,081		105,300
Corporate and other		1,118		580		603
Total depreciation and amortization	$	114,872	$	112,661	$	105,903

(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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Cash paid for capital expenditures (1):
Utica Shale	$	22,921	$	78,708	$	94,994
Williston Basin		17,309		31,541		147,477
Piceance/DJ Basins		23,714		25,719		21,144
Barnett Shale		569		3,910		6,875
Marcellus Shale		641		1,173		1,306
Total reportable segment capital expenditures		65,154		141,051		271,796
Corporate and other		59,061		1,668		429
Total cash paid for capital expenditures	$	124,215	$	142,719	$	272,225

(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:

	Year ended December 31,
	2017			2016			2015
Percentage of total revenues (1)(2):
Counterparty A - Piceance/DJ Basins	*				14	%		16	%
Counterparty B - Williston Basin		13	%	*			*
Counterparty C - Piceance/DJ Basins	*			*				14	%

(1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments (see Note 8).

(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.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Reportable segment adjusted EBITDA
Utica Shale	$	34,011	$	21,035	$	2,206
Ohio Gathering		41,246		45,602		33,667
Williston Basin		66,413		79,475		34,008
Piceance/DJ Basins		117,737		109,241		110,222
Barnett Shale		46,232		54,634		59,526
Marcellus Shale		23,888		19,203		23,214
Total of reportable segments' measures of profit or loss	$	329,527	$	329,190	$	262,843

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.

	Year ended December 31,
	2017			2016			2015
	(In thousands)
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:
Income (loss) before income taxes and income (loss) from equity method investees	$	88,614		$	(7,768	)	$	(216,268	)
Add:
Corporate and other		39,140			37,589			27,352
Interest expense		68,131			63,810			59,092
Early extinguishment of debt		22,039			—			—
Deferred Purchase Price Obligation		(200,322	)		55,854			—
Depreciation and amortization		114,872			112,661			105,903
Proportional adjusted EBITDA for equity method investees		41,246			45,602			33,667
Adjustments related to MVC shortfall payments		(41,373	)		11,600			(11,902	)
Unit-based and noncash compensation		7,951			7,985			7,017
Loss (gain) on asset sales, net		527			93			(172	)
Long-lived asset impairment		188,702			1,764			9,305
Goodwill impairment		—			—			248,851
Less:
Interest income		—			—			2
Total of reportable segments' measures of profit or loss	$	329,527		$	329,190		$	262,843

Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments

Adjustments related to MVC shortfall payments by reportable segment follow.

	Year ended December 31, 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	)	$	(3,065	)	$	—		$	(40,758	)
Expected MVC shortfall adjustments		—			(3	)		(612	)		(615	)
Total adjustments related to MVC shortfall payments	$	(37,693	)	$	(3,068	)	$	(612	)	$	(41,373	)

	Year ended December 31, 2016
	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	$	8,691	$	3,288		$	(677	)	$	11,302
Expected MVC shortfall adjustments		—		(317	)		615			298
Total adjustments related to MVC shortfall payments	$	8,691	$	2,971		$	(62	)	$	11,600

	Year Ended December 31, 2015
	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	$	11,870	$	(21,623	)	$	(1,700	)	$	(11,453	)
Expected MVC shortfall adjustments		—		33			(482	)		(449	)
Total adjustments related to MVC shortfall payments	$	11,870	$	(21,590	)	$	(2,182	)	$	(11,902	)

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of property, plant, and equipment, net

Details on property, plant and equipment follow.

	December 31,
	2017		2016
	(In thousands)
Gathering and processing systems and related equipment	$	1,973,722	$	2,026,363
Construction in progress		78,850		39,954
Land and line fill		11,735		11,442
Other		40,262		35,227
Total		2,104,569		2,112,986
Less accumulated depreciation		309,440		259,315
Property, plant and equipment, net	$	1,795,129	$	1,853,671

Schedule of long-lived asset impairments

During 2017, 2016 and 2015, we recognized the following long-lived asset impairments, by segment.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Long-lived asset impairment:
Williston Basin	$	101,961	$	569	$	7,554
Piceance/DJ Basins		697		—		1,220
Barnett Shale		—		1,195		531
Utica Shale		878		—		—

Schedule Of Depreciation Expense And Capitalized Interest Costs Table Text Block

Depreciation expense and capitalized interest follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Depreciation expense	$	75,120	$	70,770	$	63,915
Capitalized interest		2,579		3,709		3,372

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Tables)

12 Months Ended

Dec. 31, 2017

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.

	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

	December 31, 2016
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(10,795	)	$	13,400
Contract intangibles		426,464		(146,468	)		279,996
Rights-of-way		153,015		(24,959	)		128,056
Total intangible assets	$	603,674	$	(182,222	)	$	421,452

Unfavorable gas gathering contract	$	10,962	$	(6,916	)	$	4,046

Recognized amortization expense in other revenues and cost and expenses

We recognized amortization expense in other revenues as follows:

	Year ended December 31,
	2017			2016			2015
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(1,555	)	$	(1,261	)	$	(1,478	)
Amortization expense – unfavorable gas gathering contract		2,158			839			692

We recognized amortization expense in costs and expenses as follows:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Amortization expense – contract intangibles	$	34,202	$	35,416	$	35,339
Amortization expense – rights-of-way		6,153		6,053		5,863

Estimated aggregate annual amortization expected to be recognized

The estimated aggregate annual amortization expected to be recognized for as of December 31, 2017 for each of the five succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	34,026	$	1,888
2019		33,139		—
2020		32,963		—
2021		29,271		—
2022		26,204		—

GOODWILL (Tables)

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

Rollforward of goodwill by reportable segment

Accumulated goodwill impairments by reportable segment for those reporting units that have previously recognized goodwill follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Accumulated goodwill impairment:
Piceance/DJ Basins	$	45,478	$	45,478	$	45,478
Williston Basin		257,572		257,572		257,572
Total accumulated goodwill impairment	$	303,050	$	303,050	$	303,050

EQUITY METHOD INVESTMENTS (Tables)

12 Months Ended

Dec. 31, 2017

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).

	2017			2016
	(In thousands)
Investment in equity method investees, December 31	$	690,485		$	707,415
December cash distributions		4,032			3,172
December cash contributions		(3,932	)		(5,318	)
Impairment loss		1,383			—
Basis difference		(130,184	)		(143,536	)
Investment in equity method investees, net of basis difference, November 30	$	561,784		$	561,733

Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information).

	November 30, 2017				November 30, 2016
	OGC		OCC		OGC		OCC
	(In thousands)
Current assets	$	34,383	$	3,650	$	43,797	$	2,546
Noncurrent assets		1,319,448		29,156		1,330,199		31,195
Total assets	$	1,353,831	$	32,806	$	1,373,996	$	33,741

Current liabilities	$	10,882	$	3,382	$	22,067	$	3,448
Noncurrent liabilities		3,272		11,715		8,396		13,111
Total liabilities	$	14,154	$	15,097	$	30,463	$	16,559

Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information).

	Twelve months ended November 30, 2017					Twelve months ended November 30, 2016					Twelve months ended November 30, 2015
	OGC		OCC			OGC		OCC			OGC		OCC
	(In thousands)
Total revenues	$	140,679	$	8,607		$	148,662	$	15,791		$	120,623	$	9,467
Total operating expenses		111,897		8,298			96,647		111,528			96,948		15,633
Net income (loss)		28,785		(907	)		52,009		(94,230	)		23,655		(6,852	)

DEFERRED REVENUE (Tables)

12 Months Ended

Dec. 31, 2017

Deferred Revenue Disclosure [Abstract]

Rollforward of deferred revenue

A rollforward of current deferred revenue follows.

	Williston Basin		Piceance/DJ Basins		Barnett Shale		Total current
	(In thousands)
Current deferred revenue, January 1, 2016	$	—	$	—	$	677	$	677
Additions		—		11,672		—		11,672
Less revenue recognized		—		11,672		677		12,349
Current deferred revenue, December 31, 2016		—		—		—		—
Additions		—		18,294		—		18,294
Less revenue recognized		—		14,294		—		14,294
Current deferred revenue, December 31, 2017	$	—	$	4,000	$	—	$	4,000

A rollforward of noncurrent deferred revenue follows.

	Williston Basin		Piceance/DJ Basins		Barnett Shale		Total noncurrent
	(In thousands)
Noncurrent deferred revenue, January 1, 2016	$	29,002	$	16,484	$	—	$	45,486
Additions		8,691		3,700		—		12,391
Less revenue recognized		—		412		—		412
Noncurrent deferred revenue, December 31, 2016		37,693		19,772		—		57,465
Less revenue recognized		37,693		3,065		—		40,758
Less reclassification to current deferred revenue		—		4,000		—		4,000
Noncurrent deferred revenue, December 31, 2017	$	—	$	12,707	$	—	$	12,707

DEBT (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of debt and capital leases

Debt consisted of the following:

	December 31,
	2017			2016
	(In thousands)
Summit Holdings variable rate senior secured Revolving Credit Facility (4.07% at December 31, 2017 and 3.27% at December 31, 2016) due May 2022	$	261,000		$	648,000
Summit Holdings 5.5% senior unsecured notes due August 2022		300,000			300,000
Less unamortized debt issuance costs (1)		(2,910	)		(3,516	)
Summit Holdings 5.75% senior unsecured notes due April 2025		500,000			—
Less unamortized debt issuance costs (1)		(6,898	)		—
Summit Holdings 7.5% senior unsecured notes redeemed March 2017 (2)		—			300,000
Less unamortized debt issuance costs (1)(2)		—			(4,183	)
Total long-term debt	$	1,051,192		$	1,240,301

__________

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

Schedule of maturities of long-term debt

The aggregate amount of debt maturing during each of the years after December 31, 2017 are as follow (in thousands):

2018	$	—
2019		—
2020		—
2021		—
2022		561,000
Thereafter		500,000
Total long-term debt	$	1,061,000

- 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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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 4: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

FINANCIAL INSTRUMENTS (Tables)

12 Months Ended

Dec. 31, 2017

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, 2016	$	—
Addition		507,427
Change in fair value		55,854
Level 3 liability, December 31, 2016		563,281
Change in fair value		(200,322	)
Level 3 liability, December 31, 2017	$	362,959

Summary of the estimated fair value of debt instruments

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

	December 31, 2017				December 31, 2016
	Carrying value		Estimated fair value (Level 2)		Carrying value		Estimated fair value (Level 2)
	(In thousands)
Summit Holdings Revolving Credit Facility	$	261,000	$	261,000	$	648,000	$	648,000
Summit Holdings 5.5% Senior Notes ($300.0 million principal)		297,090		301,750		296,484		294,500
Summit Holdings 5.75% Senior Notes ($500.0 million principal)		493,102		501,667		—		—
Summit Holdings 7.5% Senior Notes ($300.0 million principal) (1)		—		—		295,817		316,000

(1)	Debt was extinguished following the 5.75% Senior Notes offering in February 2017. In conjunction with the early debt extinguishment, the remaining unamortized debt issuance costs were written off.

PARTNERS' CAPITAL (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Schedule of partner units activity

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

	Limited partners
	Series A Preferred Units		Common		Subordinated			General Partner
Units, January 1, 2015		—		34,426,513		24,409,850			1,200,651
Units issued in connection with the May 2015 Equity Offering		—		7,475,000		—			—
General Partner 2% contribution		—		—		—			152,551
Net units issued under SMLP LTIP		—		161,131		—			1,498
Units, December 31, 2015		—		42,062,644		24,409,850			1,354,700
Subordinated units conversion		—		24,409,850		(24,409,850	)		—
Units issued in connection with the September 2016 Equity Offering		—		5,500,000		—			—
General Partner 2% contribution		—		—		—			112,245
Net units issued under SMLP LTIP		—		138,627		—			4,242
Units, December 31, 2016		—		72,111,121		—			1,471,187
Units issued in connection with the November 2017 Equity Offering		300,000		—		—			—
Net units issued under SMLP LTIP		—		211,327		—			—
Units issued under ATM program		—		763,548		—			—
General Partner 2% contribution		—		—		—			19,812
Units, December 31, 2017		300,000		73,085,996		—			1,490,999

Schedule of Capital Distribution and Allocation to Partners

The calculation of the capital contribution and its allocation to partners' capital follows (in thousands).

Summit Investments' net investment in the 2016 Drop Down Assets	$	771,929
SMP Holdings borrowings allocated to 2016 Drop Down Assets and retained by Summit Investments	342,926
Acquired carrying value of 2016 Drop Down Assets				$	1,114,855

Deferred Purchase Price Obligation	$	507,427
Borrowings under Revolving Credit Facility	360,000
Working capital adjustment received from a subsidiary of Summit Investments	(569		)
Total consideration paid and recognized by SMLP				866,858
Excess of acquired carrying value over consideration paid and recognized				$	247,997

Allocation of capital contribution:
General Partner interest	$	4,953
Common limited partner interest	243,044
Partners' capital contribution – excess of acquired carrying value over consideration paid and recognized				$	247,997

The calculation of the capital contribution and its allocation to partners' capital follow (in thousands).

Summit Investments' net investment in Polar Midstream and Epping			$	416,044
Total net cash consideration paid to a subsidiary of Summit Investments			285,677
Excess of acquired carrying value over consideration paid			$	130,367

Allocation of capital contribution:
General Partner interest	$	2,607
Common limited partner interest	80,079
Subordinated limited partner interest	47,681
Partners' capital contribution – excess of acquired carrying value over consideration paid			$	130,367

Details of cash distributions

We paid the following per-unit distributions during the years ended December 31:

	Year ended December 31,
	2017		2016		2015
Per-unit distributions to unitholders	$	2.300	$	2.300	$	2.270

Our payment of IDRs as reported in distributions to unitholders – General Partner in the statements of partners' capital during the years ended December 31 follow.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
IDR payments	$	8,460	$	7,912	$	6,743

Schedule of partnership target distributions

The following table illustrates the percentage allocations of available cash between the unitholders and our General Partner based on the specified target distribution levels. The amounts set forth in the column Marginal Percentage Interest in Distributions are the percentage interests of our General Partner and the unitholders in any available cash we distribute up to and including the corresponding amount in the column Total Quarterly Distribution Per Unit Target Amount. The percentage interests shown for our unitholders and our General Partner for the minimum quarterly distribution are also applicable to quarterly distribution amounts that are less than the MQD. The percentage interests set forth below for our General Partner assume (i) a 2% general partner interest, (ii) that our General Partner has not transferred its IDRs and (iii) that there are no arrearages on common units.

		Marginal percentage interest in distributions
	Total quarterly distribution per unit target amount	Unitholders	General Partner
Minimum quarterly distribution	$0.40	98%	2%
First target distribution	$0.40 up to $0.46	98%	2%
Second target distribution	above $0.46 up to $0.50	85%	15%
Third target distribution	above $0.50 up to $0.60	75%	25%
Thereafter	above $0.60	50%	50%

EARNINGS PER UNIT (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedule of earnings per limited partner unit

The following table details the components of EPU.

	Year ended December 31,
	2017		2016			2015
	(In thousands, except per-unit amounts)
Numerator for basic and diluted EPU:
Allocation of net income (loss) among limited partner interests:
Net income (loss) attributable to limited partners	$	75,485	$	(48,179	)	$	(125,437	)
Net loss attributable to subordinated units (1)		—		—			(70,173	)
Less net income attributable to Series A Preferred Units		3,563		—			—
Net income (loss) attributable to common limited partners	$	71,922	$	(48,179	)	$	(195,610	)

Denominator for basic and diluted EPU:
Weighted-average common units outstanding – basic		72,705		68,264			39,217
Effect of nonvested phantom units		342		—			—
Weighted-average common units outstanding – diluted		73,047		68,264			39,217

Weighted-average subordinated units outstanding – basic and diluted							24,410

Earnings (loss) per limited partner unit:
Common unit – basic	$	0.99	$	(0.71	)	$	(3.20	)
Common unit – diluted	$	0.98	$	(0.71	)	$	(3.20	)
Subordinated unit – basic and diluted (1)						$	(2.88	)

Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		42		125			109

__________

(1) The subordination period ended on February 16, 2016 and all 24,409,850 subordinated units converted to common units on a one-for-one basis (see Note 11).

UNIT-BASED AND NONCASH COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of activity

The following table presents phantom and restricted unit activity:

	Units			Weighted-average grant date fair value
Nonvested phantom units, January 1, 2015		336,202		$	30.61
Phantom units granted		289,735			29.21
Phantom units vested		(229,497	)		27.66
Phantom units forfeited		(16,529	)		35.09
Nonvested phantom units, December 31, 2015		379,911			31.13
Phantom units granted		495,535			14.91
Phantom units vested		(178,953	)		33.80
Phantom units forfeited		(4,538	)		16.89
Nonvested phantom units, December 31, 2016		691,955			19.59
Phantom units granted		371,972			22.50
Phantom units vested		(293,222	)		24.76
Phantom units forfeited		(21,431	)		20.07
Nonvested phantom units, December 31, 2017		749,274		$	20.07

Schedule of intrinsic values

The intrinsic value of phantom and restricted units that vested during the years ended December 31, follows.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Intrinsic value of vested LTIP awards	$	6,657	$	2,957	$	5,362

Schedule of unit-based compensation recognized in general and administrative expense

Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP follows.

	Year ended December 31,
	2017		2016		2015
	(In thousands)
SMLP LTIP unit-based compensation	$	7,951	$	7,550	$	6,174

RELATED-PARTY TRANSACTIONS (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of related party transactions

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

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Operation and maintenance expense	$	27,450	$	26,485	$	25,050
General and administrative expense		30,899		31,947		26,193

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2017

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:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Rent expense	$	3,772	$	2,861	$	2,395

Future minimum lease payments due under noncancelable operating leases

Future minimum lease payments due under noncancelable operating leases at December 31, 2017, were as follows (in thousands):

2018	$	3,373
2019		3,181
2020		824
2021		389
2022		349
Thereafter		731
Total future minimum lease payments	$	8,847

Schedule of accrued environmental remediation

A rollforward of the aggregate accrued environmental remediation liabilities follows.

	Total
	(In thousands)
Accrued environmental remediation, January 1, 2016	$	13,664
Payments made, including those by affiliates		(4,211	)
Accrued environmental remediation, December 31, 2016	$	9,453
Payments made		(4,109	)
Accrued environmental remediation, December 31, 2017	$	5,344

ACQUISITIONS AND DROP DOWN TRANSACTIONS (Tables)

12 Months Ended

Dec. 31, 2017

Business Combinations [Abstract]

Fair Value of Assets Acquired and Liabilities Assumed

The fair values of the assets acquired and liabilities assumed as of February 15, 2013, were as follows (in thousands):

Purchase price assigned to Meadowlark Midstream			$	25,376
Current assets	$	2,227
Property, plant and equipment		18,795
Other noncurrent assets		4,354
Total assets acquired		25,376
Total liabilities assumed	$	—
Net identifiable assets acquired			$	25,376

Business Combination, Revenue and Net Income Disclosures

Revenues and net income for the previously separate entities and the combined amounts, as presented in these consolidated financial statements follow.

	Year ended December 31,
	2016			2015
	(In thousands)
SMLP revenues	$	393,495		$	358,046
2016 Drop Down Assets revenues (1)		8,867			29,238
Polar and Divide revenues (1)		—			13,273
Combined revenues	$	402,362		$	400,557

SMLP net loss	$	(40,932	)	$	(192,212	)
2016 Drop Down Assets net income (loss) (1)		2,745			(35,419	)
Polar and Divide net income (1)		—			5,403
Combined net loss	$	(38,187	)	$	(222,228	)

_______

(1) Results are fully reflected in SMLP's results of operations subsequent to closing the respective drop down.

CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Tables)

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Condensed Balance Sheet

Balance sheets as of December 31, 2017 and 2016 follow.

	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

	December 31, 2016
	SMLP		Co-Issuers		Guarantor Subsidiaries		Non-Guarantor Subsidiaries		Consolidating adjustments			Total
	(In thousands)
Assets
Cash and cash equivalents	$	698	$	51	$	5,768	$	911	$	—		$	7,428
Accounts receivable		53		—		91,152		6,159		—			97,364
Other current assets		1,526		—		2,428		355		—			4,309
Due from affiliate		11,768		38,013		366,867		—		(416,648	)		—
Total current assets		14,045		38,064		466,215		7,425		(416,648	)		109,101
Property, plant and equipment, net		2,266		—		1,508,824		342,581		—			1,853,671
Intangible assets, net		—		—		398,992		22,460		—			421,452
Goodwill		—		—		16,211		—		—			16,211
Investment in equity method investees		—		—		—		707,415		—			707,415
Other noncurrent assets		1,993		5,198		138		—		—			7,329
Investment in subsidiaries		2,132,757		3,347,393		—		—		(5,480,150	)		—
Total assets	$	2,151,061	$	3,390,655	$	2,390,380	$	1,079,881	$	(5,896,798	)	$	3,115,179

Liabilities and Partners' Capital
Trade accounts payable	$	978	$	—	$	10,640	$	4,633	$	—		$	16,251
Accrued expenses		2,399		114		6,284		2,592		—			11,389
Due to affiliate		405,138		—		—		11,768		(416,648	)		258
Ad valorem taxes payable		16		—		9,847		725		—			10,588
Accrued interest		—		17,483		—		—		—			17,483
Accrued environmental remediation		—		—		—		4,301		—			4,301
Other current liabilities		6,717		—		4,047		707		—			11,471
Total current liabilities		415,248		17,597		30,818		24,726		(416,648	)		71,741
Long-term debt		—		1,240,301		—		—		—			1,240,301
Deferred Purchase Price Obligation		563,281		—		—		—		—			563,281
Deferred revenue		—		—		57,465		—		—			57,465
Noncurrent accrued environmental remediation		—		—		—		5,152		—			5,152
Other noncurrent liabilities		2,859		—		4,602		105		—			7,566
Total liabilities		981,388		1,257,898		92,885		29,983		(416,648	)		1,945,506

Total partners' capital		1,169,673		2,132,757		2,297,495		1,049,898		(5,480,150	)		1,169,673
Total liabilities and partners' capital	$	2,151,061	$	3,390,655	$	2,390,380	$	1,079,881	$	(5,896,798	)	$	3,115,179

Condensed Statement of Operations

Statements of operations for the years ended December 31, 2017, 2016 and 2015 follow.

	Year ended December 31, 2017
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments		Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	334,625		$	59,802		$	—	$	394,427
Natural gas, NGLs and condensate sales		—			—			68,459			—			—		68,459
Other revenues		—			—			25,583			272			—		25,855
Total revenues		—			—			428,667			60,074			—		488,741
Costs and expenses:
Cost of natural gas and NGLs		—			—			57,237			—			—		57,237
Operation and maintenance		—			—			85,599			8,283			—		93,882
General and administrative		—			—			47,878			6,803			—		54,681
Depreciation and amortization		1,101			—			101,319			13,055			—		115,475
Transaction costs		73			—			—			—			—		73
(Gain) loss on asset sales, net		—			—			(15	)		542			—		527
Long-lived asset impairment		—			—			187,823			879			—		188,702
Total costs and expenses		1,174			—			479,841			29,562			—		510,577
Other income		298			—			—			—			—		298
Interest expense		—			(68,131	)		—			—			—		(68,131	)
Early extinguishment of debt		—			(22,039	)		—			—			—		(22,039	)
Deferred Purchase Price Obligation		200,322			—			—			—			—		200,322
Income (loss) before income taxes and loss from equity method investees		199,446			(90,170	)		(51,174	)		30,512			—		88,614
Income tax expense		(341	)		—			—			—			—		(341	)
Loss from equity method investees		—			—			—			(2,223	)		—		(2,223	)
Equity in loss of consolidated subsidiaries		(113,055	)		(22,885	)		—			—			135,940		—
Net income	$	86,050		$	(113,055	)	$	(51,174	)	$	28,289		$	135,940	$	86,050

	Year ended December 31, 2016
	SMLP			Co-Issuers			Guarantor Subsidiaries		Non-Guarantor Subsidiaries			Consolidating adjustments			Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	294,837	$	51,124		$	—		$	345,961
Natural gas, NGLs and condensate sales		—			—			35,833		—			—			35,833
Other revenues		—			—			20,227		341			—			20,568
Total revenues		—			—			350,897		51,465			—			402,362
Costs and expenses:
Cost of natural gas and NGLs		—			—			27,421		—			—			27,421
Operation and maintenance		—			—			89,031		6,303			—			95,334
General and administrative		—			—			44,922		7,488			—			52,410
Depreciation and amortization		580			—			101,415		10,244			—			112,239
Transaction costs		1,321			—			—		—			—			1,321
Loss (gain) on asset sales, net		—			—			97		(4	)		—			93
Long-lived asset impairment		—			—			1,235		529			—			1,764
Total costs and expenses		1,901			—			264,121		24,560			—			290,582
Other income		116			—			—		—			—			116
Interest expense		(1,441	)		(62,369	)		—		—			—			(63,810	)
Deferred Purchase Price Obligation		(55,854	)		—			—		—			—			(55,854	)
(Loss) income before income taxes and loss from equity method investees		(59,080	)		(62,369	)		86,776		26,905			—			(7,768	)
Income tax expense		(75	)		—			—		—			—			(75	)
Loss from equity method investees		—			—			—		(30,344	)		—			(30,344	)
Equity in earnings of consolidated subsidiaries		20,968			83,337			—		—			(104,305	)		—
Net (loss) income	$	(38,187	)	$	20,968		$	86,776	$	(3,439	)	$	(104,305	)	$	(38,187	)

	Year ended December 31, 2015
	SMLP			Co-Issuers			Guarantor Subsidiaries			Non-Guarantor Subsidiaries			Consolidating adjustments		Total
	(In thousands)
Revenues:
Gathering services and related fees	$	—		$	—		$	320,454		$	17,365		$	—	$	337,819
Natural gas, NGLs and condensate sales		—			—			42,079			—			—		42,079
Other revenues		—			—			20,283			376			—		20,659
Total revenues		—			—			382,816			17,741			—		400,557
Costs and expenses:
Cost of natural gas and NGLs		—			—			31,398			—			—		31,398
Operation and maintenance		—			—			91,763			3,223			—		94,986
General and administrative		—			—			40,134			4,974			—		45,108
Depreciation and amortization		603			—			98,002			6,512			—		105,117
Transaction costs		1,342			—			—			—			—		1,342
Environmental remediation		—			—			—			21,800			—		21,800
Gain on asset sales, net		—			—			(172	)		—			—		(172	)
Long-lived asset impairment		—			—			9,305			—			—		9,305
Goodwill impairment		—			—			248,851			—			—		248,851
Total costs and expenses		1,945			—			519,281			36,509			—		557,735
Other income		2			—			—			—			—		2
Interest expense		(10,494	)		(48,598	)		—			—			—		(59,092	)
Loss before income taxes and loss from equity method investees		(12,437	)		(48,598	)		(136,465	)		(18,768	)		—		(216,268	)
Income tax benefit		603			—			—			—			—		603
Loss from equity method investees		—			—			—			(6,563	)		—		(6,563	)
Equity in loss of consolidated subsidiaries		(210,394	)		(161,796	)		—			—			372,190		—
Net loss	$	(222,228	)	$	(210,394	)	$	(136,465	)	$	(25,331	)	$	372,190	$	(222,228	)

Condensed Cash Flow Statement

Statements of cash flows for the years ended December 31, 2017 and 2016 follow.

	Year ended December 31, 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	$	7,122		$	(68,915	)	$	221,115		$	78,510		$	—		$	237,832

Cash flows from investing activities:
Capital expenditures		(3,041	)		—			(98,926	)		(22,248	)		—			(124,215	)
Proceeds from asset sales		—			—			—			2,300			—			2,300
Contributions to equity method investees		—			—			—			(25,513	)		—			(25,513	)
Purchase of noncontrolling interest		(797	)		—			—			—			—			(797	)
Other, net		(458	)		—			—			—			—			(458	)
Advances to affiliates		(278,493	)		—			(126,198	)		(22,031	)		426,722			—
Net cash used in investing activities		(282,789	)		—			(225,124	)		(67,492	)		426,722			(148,683	)

Cash flows from financing activities:
Distributions to unitholders		(181,478	)		—			—			—			—			(181,478	)
Borrowings under Revolving Credit Facility		—			247,500			—			—			—			247,500
Repayments under Revolving Credit Facility		—			(634,500	)		—			—			—			(634,500	)
Debt issuance costs		—			(16,390	)		—			—			—			(16,390	)
Payment of redemption and call premiums on senior notes		—			(17,932	)		—			—			—			(17,932	)
Proceeds from ATM Program common unit issuances, net of costs		17,078			—			—			—			—			17,078
Proceeds from issuance of Series A Preferred Units, net of costs		293,238			—			—			—			—			293,238
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		(2,437	)		—			(621	)		(70	)		—			(3,128	)
Advances from affiliates		148,229			290,261			—			(11,768	)		(426,722	)		—
Net cash provided by (used in) financing activities		275,095			68,939			(621	)		(11,838	)		(426,722	)		(95,147	)
Net change in cash and cash equivalents		(572	)		24			(4,630	)		(820	)		—			(5,998	)
Cash and cash equivalents, beginning of period		698			51			5,768			911			—			7,428
Cash and cash equivalents, end of period	$	126		$	75		$	1,138		$	91		$	—		$	1,430

	Year ended December 31, 2016
	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	$	9,691		$	(58,254	)	$	201,516		$	77,542		$	—		$	230,495

Cash flows from investing activities:
Capital expenditures		(1,668	)		—			(55,185	)		(85,866	)		—			(142,719	)
Contributions to equity method investees		—			—			—			(31,582	)		—			(31,582	)
Acquisitions of gathering systems from affiliate		(359,431	)		—			—			—			—			(359,431	)
Other, net		(394	)		—			—			—			—			(394	)
Advances to affiliates		(15,697	)		(255,070	)		(150,775	)		—			421,542			—
Net cash used in investing activities		(377,190	)		(255,070	)		(205,960	)		(117,448	)		421,542			(534,126	)

Cash flows from financing activities:
Distributions to unitholders		(167,504	)		—			—			—			—			(167,504	)
Borrowings under Revolving Credit Facility		12,000			508,300			—			—			—			520,300
Repayments under Revolving Credit Facility		—			(204,300	)		—			—			—			(204,300	)
Debt issuance costs		—			(3,032	)		—			—			—			(3,032	)
Proceeds from underwritten issuance of common units, net of costs		125,233			—			—			—			—			125,233
Contribution from General Partner		2,702			—			—			—			—			2,702
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		(12,000	)		—			3,223			20,991			—			12,214
Expenses paid by Summit Investments on behalf of contributed subsidiaries		3,030			—			—			1,791			—			4,821
Other, net		(1,182	)		—			(121	)		135			—			(1,168	)
Advances from affiliates		405,845			—			—			15,697			(421,542	)		—
Net cash provided by (used in) financing activities		368,124			300,968			3,102			38,614			(421,542	)		289,266
Net change in cash and cash equivalents		625			(12,356	)		(1,342	)		(1,292	)		—			(14,365	)
Cash and cash equivalents, beginning of period		73			12,407			7,110			2,203			—			21,793
Cash and cash equivalents, end of period	$	698		$	51		$	5,768		$	911		$	—		$	7,428

	Year ended December 31, 2015
	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	$	409		$	(46,716	)	$	207,684		$	29,998		$	—		$	191,375

Cash flows from investing activities:
Capital expenditures		(429	)		—			(120,784	)		(151,012	)		—			(272,225	)
Contributions to equity method investees		—			—			—			(86,200	)		—			(86,200	)
Acquisitions of gathering systems from affiliate		(288,618	)		—			—			—			—			(288,618	)
Other, net		—			—			323			—			—			323
Advances to affiliates		(2,589	)		(88,221	)		(110,003	)		—			200,813			—
Net cash used in investing activities		(291,636	)		(88,221	)		(230,464	)		(237,212	)		200,813			(646,720	)

Cash flows from financing activities:
Distributions to unitholders		(152,074	)		—			—			—			—			(152,074	)
Borrowings under Revolving Credit Facility		180,000			187,000			—			—			—			367,000
Repayments under Revolving Credit Facility		(100,000	)		(51,000	)		—			—			—			(151,000	)
Repayments under term loan		(182,500	)		—			—			—			—			(182,500	)
Debt issuance costs		(135	)		(277	)		—			—			—			(412	)
Proceeds from underwritten issuance of common units, net of costs		221,977			—			—			—			—			221,977
Contribution from General Partner		4,737			—			—			—			—			4,737
Cash advance from Summit Investments to contributed subsidiaries, net		102,500			—			18,811			199,216			—			320,527
Expenses paid by Summit Investments on behalf of contributed subsidiaries		12,655			—			3,864			6,360			—			22,879
Other, net		(1,615	)		—			(192	)		—			—			(1,807	)
Advances from affiliates		198,224			—			—			2,589			(200,813	)		—
Net cash provided by financing activities		283,769			135,723			22,483			208,165			(200,813	)		449,327
Net change in cash and cash equivalents		(7,458	)		786			(297	)		951			—			(6,018	)
Cash and cash equivalents, beginning of period		7,531			11,621			7,407			1,252			—			27,811
Cash and cash equivalents, end of period	$	73		$	12,407		$	7,110		$	2,203		$	—		$	21,793

UNAUDITED QUARTERLY FINANCIAL DATA (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly summarized information on the consolidated results of operations

Summarized information on the consolidated results of operations for each of the quarters during the two-year period ended December 31, 2017, follows.

	Quarter ended
	December 31, 2017			September 30, 2017		June 30, 2017		March 31, 2017
	(In thousands, except per-unit amounts)
Total revenues	$	126,199		$	124,945	$	101,792	$	135,805

Net (loss) income attributable to SMLP	$	(18,331	)	$	93,546	$	11,157	$	(685	)
Less net (loss) income and IDRs attributable to General Partner		1,760			3,999		2,351		2,092
Less net income attributable to Series A Preferred Units		3,563			—		—		—
Net (loss) income attributable to common limited partners	$	(23,654	)	$	89,547	$	8,806	$	(2,777	)

(Loss) earnings per limited partner unit:
Common unit - basic	$	(0.32	)	$	1.23	$	0.12	$	(0.04	)
Common unit - diluted	$	(0.32	)	$	1.22	$	0.12	$	(0.04	)

	Quarter ended
	December 31, 2016		September 30, 2016			June 30, 2016			March 31, 2016
	(In thousands, except per-unit amounts)
Total revenues	$	127,083	$	95,073		$	89,635		$	90,571

Net income (loss) attributable to SMLP	$	13,901	$	1,922		$	(50,287	)	$	(6,454	)
Less net income (loss) and IDRs attributable to General Partner		2,379		2,137			935			1,810
Net income (loss) attributable to common limited partners	$	11,522	$	(215	)	$	(51,222	)	$	(8,264	)

Earnings (loss) per limited partner unit:
Common unit - basic	$	0.16	$	0.00		$	(0.77	)	$	(0.12	)
Common unit - diluted	$	0.16	$	0.00		$	(0.77	)	$	(0.12	)

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION - Narrative (Details) - shares	1 Months Ended		12 Months Ended
	Sep. 30, 2016	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)			73,086,000	72,111,000
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%
Drop Down Assets 2016 Acquisition
Organization And Business Operations [Line Items]
Ownership interest			1.00%
Subsidiary of Summit Investments
Organization And Business Operations [Line Items]
Ownership interest			1.00%		1.00%
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%		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 - Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Gathering and processing systems and related equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	12 years
Gathering and processing systems and related equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	30 years
Other \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	4 years
Other \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	15 years

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Amortizing Intangibles (Details)	12 Months Ended
	Dec. 31, 2017
Favorable gas gathering contracts \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	10 years
Favorable gas gathering contracts \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	20 years
Other Gas Gathering Contract \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	3 years
Other Gas Gathering Contract \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	25 years
Rights-of-way \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	20 years
Rights-of-way \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful lives (In years)	30 years

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Noncontrolling Interest (Details)	Dec. 31, 2017	Jun. 30, 2016
Drop Down Assets 2016 Acquisition
Entity Information [Line Items]
Ownership percentage by noncontrolling owners	1.00%	1.00%

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings or Loss Per Unit (Details)	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017
Drop Down Assets 2016 Acquisition
Entity Information [Line Items]
Ownership interest		1.00%
General Partner
Entity Information [Line Items]
General partner interest	2.00%	2.00%

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - Accounting Standards Update 2014-09 $ in Millions	Dec. 31, 2017 USD ($)
Property, Plant and Equipment, Net \| Contributions In Aid of Construction
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	$ 34.8
Partners Capital \| Contributions In Aid of Construction
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	8.7
Partners Capital \| Facility Fee
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	(16.2)
Deferred Revenue \| Contributions In Aid of Construction
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	26.1
Deferred Revenue \| Facility Fee
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	16.2
Deferred Revenue and Retained Earnings \| MVC Arrangements with Banking Mechanisms
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative expected impact upon adoption of new accounting principle	$ 16.7

SEGMENT INFORMATION - Assets, Revenues, Depreciation and Amortization, and Capital Expenditures by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Assets	$ 2,894,793				$ 3,115,179				$ 2,894,793	$ 3,115,179	$ 3,164,672
Revenues	126,199	$ 124,945	$ 101,792	$ 135,805	127,083	$ 95,073	$ 89,635	$ 90,571	488,741	402,362	400,557
Depreciation and amortization									114,872	112,661	105,903
Capital expenditures									124,215	142,719	272,225
Corporate
Segment Reporting Information [Line Items]
Capital expenditures									3,000
Reportable Segments
Segment Reporting Information [Line Items]
Assets	2,815,046				3,103,354				2,815,046	3,103,354	3,142,382
Revenues									469,224	402,407	400,557
Depreciation and amortization									113,754	112,081	105,300
Capital expenditures									65,154	141,051	271,796
Reportable Segments \| Utica Shale
Segment Reporting Information [Line Items]
Assets	212,311				199,392				212,311	199,392	135,056
Revenues									38,907	24,263	4,700
Depreciation and amortization									7,009	4,331	1,417
Capital expenditures									22,921	78,708	94,994
Reportable Segments \| Ohio Gathering
Segment Reporting Information [Line Items]
Assets	690,485				707,415				690,485	707,415	751,168
Reportable Segments \| Williston Basin
Segment Reporting Information [Line Items]
Assets	512,860				724,084				512,860	724,084	740,361
Revenues									161,503	122,174	98,929
Depreciation and amortization									33,772	33,676	31,376
Capital expenditures									17,309	31,541	147,477
Reportable Segments \| Piceance/DJ Basins
Segment Reporting Information [Line Items]
Assets	798,722				843,440				798,722	843,440	866,095
Revenues									166,753	149,903	180,418
Depreciation and amortization									48,925	49,140	47,433
Capital expenditures									23,714	25,719	21,144
Reportable Segments \| Barnett Shale
Segment Reporting Information [Line Items]
Assets	383,306				404,314				383,306	404,314	416,586
Revenues									71,667	79,956	88,042
Depreciation and amortization									15,001	16,093	16,392
Capital expenditures									569	3,910	6,875
Reportable Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Assets	217,362				224,709				217,362	224,709	233,116
Revenues									30,394	26,111	28,468
Depreciation and amortization									9,047	8,841	8,682
Capital expenditures									641	1,173	1,306
Corporate and other
Segment Reporting Information [Line Items]
Assets	79,996				12,294				79,996	12,294	22,290
Revenues									26,446	412
Depreciation and amortization									1,118	580	603
Capital expenditures									59,061	1,668	429
Eliminations
Segment Reporting Information [Line Items]
Assets	$ (249)				$ (469)				(249)	(469)	$ 0
Revenues									$ (6,929)	$ (457)

SEGMENT INFORMATION - Concentration Risk (Details) - Revenue - Customer concentration	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Counterparty A - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	14.00%	16.00%
Counterparty B - Williston Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	13.00%	0.00%	0.00%
Counterparty C - Piceance/DJ Basins
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	0.00%	14.00%

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 329,527	$ 329,190	$ 262,843
Utica Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	34,011	21,035	2,206
Ohio Gathering
Segment Reporting Information [Line Items]
Adjusted EBITDA	41,246	45,602	33,667
Williston Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	66,413	79,475	34,008
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Adjusted EBITDA	117,737	109,241	110,222
Barnett Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	46,232	54,634	59,526
Marcellus Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 23,888	$ 19,203	$ 23,214

SEGMENT INFORMATION - Reconciliation of Net Income to Adjusted EBITDA (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Income (loss) before income taxes and income (loss) from equity method investees	$ 88,614,000	$ (7,768,000)	$ (216,268,000)
Add:
Corporate and other	39,140,000	37,589,000	27,352,000
Interest expense	68,131,000	63,810,000	59,092,000
Early extinguishment of debt	22,039,000	0	0
Deferred Purchase Price Obligation	(200,322,000)	55,854,000	0
Depreciation and amortization	114,872,000	112,661,000	105,903,000
Proportional adjusted EBITDA for equity method investees	41,246,000	45,602,000	33,667,000
Adjustments related to MVC shortfall payments	(41,373,000)	11,600,000	(11,902,000)
Unit-based and noncash compensation	7,951,000	7,985,000	7,017,000
Loss (gain) on asset sales, net	527,000	93,000	(172,000)
Long-lived asset impairment	188,702,000	1,764,000	9,305,000
Goodwill impairment	0	0	248,851,000
Less:
Interest income	0	0	2,000
Total of reportable segments' measures of profit or loss	$ 329,527,000	$ 329,190,000	$ 262,843,000

SEGMENT INFORMATION - Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	$ (40,758)	$ 11,302	$ (11,453)
Expected MVC shortfall adjustments	(615)	298	(449)
Total adjustments related to MVC shortfall payments	(41,373)	11,600	(11,902)
Williston Basin
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	(37,693)	8,691	11,870
Expected MVC shortfall adjustments	0	0	0
Total adjustments related to MVC shortfall payments	(37,693)	8,691	11,870
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	(3,065)	3,288	(21,623)
Expected MVC shortfall adjustments	(3)	(317)	33
Total adjustments related to MVC shortfall payments	(3,068)	2,971	(21,590)
Barnett Shale
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	(677)	(1,700)
Expected MVC shortfall adjustments	(612)	615	(482)
Total adjustments related to MVC shortfall payments	$ (612)	$ (62)	$ (2,182)

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Property, Plant, and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,104,569	$ 2,112,986
Less accumulated depreciation	309,440	259,315
Property, plant and equipment, net	1,795,129	1,853,671
Gathering and processing systems and related equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,973,722	2,026,363
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	78,850	39,954
Land and line fill
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,735	11,442
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 40,262	$ 35,227

PROPERTY, PLANT, AND EQUIPMENT, NET - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Long-lived assets impairment charge		$ 188,702	$ 1,764	$ 9,305
Williston Basin
Property, Plant and Equipment [Line Items]
Long-lived assets impairment charge	$ 101,900	$ 101,961	$ 569	$ 7,554

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Long-Lived Asset Impairments (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Long-lived asset impairment		$ 188,702	$ 1,764	$ 9,305
Williston Basin
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	$ 101,900	101,961	569	7,554
Piceance/DJ Basins
Property, Plant and Equipment [Line Items]
Long-lived asset impairment		697	0	1,220
Barnett Shale
Property, Plant and Equipment [Line Items]
Long-lived asset impairment		0	1,195	531
Utica Shale
Property, Plant and Equipment [Line Items]
Long-lived asset impairment		$ 878	$ 0	$ 0

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Depreciation Expense and Capitalized Interest (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation expense	$ 75,120	$ 70,770	$ 63,915
Capitalized interest	$ 2,579	$ 3,709	$ 3,372

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 462,629	$ 603,674
Accumulated amortization	(161,284)	(182,222)
Net	301,345	421,452
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(12,350)	(10,795)
Net	11,845	13,400
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	426,464
Accumulated amortization	(117,821)	(146,468)
Net	160,627	279,996
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	159,986	153,015
Accumulated amortization	(31,113)	(24,959)
Net	128,873	128,056
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	(9,074)	(6,916)
Net	$ 1,888	$ 4,046

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Narrative) (Details) $ in Millions	1 Months Ended
	Dec. 31, 2017 USD ($)
Williston Basin \| Contract Intangible Assets and Rights-of-Way Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Impairment charge for intangible assets	$ 85.2

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Recognized Amortization Expense in Other Revenues and Cost and Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Revenue \| Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ (1,555)	$ (1,261)	$ (1,478)
Other Revenue \| Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Amortization expense	2,158	839	692
Costs And Expenses \| Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization expense	34,202	35,416	35,339
Costs And Expenses \| Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 6,153	$ 6,053	$ 5,863

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Intangible assets
2018	$ 34,026
2019	33,139
2020	32,963
2021	29,271
2022	26,204
Unfavorable gas gathering contract
2018	$ 1,888

GOODWILL - Narrative (Details) - USD ($)	12 Months Ended
GOODWILL - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Goodwill	$ 16,211,000	$ 16,211,000
Goodwill impairment	0	0	$ 248,851,000
Piceance/DJ Basins
Segment Reporting Information [Line Items]
Goodwill impairment			45,500,000
Williston Basin
Segment Reporting Information [Line Items]
Goodwill impairment			$ 203,400,000
Mountaineer Midstream \| Reportable Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Goodwill	$ 16,200,000	$ 16,200,000

GOODWILL - Schedule of goodwill (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Accumulated goodwill impairment	$ 303,050	$ 303,050	$ 303,050
Piceance/DJ Basins
Goodwill [Line Items]
Accumulated goodwill impairment	45,478	45,478	45,478
Williston Basin
Goodwill [Line Items]
Accumulated goodwill impairment	$ 257,572	$ 257,572	$ 257,572

EQUITY METHOD INVESTMENTS - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		11 Months Ended		12 Months Ended
	May 31, 2014	Jan. 31, 2014	Nov. 30, 2017	Nov. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Equity Method Investments [Line Items]
Acquisition of initial interest in Ohio Gathering					$ 25,513	$ 31,582	$ 86,200
OGC
Schedule Of Equity Method Investments [Line Items]
Non-affiliated owners' interest percent	60.00%
OGC \| Principal Owner
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest					40.00%
Equity Method Investment [Roll Forward]
Asset impairment					$ 1,400
OGC \| Summit Midstream Partners, LLC
Schedule Of Equity Method Investments [Line Items]
Cost method ownership interest		1.00%
Ownership interest value		$ 190,000
Capital contribution		$ 8,400
Equity method potential ownership interest		40.00%
Percentage of ownership interest	40.00%
Capital contributions true-ups	$ 326,600
Acquisition of initial interest in Ohio Gathering	50,400
Management fee	$ 5,400
Option value		$ 7,500
Equity Method Investment [Roll Forward]
Asset impairment			$ 1,383	$ 0

EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	11 Months Ended		12 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investment [Roll Forward]
Investment in equity method investees, December 31	$ 707,415		$ 707,415
December cash distributions			40,220	$ 44,991	$ 34,641
Investment in equity method investees, net of basis difference, November 30			690,485	707,415
OGC \| Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, December 31	690,485	$ 707,415	$ 690,485	707,415
December cash distributions	4,032	3,172
December cash contributions	(3,932)	(5,318)
Impairment loss	1,383	0
Basis difference	(130,184)	(143,536)
Investment in equity method investees, net of basis difference, November 30	$ 561,784	$ 561,733		$ 690,485	$ 707,415

EQUITY METHOD INVESTMENTS - Balance Sheet Information (Details) - USD ($) $ in Thousands	Nov. 30, 2017	Nov. 30, 2016
OGC
Schedule Of Equity Method Investments [Line Items]
Current assets	$ 34,383	$ 43,797
Noncurrent assets	1,319,448	1,330,199
Total assets	1,353,831	1,373,996
Current liabilities	10,882	22,067
Noncurrent liabilities	3,272	8,396
Total liabilities	14,154	30,463
OCC
Schedule Of Equity Method Investments [Line Items]
Current assets	3,650	2,546
Noncurrent assets	29,156	31,195
Total assets	32,806	33,741
Current liabilities	3,382	3,448
Noncurrent liabilities	11,715	13,111
Total liabilities	$ 15,097	$ 16,559

EQUITY METHOD INVESTMENTS - Statements of Operations Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Nov. 30, 2015
OGC
Schedule Of Equity Method Investments [Line Items]
Total revenues	$ 140,679	$ 148,662	$ 120,623
Total operating expenses	111,897	96,647	96,948
Net income (loss)	28,785	52,009	23,655
OCC
Schedule Of Equity Method Investments [Line Items]
Total revenues	8,607	15,791	9,467
Total operating expenses	8,298	111,528	15,633
Net income (loss)	$ (907)	$ (94,230)	$ (6,852)

DEFERRED REVENUE - Rollforward of deferred revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 0	$ 677
Current deferred revenue, Additions	18,294	11,672
Current deferred revenue, Less revenue recognized	14,294	12,349
Current deferred revenue, ending balance	4,000	0
Noncurrent deferred revenue, beginning balance	57,465	45,486
Noncurrent deferred revenue, Additions		12,391
Noncurrent deferred revenue, Less revenue recognized	40,758	412
Noncurrent deferred revenue, Less reclassification to current deferred revenue	4,000
Noncurrent deferred revenue, ending balance	12,707	57,465
Williston Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	0	0
Current deferred revenue, Less revenue recognized	0	0
Current deferred revenue, ending balance	0	0
Noncurrent deferred revenue, beginning balance	37,693	29,002
Noncurrent deferred revenue, Additions		8,691
Noncurrent deferred revenue, Less revenue recognized	37,693	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	0
Noncurrent deferred revenue, ending balance	0	37,693
Piceance/DJ Basins
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	18,294	11,672
Current deferred revenue, Less revenue recognized	14,294	11,672
Current deferred revenue, ending balance	4,000	0
Noncurrent deferred revenue, beginning balance	19,772	16,484
Noncurrent deferred revenue, Additions		3,700
Noncurrent deferred revenue, Less revenue recognized	3,065	412
Noncurrent deferred revenue, Less reclassification to current deferred revenue	4,000
Noncurrent deferred revenue, ending balance	12,707	19,772
Barnett Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	677
Current deferred revenue, Additions	0	0
Current deferred revenue, Less revenue recognized	0	677
Current deferred revenue, ending balance	0	0
Noncurrent deferred revenue, beginning balance	0	0
Noncurrent deferred revenue, Additions		0
Noncurrent deferred revenue, Less revenue recognized	0	0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	0
Noncurrent deferred revenue, ending balance	$ 0	$ 0

DEFERRED REVENUE - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2017
Williston Basin \| Gathering Services and Related Fees
Deferred Revenue Arrangement [Line Items]
Recognized amount of deferred revenue	$ 37.7
Accrued Income Receivable
Deferred Revenue Arrangement [Line Items]
Shortfall payment billings included in accounts receivable		$ 18.4

DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,051,192	$ 1,240,301
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,051,192	1,240,301
SMP Holdings \| Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	261,000	648,000
SMP Holdings \| Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(2,910)	(3,516)
SMP Holdings \| Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	0
Less unamortized debt issuance costs	(6,898)	0
SMP Holdings \| Senior Notes Due March 2017
Line of Credit Facility [Line Items]
Unsecured notes	0	300,000
Less unamortized debt issuance costs	$ 0	$ (4,183)

DEBT - Components of Long-Term Debt (Parenthetical) (Details) - SMP Holdings	Dec. 31, 2017	Dec. 31, 2016
Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate	4.07%	3.27%
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%
Senior Notes Due March 2017
Line of Credit Facility [Line Items]
Stated interest rate		7.50%

DEBT - Maturities (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Debt
2018	$ 0
2019	0
2020	0
2021	0
2022	561,000
Thereafter	500,000
Total long-term debt	$ 1,061,000

DEBT - Revolving Credit Facility (Narrative) (Details)	1 Months Ended	12 Months Ended
DEBT - Revolving Credit Facility (Narrative) (Details)	May 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	May 26, 2017	May 25, 2017
Line of Credit Facility [Line Items]
Debt issuance costs		$ 8,100,000
Revolving credit facility
Line of Credit Facility [Line Items]
Borrowing capacity	$ 1,250,000,000
Revolving credit facility maturity date	2022-05
Accordion feature	$ 250,000,000
Leverage ratio			5.50	5.00
Senior secured leverage ratio	3.75
Commitment fee on unused portion of the facility		0.50%
Interest rate at period end		4.07%
Unused portion under the facility		$ 989,000,000
Cumulative payment obligations, maximum		50,000,000
Debt issuance costs		10,900,000
Debt defaults		$ 0
Revolving credit facility \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin		2.50%
Revolving credit facility \| Minimum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility		0.30%
Ratio of consolidated trailing 12-month EBITDA to net interest expense		2.5
Revolving credit facility \| Minimum \| ABR
Line of Credit Facility [Line Items]
Applicable margin		0.75%
Revolving credit facility \| Minimum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin		1.75%
Revolving credit facility \| Maximum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility		0.50%
Ratio of total net indebtedness to consolidated trailing 12-month EBITDA		5.50
Ratio of first lien net indebtedness to consolidated trailing 12-month EBITDA		3.75
Revolving credit facility \| Maximum \| ABR
Line of Credit Facility [Line Items]
Applicable margin		1.75%
Revolving credit facility \| Maximum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin		2.75%

DEBT - Senior Notes (Narrative) (Details) - USD ($)				1 Months Ended			12 Months Ended
DEBT - Senior Notes (Narrative) (Details) - USD ($)	Feb. 28, 2017	Feb. 16, 2017	Feb. 14, 2017	Feb. 28, 2017	Jul. 31, 2014	Jun. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 08, 2017
Debt Instrument [Line Items]
Early extinguishment of debt							$ 302,000	$ 0	$ 727,000
Revolving credit facility
Debt Instrument [Line Items]
Extinguishment of debt				$ 172,000,000
5.75% Senior Notes
Debt Instrument [Line Items]
Stated interest rate					5.75%					5.75%
Debt face amount										$ 500,000,000
Unsecured notes	$ 500,000,000			$ 500,000,000
Deferred loan costs										$ 7,700
Default period				30 days
Compliance notice period				30 days
Final judgment				$ 75,000,000
Percent of principal (at least)				25.00%
5.75% Senior Notes \| Redemption period one
Debt Instrument [Line Items]
Percent of principal redeemed				35.00%
Redemption price, expressed as percentage of principal amount				105.75%
5.75% Senior Notes \| Redemption period two
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount				104.313%
5.75% Senior Notes \| Redemption period three
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount				100.00%
5.5% Senior Notes
Debt Instrument [Line Items]
Stated interest rate							5.50%
Debt issuance costs					$ 5,100,000
Events of default, period of payment default					30 days
Events of default, failure to comply with covenants, period after notice					180 days
Events of default, failure to comply with other agreements in the indenture, period after notice					30 days
Events of default, failure to pay final judgments, in excess of 20.0 million					$ 20,000,000
Declaration of immediate payment, holding percent of principal amount required					25.00%
Summit Holdings and Finance Corporation \| 7.5% Senior Notes
Debt Instrument [Line Items]
Senior unsecured notes						$ 300,000,000
Stated interest rate						7.50%
Extinguishment of debt		$ 23,100,000	$ 276,900,000
Redemption and call premiums	17,900,000			$ 17,900,000
Early extinguishment of debt							$ 4,100,000
Unsecured notes	$ 300,000,000			$ 300,000,000
Summit Holdings and Finance Corporation \| Senior Notes Due August 2022
Debt Instrument [Line Items]
Senior unsecured notes					$ 300,000,000
Stated interest rate					5.50%
Summit Holdings and Finance Corporation \| 5.5% Senior Notes \| Redemption period one
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount					104.125%
Summit Holdings and Finance Corporation \| 5.5% Senior Notes \| Redemption period two
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount					100.00%
Finance Corp \| Summit Holdings
Debt Instrument [Line Items]
Cumulative percentage ownership in subsidiary						100.00%

DEBT - SMP Holdings Credit Facility (Narrative) (Details) - USD ($)	Dec. 31, 2017	May 31, 2017
Revolving credit facility
Debt Instrument [Line Items]
Accordion feature		$ 250,000,000
SMP Holdings \| Term Loan Credit Facility
Debt Instrument [Line Items]
Debt face amount	$ 200,000,000
SMP Holdings \| Revolving credit facility
Debt Instrument [Line Items]
Accordion feature	$ 250,000,000

FINANCIAL INSTRUMENTS - Concentration Risk - Narrative (Details) - Customer	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
CONCENTRATIONS OF RISK
Number of top customers	5
Five Largest Customers \| Accounts receivable \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	44.00%	62.00%

FINANCIAL INSTRUMENTS - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 liability, beginning of period	$ 563,281	$ 0
Addition		507,427
Change in fair value	(200,322)	55,854
Level 3 liability, end of period	$ 362,959	$ 563,281

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying value \| Revolving credit facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility	$ 261,000	$ 648,000
Carrying value \| 5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	297,090	296,484
Carrying value \| Senior Notes 5.75%
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	493,102
Carrying value \| 7.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		295,817
Fair Value \| Revolving credit facility \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility	261,000	648,000
Fair Value \| 5.5% Senior Notes \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	301,750	294,500
Fair Value \| Senior Notes 5.75% \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 501,667
Fair Value \| 7.5% Senior Notes \| Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		$ 316,000

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Parenthetical) (Details) - Summit Holdings - USD ($)	Dec. 31, 2017	Dec. 31, 2016
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	$ 300,000,000
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
7.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate		7.50%
Debt face amount		$ 300,000,000

PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	0	0	0
Subordinated units conversion		0
Units issued in connection with offering	300,000	0	0
Net units issued under SMLP LTIP (in shares)	0	0	0
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	300,000	0	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	72,111,121	42,062,644	34,426,513
Subordinated units conversion		24,409,850
Units issued in connection with offering	0	5,500,000	7,475,000
Net units issued under SMLP LTIP (in shares)	211,327	138,627	161,131
Units issued under ATM program (in shares)	763,548		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	73,085,996	72,111,121	42,062,644
Limited partners, Subordinated
Rollforwards of the number of partner units
Units, beginning balance (in shares)	0	24,409,850	24,409,850
Subordinated units conversion		(24,409,850)
Units issued in connection with offering	0	0	0
Net units issued under SMLP LTIP (in shares)	0	0	0
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	0	0	24,409,850
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,471,187	1,354,700	1,200,651
Subordinated units conversion		0
Units issued in connection with offering	0	0	0
Net units issued under SMLP LTIP (in shares)	0	4,242	1,498
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	19,812	112,245	152,551
Units, ending balance (in shares)	1,490,999	1,471,187	1,354,700

PARTNERS' CAPITAL - Narrative (Details)							1 Months Ended				3 Months Ended	12 Months Ended
PARTNERS' CAPITAL - Narrative (Details)	Jan. 25, 2018 USD ($) $ / shares	Dec. 15, 2017 USD ($) $ / shares	Mar. 03, 2016 USD ($)	May 31, 2015 $ / shares shares	May 22, 2015 $ / shares shares	May 18, 2015 USD ($)	Nov. 30, 2017 USD ($) $ / shares shares	Feb. 28, 2017 USD ($) shares	Sep. 30, 2016 $ / shares shares	Jun. 30, 2016 USD ($)	Dec. 31, 2015	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Partners' Capital [Line Items]
Aggregate gross proceeds												$ 17,078,000	$ 0	$ 0
Preferred units issued \| shares												300,000
Repayment of outstanding revolving credit facility												$ 634,500,000	204,300,000	$ 151,000,000
Net investment												$ 0	$ 0
Maximum period following end of quarter to distribute all available cash												45 days
Minimum quarterly distributions (in dollars per share) \| $ / shares												$ 0.40
Minimum annual distributions (in dollars per share) \| $ / shares												1.60
Per-unit annual distributions to unitholders (in dollars per unit) \| $ / shares												$ 2.300	$ 2.300	$ 2.270
Subsequent Event
Partners' Capital [Line Items]
Per-unit annual distributions to unitholders (in dollars per unit) \| $ / shares	$ 0.575
Cash paid to unit holders	$ 45,100,000
Cash distribution date	Feb. 14, 2018
Cash distribution date of record	Feb. 07, 2018
Polar Midstream and Epping
Partners' Capital [Line Items]
Percent of membership interest acquired						100.00%
Purchase price						$ 285,700,000
Summit Investments \| Polar Midstream and Epping
Partners' Capital [Line Items]
Net investment						$ 416,000,000
Drop Down Assets 2016 Acquisition
Partners' Capital [Line Items]
Initial payment			$ 360,000,000
Deferred purchase price obligation			507,427,000
Net investment			1,114,855,000
Working capital adjustment			$ (569,000)							$ 600,000
Revolving credit facility
Partners' Capital [Line Items]
Repayment of outstanding revolving credit facility							$ 293,200,000
Subordinated Units
Partners' Capital [Line Items]
Common unit per subordinated unit upon conversion at end of subordination period											1
Series A Preferred Units
Partners' Capital [Line Items]
Preferred units issued \| shares							300,000
Interest in partnership per unit \| $ / shares							$ 1,000
Distribution rate							9.50%
Liquidation preference per unit \| $ / shares							$ 1,000
Distribution percentage of liquidation preference rate description												Three-month LIBOR plus a spread of 7.43%.
Pro-rated initial distribution per share \| $ / shares		$ 7.9167
Pro-rated initial distribution		$ 2,400,000
Series A Preferred Units \| LIBOR
Partners' Capital [Line Items]
Distribution percentage of liquidation preference rate							7.43%
Common units
Partners' Capital [Line Items]
Units issued under ATM program (in shares) \| shares									5,500,000
Common units sold in public offering, price per share \| $ / shares									$ 23.20
General Partner
Partners' Capital [Line Items]
General partner interest									2.00%			2.00%
Percentage interest in distributions												2.00%
Percentage interest in distributions												2.00%
Common
Partners' Capital [Line Items]
Units issued under ATM program (in shares) \| shares												763,548		0
Units issued in connection with offering \| shares												0	5,500,000	7,475,000
Public Offering \| Common
Partners' Capital [Line Items]
Units issued under ATM program (in shares) \| shares				6,500,000
Common units sold in public offering, price per share \| $ / shares				$ 30.75
Public Offering \| Common \| 2016 SRS
Partners' Capital [Line Items]
Units issued under ATM program (in shares) \| shares								4,000,000
Proceeds from Issuance of common stock								$ 0
Public Offering \| Common
Partners' Capital [Line Items]
Aggregate offering price								$ 150,000,000
Units issued in connection with offering \| shares												763,548
Aggregate gross proceeds												$ 17,700,000
Compensation paid to sales agents												200,000
Aggregate offering price												$ 132,300,000
Over-Allotment Option \| Common
Partners' Capital [Line Items]
Units issued under ATM program (in shares) \| shares					975,000
Common units sold in public offering, price per share \| $ / shares					$ 30.75

PARTNERS' CAPITAL - Schedule of Capital Distribution and Allocation to Partners (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 03, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Subsidiary or Equity Method Investee [Line Items]
Excess of acquired carrying value over consideration paid and recognized			$ 0
Drop Down Assets 2016 Acquisition
Subsidiary or Equity Method Investee [Line Items]
Summit Investments' net investment in the 2016 Drop Down Assets	$ 771,929
SMP Holdings borrowings allocated to 2016 Drop Down Assets and retained by Summit Investments	342,926
Acquired carrying value of 2016 Drop Down Assets	1,114,855
Deferred purchase price obligation	507,427
Borrowings under Revolving Credit Facility	360,000
Working capital adjustment received from a subsidiary of Summit Investments	(569)	$ 600
Total consideration paid and recognized by SMLP	866,858
Excess of acquired carrying value over consideration paid and recognized	247,997
General Partner
Subsidiary or Equity Method Investee [Line Items]
Excess of acquired carrying value over consideration paid and recognized	4,953		4,953	$ 2,607
Common
Subsidiary or Equity Method Investee [Line Items]
Excess of acquired carrying value over consideration paid and recognized	243,044		$ 243,044	$ 80,079
Summit Investments' equity in contributed subsidiaries
Subsidiary or Equity Method Investee [Line Items]
Excess of acquired carrying value over consideration paid and recognized	$ 247,997

PARTNERS' CAPITAL - Schedule of Polar and Divide (Details) - USD ($) $ in Thousands		12 Months Ended
	May 18, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Net investment		$ 0	$ 0
Partners' capital contribution		465	2,702	$ 4,737
Polar Midstream and Epping
Investment [Line Items]
Purchase price	$ 285,677
Partners' capital contribution	130,367
Summit Investments \| Polar Midstream and Epping
Investment [Line Items]
Net investment	416,044
General Partner
Investment [Line Items]
Partners' capital contribution		465	2,702	$ 4,737
General Partner \| Polar Midstream and Epping
Investment [Line Items]
Partners' capital contribution	2,607
Limited partners, Common
Investment [Line Items]
Partners' capital contribution		$ 0	0
Limited partners, Common \| Polar Midstream and Epping
Investment [Line Items]
Partners' capital contribution	80,079
Limited partners, Subordinated
Investment [Line Items]
Partners' capital contribution			$ 0
Limited partners, Subordinated \| Polar Midstream and Epping
Investment [Line Items]
Partners' capital contribution	$ 47,681

PARTNERS' CAPITAL - Cash Distributions Paid and Declared (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 2.300	$ 2.300	$ 2.270

PARTNERS' CAPITAL - Percentage Allocations of Available Cash (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Partnership Target Distributions [Line Items]
IDR payments	$ 8,460	$ 7,912	$ 6,743
Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	$ 0.40
Thereafter
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.60
Minimum \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.40
Minimum \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.46
Minimum \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.50
Maximum \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.46
Maximum \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	0.50
Maximum \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Total quarterly distribution per unit target amount	$ 0.60
Unitholders \| Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	98.00%
Unitholders \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	98.00%
Unitholders \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	85.00%
Unitholders \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	75.00%
Unitholders \| Thereafter
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	50.00%
General Partner
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
General Partner \| Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
General Partner \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
General Partner \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	15.00%
General Partner \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	25.00%
General Partner \| Thereafter
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	50.00%

EARNINGS PER UNIT (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Feb. 16, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		$ (23,654)	$ 89,547	$ 8,806	$ (2,777)	$ 11,522	$ (215)	$ (51,222)	$ (8,264)	$ 75,485	$ (48,179)	$ (195,610)
Phantom Units
Earnings (loss) per limited partner unit:
Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU (in shares)										42,000	125,000	109,000
Limited Partner Interest Before Subordinated Units Converted to Common Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners												$ (125,437)
Subordinated Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners										$ 0	$ 0	$ (70,173)
Weighted-average units outstanding (in shares)										0	0	24,410,000
Weighted-average units outstanding – diluted (shares)										0	0	24,410,000
Earnings (loss) per limited partner unit:
Basic (in dollars per share)										$ 0	$ 0	$ (2.88)
Diluted (in dollars per share)										$ 0	$ 0	$ (2.88)
Number of units converted (in shares)	24,409,850
Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners										$ 71,922	$ (48,179)	$ (195,610)
Weighted-average units outstanding (in shares)										72,705,000	68,264,000	39,217,000
Effect of nonvested phantom units (in shares)										342,000	0	0
Weighted-average units outstanding – diluted (shares)										73,047,000	68,264,000	39,217,000
Earnings (loss) per limited partner unit:
Basic (in dollars per share)		$ (0.32)	$ 1.23	$ 0.12	$ (0.04)	$ 0.16	$ 0.00	$ (0.77)	$ (0.12)	$ 0.99	$ (0.71)	$ (3.20)
Diluted (in dollars per share)		$ (0.32)	$ 1.22	$ 0.12	$ (0.04)	$ 0.16	$ 0.00	$ (0.77)	$ (0.12)	$ 0.98	$ (0.71)	$ (3.20)
Series A Preferred Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		$ 3,563								$ 3,563	$ 0	$ 0

UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2013
Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
SMLP LTIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserved for issuance (in shares)	5,000,000
Units remaining available	3,600,000
Unrecognized unit-based compensation	$ 5.7
Remaining vesting period	2 years 2 months 12 days
Forfeitures	0

UNIT-BASED AND NONCASH COMPENSATION - SMLP Long-Term Incentive Plan (Details) - SMLP LTIP - Phantom Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Units
Nonvested units(in shares)	691,955	379,911	336,202
Units granted (in shares)	371,972	495,535	289,735
Unit vested (in shares)	(293,222)	(178,953)	(229,497)
Units forfeited (in shares)	(21,431)	(4,538)	(16,529)
Nonvested units(in shares)	749,274	691,955	379,911
Weighted-average grant date fair value
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 19.59	$ 31.13	$ 30.61
Units granted, Weighted-average grant date fair value (in dollars per unit)	22.50	14.91	29.21
Units vested, Weighted-average grant date fair value (in dollars per unit)	24.76	33.80	27.66
Units forfeited, Weighted-average grant date fair value (in dollars per unit)	20.07	16.89	35.09
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 20.07	$ 19.59	$ 31.13

UNIT-BASED AND NONCASH COMPENSATION - Schedule of Intrinsic Values (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SMLP LTIP \| Phantom and Restricted Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of vested LTIP awards	$ 6,657	$ 2,957	$ 5,362

UNIT-BASED AND NONCASH COMPENSATION - SMP Net Profits Interests (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 7,878	$ 7,550	$ 6,174
SMLP LTIP \| General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 7,951	$ 7,550	$ 6,174

RELATED-PARTY TRANSACTIONS - Narrative (Details) - SMP Holdings	1 Months Ended
	Feb. 28, 2017 $ / shares shares
RELATED-PARTY TRANSACTIONS
Shares sold (in shares) \| shares	4,000,000
Sale price (in dollars per share) \| $ / shares	$ 24.00

RELATED-PARTY TRANSACTIONS - Expenses Incurred by the General Partner and Reimbursed (Details) - General Partner - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operation and maintenance expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 27,450	$ 26,485	$ 25,050
General and administrative expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 30,899	$ 31,947	$ 26,193

COMMITMENTS AND CONTINGENCIES - Rent Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 3,772	$ 2,861	$ 2,395

COMMITMENTS AND CONTINGENCIES - Schedule of Future Minimum Rental Payments for Operating Leases (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 3,373
2019	3,181
2020	824
2021	389
2022	349
Thereafter	731
Total future minimum lease payments	$ 8,847

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2015
Maximum \| Meadowlark Midstream Gathering System \| Principal Owner
Loss Contingencies [Line Items]
Coverage from pollution liability insurance policy		$ 25,000,000	$ 25,000,000
Coverage from property and business interruption insurance policy		$ 200,000,000
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 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 9,453	$ 13,664
Payments made, including those by affiliates	(4,109)	(4,211)
Accrued environmental remediation, ending balance	$ 5,344	$ 9,453

ACQUISITIONS AND DROP DOWN TRANSACTIONS - 2016 Drop Down (Details) $ in Thousands		1 Months Ended
	Mar. 03, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Deferred purchase price obligation			$ 362,959	$ 563,281
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Initial payment	$ 360,000
Working capital adjustment	$ (569)	$ 600
Deferred Purchase price obligation, multiple	6.5
Estimated future payment obligation	$ 860,300		454,400
Deferred purchase price obligation	$ 507,400		$ 363,000
Discount rate	13.00%		10.50%

ACQUISITIONS AND DROP DOWN TRANSACTIONS - Meadowlark Midstream (Details) - Principal Owner - Meadowlark Midstream Company, LLC $ in Thousands	Feb. 15, 2013 USD ($)
Business Acquisition [Line Items]
Purchase price	$ 25,376
Current assets	2,227
Property, plant and equipment	18,795
Other noncurrent assets	4,354
Total assets acquired	25,376
Total liabilities assumed	0
Net identifiable assets acquired	$ 25,376

ACQUISITIONS AND DROP DOWN TRANSACTIONS - Polar and Divide Drop Down (Details) - Polar Midstream and Epping - USD ($) $ in Thousands	Jul. 31, 2015	May 18, 2015
Business Acquisition [Line Items]
Purchase price		$ 285,677
Liabilities incurred for acquisition		92,000
Cash payment for working capital and capital expenditure adjustments	$ 4,300
Common units
Business Acquisition [Line Items]
Issuance of stock		193,400
General Partner Units
Business Acquisition [Line Items]
Issuance of stock		4,100
Previously Reported
Business Acquisition [Line Items]
Purchase price		$ 290,000

ACQUISITIONS AND DROP DOWN TRANSACTIONS - Revenue and Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Revenues	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 127,083	$ 95,073	$ 89,635	$ 90,571	$ 488,741	$ 402,362	$ 400,557
Net (loss) income									$ 86,050	(38,187)	(222,228)
Summit Midstream Partners, LP
Business Acquisition [Line Items]
Revenues										393,495	358,046
Net (loss) income										(40,932)	(192,212)
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Revenues										8,867	29,238
Net (loss) income										$ 2,745	(35,419)
Polar Midstream and Epping
Business Acquisition [Line Items]
Revenues											13,273
Net (loss) income											$ 5,403

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Narrative (Details) - Summit Midstream Niobrara, LLC $ in Millions	1 Months Ended
	Dec. 31, 2017 USD ($)
Grand River Gathering, LLC
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest	100.00%
Summit Midstream Partners, LP
Schedule Of Equity Method Investments [Line Items]
Cost method ownership interest	1.00%
Purchase of Niobrara interest from Summit Epping LLC	$ 0.8

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 1,430	$ 7,428	$ 21,793	$ 27,811
Accounts receivable	72,301	97,364
Other current assets	4,327	4,309
Due from affiliate	0	0
Total current assets	78,058	109,101
Property, plant and equipment, net	1,795,129	1,853,671
Intangible assets, net	301,345	421,452
Goodwill	16,211	16,211
Investment in equity method investees	690,485	707,415
Other noncurrent assets	13,565	7,329
Investment in subsidiaries	0	0
Total assets	2,894,793	3,115,179	3,164,672
Liabilities and Partners' Capital
Trade accounts payable	16,375	16,251
Accrued expenses	12,499	11,389
Due to affiliate	1,088	258
Deferred revenue	4,000	0	677
Ad valorem taxes payable	8,329	10,588
Accrued interest	12,310	17,483
Accrued environmental remediation	3,130	4,301
Other current liabilities	11,258	11,471
Total current liabilities	68,989	71,741
Long-term debt	1,051,192	1,240,301
Deferred Purchase Price Obligation	362,959	563,281
Deferred revenue	12,707	57,465	45,486
Noncurrent accrued environmental remediation	2,214	5,152
Other noncurrent liabilities	7,063	7,566
Total liabilities	1,505,124	1,945,506
Total partners' capital	1,389,669	1,169,673	1,747,299	1,830,678
Total liabilities and partners' capital	2,894,793	3,115,179
Consolidating adjustments
Assets
Cash and cash equivalents	0	0	0	0
Accounts receivable	0	0
Other current assets	0	0
Due from affiliate	(515,097)	(416,648)
Total current assets	(515,097)	(416,648)
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	(5,344,164)	(5,480,150)
Total assets	(5,859,261)	(5,896,798)
Liabilities and Partners' Capital
Trade accounts payable	0	0
Accrued expenses	0	0
Due to affiliate	(515,097)	(416,648)
Deferred revenue	0
Ad valorem taxes payable	0	0
Accrued interest	0	0
Accrued environmental remediation	0	0
Other current liabilities	0	0
Total current liabilities	(515,097)	(416,648)
Long-term debt	0	0
Deferred Purchase Price Obligation	0	0
Deferred revenue	0	0
Noncurrent accrued environmental remediation	0	0
Other noncurrent liabilities	0	0
Total liabilities	(515,097)	(416,648)
Total partners' capital	(5,344,164)	(5,480,150)
Total liabilities and partners' capital	(5,859,261)	(5,896,798)
Summit Midstream Partners, LP \| Reportable Legal Entities
Assets
Cash and cash equivalents	126	698	73	7,531
Accounts receivable	22	53
Other current assets	1,188	1,526
Due from affiliate	0	11,768
Total current assets	1,336	14,045
Property, plant and equipment, net	4,206	2,266
Intangible assets, net	0	0
Goodwill	0	0
Investment in equity method investees	0	0
Other noncurrent assets	2,547	1,993
Investment in subsidiaries	2,019,700	2,132,757
Total assets	2,027,789	2,151,061
Liabilities and Partners' Capital
Trade accounts payable	209	978
Accrued expenses	928	2,399
Due to affiliate	263,935	405,138
Deferred revenue	0
Ad valorem taxes payable	0	16
Accrued interest	0	0
Accrued environmental remediation	0	0
Other current liabilities	6,395	6,717
Total current liabilities	271,467	415,248
Long-term debt	0	0
Deferred Purchase Price Obligation	362,959	563,281
Deferred revenue	0	0
Noncurrent accrued environmental remediation	0	0
Other noncurrent liabilities	3,694	2,859
Total liabilities	638,120	981,388
Total partners' capital	1,389,669	1,169,673
Total liabilities and partners' capital	2,027,789	2,151,061
Co-Issuers \| Reportable Legal Entities
Assets
Cash and cash equivalents	75	51	12,407	11,621
Accounts receivable	0	0
Other current assets	0	0
Due from affiliate	0	38,013
Total current assets	75	38,064
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	10,913	5,198
Investment in subsidiaries	3,324,464	3,347,393
Total assets	3,335,452	3,390,655
Liabilities and Partners' Capital
Trade accounts payable	0	0
Accrued expenses	0	114
Due to affiliate	252,250	0
Deferred revenue	0
Ad valorem taxes payable	0	0
Accrued interest	12,310	17,483
Accrued environmental remediation	0	0
Other current liabilities	0	0
Total current liabilities	264,560	17,597
Long-term debt	1,051,192	1,240,301
Deferred Purchase Price Obligation	0	0
Deferred revenue	0	0
Noncurrent accrued environmental remediation	0	0
Other noncurrent liabilities	0	0
Total liabilities	1,315,752	1,257,898
Total partners' capital	2,019,700	2,132,757
Total liabilities and partners' capital	3,335,452	3,390,655
Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	1,138	5,768	7,110	7,407
Accounts receivable	65,842	91,152
Other current assets	2,697	2,428
Due from affiliate	493,067	366,867
Total current assets	562,744	466,215
Property, plant and equipment, net	1,442,333	1,508,824
Intangible assets, net	278,958	398,992
Goodwill	16,211	16,211
Investment in equity method investees	0	0
Other noncurrent assets	105	138
Investment in subsidiaries	0	0
Total assets	2,300,351	2,390,380
Liabilities and Partners' Capital
Trade accounts payable	11,283	10,640
Accrued expenses	10,592	6,284
Due to affiliate	0	0
Deferred revenue	4,000
Ad valorem taxes payable	7,809	9,847
Accrued interest	0	0
Accrued environmental remediation	0	0
Other current liabilities	4,385	4,047
Total current liabilities	38,069	30,818
Long-term debt	0	0
Deferred Purchase Price Obligation	0	0
Deferred revenue	12,707	57,465
Noncurrent accrued environmental remediation	0	0
Other noncurrent liabilities	3,293	4,602
Total liabilities	54,069	92,885
Total partners' capital	2,246,282	2,297,495
Total liabilities and partners' capital	2,300,351	2,390,380
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Assets
Cash and cash equivalents	91	911	$ 2,203	$ 1,252
Accounts receivable	6,437	6,159
Other current assets	442	355
Due from affiliate	22,030	0
Total current assets	29,000	7,425
Property, plant and equipment, net	348,590	342,581
Intangible assets, net	22,387	22,460
Goodwill	0	0
Investment in equity method investees	690,485	707,415
Other noncurrent assets	0	0
Investment in subsidiaries	0	0
Total assets	1,090,462	1,079,881
Liabilities and Partners' Capital
Trade accounts payable	4,883	4,633
Accrued expenses	979	2,592
Due to affiliate	0	11,768
Deferred revenue	0
Ad valorem taxes payable	520	725
Accrued interest	0	0
Accrued environmental remediation	3,130	4,301
Other current liabilities	478	707
Total current liabilities	9,990	24,726
Long-term debt	0	0
Deferred Purchase Price Obligation	0	0
Deferred revenue	0	0
Noncurrent accrued environmental remediation	2,214	5,152
Other noncurrent liabilities	76	105
Total liabilities	12,280	29,983
Total partners' capital	1,078,182	1,049,898
Total liabilities and partners' capital	$ 1,090,462	$ 1,079,881

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Statement of Operations (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Gathering services and related fees									$ 394,427,000	$ 345,961,000	$ 337,819,000
Natural gas, NGLs and condensate sales									68,459,000	35,833,000	42,079,000
Other revenues									25,855,000	20,568,000	20,659,000
Total revenues	$ 126,199,000	$ 124,945,000	$ 101,792,000	$ 135,805,000	$ 127,083,000	$ 95,073,000	$ 89,635,000	$ 90,571,000	488,741,000	402,362,000	400,557,000
Costs and expenses:
Cost of natural gas and NGLs									57,237,000	27,421,000	31,398,000
Operation and maintenance									93,882,000	95,334,000	94,986,000
General and administrative									54,681,000	52,410,000	45,108,000
Depreciation and amortization									115,475,000	112,239,000	105,117,000
Transaction costs									73,000	1,321,000	1,342,000
Environmental remediation									0	0	21,800,000
Loss (gain) on asset sales, net									527,000	93,000	(172,000)
Long-lived asset impairment									188,702,000	1,764,000	9,305,000
Goodwill impairment									0	0	248,851,000
Total costs and expenses									510,577,000	290,582,000	557,735,000
Other income									298,000	116,000	2,000
Interest expense									(68,131,000)	(63,810,000)	(59,092,000)
Early extinguishment of debt									(22,039,000)	0	0
Deferred Purchase Price Obligation									200,322,000	(55,854,000)	0
Income (loss) before income taxes and loss from equity method investees									88,614,000	(7,768,000)	(216,268,000)
Income tax (expense) benefit									(341,000)	(75,000)	603,000
Loss from equity method investees									(2,223,000)	(30,344,000)	(6,563,000)
Equity in earnings (loss) of consolidated subsidiaries									0	0	0
Net income (loss)									86,050,000	(38,187,000)	(222,228,000)
Consolidating adjustments
Revenues:
Gathering services and related fees									0	0	0
Natural gas, NGLs and condensate sales									0	0	0
Other revenues									0	0	0
Total revenues									0	0	0
Costs and expenses:
Cost of natural gas and NGLs									0	0	0
Operation and maintenance									0	0	0
General and administrative									0	0	0
Depreciation and amortization									0	0	0
Transaction costs									0	0	0
Environmental remediation											0
Loss (gain) on asset sales, net									0	0	0
Long-lived asset impairment									0	0	0
Goodwill impairment											0
Total costs and expenses									0	0	0
Other income									0	0	0
Interest expense									0	0	0
Early extinguishment of debt									0
Deferred Purchase Price Obligation									0	0
Income (loss) before income taxes and loss from equity method investees									0	0	0
Income tax (expense) benefit									0	0	0
Loss from equity method investees									0	0	0
Equity in earnings (loss) of consolidated subsidiaries									135,940,000	(104,305,000)	372,190,000
Net income (loss)									135,940,000	(104,305,000)	372,190,000
Summit Midstream Partners, LP
Revenues:
Total revenues										393,495,000	358,046,000
Costs and expenses:
Net income (loss)										(40,932,000)	(192,212,000)
Summit Midstream Partners, LP \| Reportable Legal Entities
Revenues:
Gathering services and related fees									0	0	0
Natural gas, NGLs and condensate sales									0	0	0
Other revenues									0	0	0
Total revenues									0	0	0
Costs and expenses:
Cost of natural gas and NGLs									0	0	0
Operation and maintenance									0	0	0
General and administrative									0	0	0
Depreciation and amortization									1,101,000	580,000	603,000
Transaction costs									73,000	1,321,000	1,342,000
Environmental remediation											0
Loss (gain) on asset sales, net									0	0	0
Long-lived asset impairment									0	0	0
Goodwill impairment											0
Total costs and expenses									1,174,000	1,901,000	1,945,000
Other income									298,000	116,000	2,000
Interest expense									0	(1,441,000)	(10,494,000)
Early extinguishment of debt									0
Deferred Purchase Price Obligation									200,322,000	(55,854,000)
Income (loss) before income taxes and loss from equity method investees									199,446,000	(59,080,000)	(12,437,000)
Income tax (expense) benefit									(341,000)	(75,000)	603,000
Loss from equity method investees									0	0	0
Equity in earnings (loss) of consolidated subsidiaries									(113,055,000)	20,968,000	(210,394,000)
Net income (loss)									86,050,000	(38,187,000)	(222,228,000)
Co-Issuers \| Reportable Legal Entities
Revenues:
Gathering services and related fees									0	0	0
Natural gas, NGLs and condensate sales									0	0	0
Other revenues									0	0	0
Total revenues									0	0	0
Costs and expenses:
Cost of natural gas and NGLs									0	0	0
Operation and maintenance									0	0	0
General and administrative									0	0	0
Depreciation and amortization									0	0	0
Transaction costs									0	0	0
Environmental remediation											0
Loss (gain) on asset sales, net									0	0	0
Long-lived asset impairment									0	0	0
Goodwill impairment											0
Total costs and expenses									0	0	0
Other income									0	0	0
Interest expense									(68,131,000)	(62,369,000)	(48,598,000)
Early extinguishment of debt									(22,039,000)
Deferred Purchase Price Obligation									0	0
Income (loss) before income taxes and loss from equity method investees									(90,170,000)	(62,369,000)	(48,598,000)
Income tax (expense) benefit									0	0	0
Loss from equity method investees									0	0	0
Equity in earnings (loss) of consolidated subsidiaries									(22,885,000)	83,337,000	(161,796,000)
Net income (loss)									(113,055,000)	20,968,000	(210,394,000)
Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Gathering services and related fees									334,625,000	294,837,000	320,454,000
Natural gas, NGLs and condensate sales									68,459,000	35,833,000	42,079,000
Other revenues									25,583,000	20,227,000	20,283,000
Total revenues									428,667,000	350,897,000	382,816,000
Costs and expenses:
Cost of natural gas and NGLs									57,237,000	27,421,000	31,398,000
Operation and maintenance									85,599,000	89,031,000	91,763,000
General and administrative									47,878,000	44,922,000	40,134,000
Depreciation and amortization									101,319,000	101,415,000	98,002,000
Transaction costs									0	0	0
Environmental remediation											0
Loss (gain) on asset sales, net									(15,000)	97,000	(172,000)
Long-lived asset impairment									187,823,000	1,235,000	9,305,000
Goodwill impairment											248,851,000
Total costs and expenses									479,841,000	264,121,000	519,281,000
Other income									0	0	0
Interest expense									0	0	0
Early extinguishment of debt									0
Deferred Purchase Price Obligation									0	0
Income (loss) before income taxes and loss from equity method investees									(51,174,000)	86,776,000	(136,465,000)
Income tax (expense) benefit									0	0	0
Loss from equity method investees									0	0	0
Equity in earnings (loss) of consolidated subsidiaries									0	0	0
Net income (loss)									(51,174,000)	86,776,000	(136,465,000)
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Revenues:
Gathering services and related fees									59,802,000	51,124,000	17,365,000
Natural gas, NGLs and condensate sales									0	0	0
Other revenues									272,000	341,000	376,000
Total revenues									60,074,000	51,465,000	17,741,000
Costs and expenses:
Cost of natural gas and NGLs									0	0	0
Operation and maintenance									8,283,000	6,303,000	3,223,000
General and administrative									6,803,000	7,488,000	4,974,000
Depreciation and amortization									13,055,000	10,244,000	6,512,000
Transaction costs									0	0	0
Environmental remediation											21,800,000
Loss (gain) on asset sales, net									542,000	(4,000)	0
Long-lived asset impairment									879,000	529,000	0
Goodwill impairment											0
Total costs and expenses									29,562,000	24,560,000	36,509,000
Other income									0	0	0
Interest expense									0	0	0
Early extinguishment of debt									0
Deferred Purchase Price Obligation									0	0
Income (loss) before income taxes and loss from equity method investees									30,512,000	26,905,000	(18,768,000)
Income tax (expense) benefit									0	0	0
Loss from equity method investees									(2,223,000)	(30,344,000)	(6,563,000)
Equity in earnings (loss) of consolidated subsidiaries									0	0	0
Net income (loss)									$ 28,289,000	$ (3,439,000)	$ (25,331,000)

CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Cash Flow Statement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 237,832	$ 230,495	$ 191,375
Cash flows from investing activities:
Capital expenditures	(124,215)	(142,719)	(272,225)
Proceeds from asset sale	2,300	0	0
Contributions to equity method investees	(25,513)	(31,582)	(86,200)
Acquisitions of gathering systems from affiliate	0	(359,431)	(288,618)
Purchase of noncontrolling interest	797	0	0
Other, net	(458)	(394)	323
Advances to affiliates	0	0	0
Net cash provided by (used in) investing activities	(148,683)	(534,126)	(646,720)
Cash flows from financing activities:
Distributions to unitholders	(181,478)	(167,504)	(152,074)
Borrowings under Revolving Credit Facility	247,500	520,300	367,000
Repayments under Revolving Credit Facility	(634,500)	(204,300)	(151,000)
Repayments under term loan	0	0	(182,500)
Debt issuance costs	(16,390)	(3,032)	(412)
Payment of redemption and call premiums on senior notes	(17,932)	0	0
Proceeds from ATM Program common unit issuances, net of costs	17,078	0	0
Proceeds from underwritten issuance of common units, net of costs	0	125,233	221,977
Proceeds from issuance of Series A Preferred Units, net of costs	293,238	0	0
Contribution from General Partner	465	2,702	4,737
Issuance of senior notes	500,000	0	0
Tender and redemption of senior notes	(300,000)	0	0
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net	0	12,214	320,527
Expenses paid by Summit Investments on behalf of contributed subsidiaries	0	4,821	22,879
Other, net	(3,128)	(1,168)	(1,807)
Advances from affiliates	0	0	0
Net cash (used in) provided by financing activities	(95,147)	289,266	449,327
Net change in cash and cash equivalents	(5,998)	(14,365)	(6,018)
Cash and cash equivalents, beginning of period	7,428	21,793	27,811
Cash and cash equivalents, end of period	1,430	7,428	21,793
Consolidating adjustments
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	0	0	0
Cash flows from investing activities:
Capital expenditures	0	0	0
Proceeds from asset sale	0
Contributions to equity method investees	0	0	0
Acquisitions of gathering systems from affiliate		0	0
Purchase of noncontrolling interest	0
Other, net	0	0	0
Advances to affiliates	426,722	421,542	200,813
Net cash provided by (used in) investing activities	426,722	421,542	200,813
Cash flows from financing activities:
Distributions to unitholders	0	0	0
Borrowings under Revolving Credit Facility	0	0	0
Repayments under Revolving Credit Facility	0	0	0
Repayments under term loan			0
Debt issuance costs	0	0	0
Payment of redemption and call premiums on senior notes	0
Proceeds from ATM Program common unit issuances, net of costs	0
Proceeds from underwritten issuance of common units, net of costs		0	0
Proceeds from issuance of Series A Preferred Units, net of costs	0
Contribution from General Partner	0	0	0
Issuance of senior notes	0
Tender and redemption of senior notes	0
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		0	0
Expenses paid by Summit Investments on behalf of contributed subsidiaries		0	0
Other, net	0	0	0
Advances from affiliates	(426,722)	(421,542)	(200,813)
Net cash (used in) provided by financing activities	(426,722)	(421,542)	(200,813)
Net change in cash and cash equivalents	0	0	0
Cash and cash equivalents, beginning of period	0	0	0
Cash and cash equivalents, end of period	0	0	0
Summit Midstream Partners, LP \| Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	7,122	9,691	409
Cash flows from investing activities:
Capital expenditures	(3,041)	(1,668)	(429)
Proceeds from asset sale	0
Contributions to equity method investees	0	0	0
Acquisitions of gathering systems from affiliate		(359,431)	(288,618)
Purchase of noncontrolling interest	797
Other, net	(458)	(394)	0
Advances to affiliates	(278,493)	(15,697)	(2,589)
Net cash provided by (used in) investing activities	(282,789)	(377,190)	(291,636)
Cash flows from financing activities:
Distributions to unitholders	(181,478)	(167,504)	(152,074)
Borrowings under Revolving Credit Facility	0	12,000	180,000
Repayments under Revolving Credit Facility	0	0	(100,000)
Repayments under term loan			(182,500)
Debt issuance costs	0	0	(135)
Payment of redemption and call premiums on senior notes	0
Proceeds from ATM Program common unit issuances, net of costs	17,078
Proceeds from underwritten issuance of common units, net of costs		125,233	221,977
Proceeds from issuance of Series A Preferred Units, net of costs	293,238
Contribution from General Partner	465	2,702	4,737
Issuance of senior notes	0
Tender and redemption of senior notes	0
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		(12,000)	102,500
Expenses paid by Summit Investments on behalf of contributed subsidiaries		3,030	12,655
Other, net	(2,437)	(1,182)	(1,615)
Advances from affiliates	148,229	405,845	198,224
Net cash (used in) provided by financing activities	275,095	368,124	283,769
Net change in cash and cash equivalents	(572)	625	(7,458)
Cash and cash equivalents, beginning of period	698	73	7,531
Cash and cash equivalents, end of period	126	698	73
Co-Issuers \| Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(68,915)	(58,254)	(46,716)
Cash flows from investing activities:
Capital expenditures	0	0	0
Proceeds from asset sale	0
Contributions to equity method investees	0	0	0
Acquisitions of gathering systems from affiliate		0	0
Purchase of noncontrolling interest	0
Other, net	0	0	0
Advances to affiliates	0	(255,070)	(88,221)
Net cash provided by (used in) investing activities	0	(255,070)	(88,221)
Cash flows from financing activities:
Distributions to unitholders	0	0	0
Borrowings under Revolving Credit Facility	247,500	508,300	187,000
Repayments under Revolving Credit Facility	(634,500)	(204,300)	(51,000)
Repayments under term loan			0
Debt issuance costs	(16,390)	(3,032)	(277)
Payment of redemption and call premiums on senior notes	(17,932)
Proceeds from ATM Program common unit issuances, net of costs	0
Proceeds from underwritten issuance of common units, net of costs		0	0
Proceeds from issuance of Series A Preferred Units, net of costs	0
Contribution from General Partner	0	0	0
Issuance of senior notes	500,000
Tender and redemption of senior notes	(300,000)
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		0	0
Expenses paid by Summit Investments on behalf of contributed subsidiaries		0	0
Other, net	0	0	0
Advances from affiliates	290,261	0	0
Net cash (used in) provided by financing activities	68,939	300,968	135,723
Net change in cash and cash equivalents	24	(12,356)	786
Cash and cash equivalents, beginning of period	51	12,407	11,621
Cash and cash equivalents, end of period	75	51	12,407
Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	221,115	201,516	207,684
Cash flows from investing activities:
Capital expenditures	(98,926)	(55,185)	(120,784)
Proceeds from asset sale	0
Contributions to equity method investees	0	0	0
Acquisitions of gathering systems from affiliate		0	0
Purchase of noncontrolling interest	0
Other, net	0	0	323
Advances to affiliates	(126,198)	(150,775)	(110,003)
Net cash provided by (used in) investing activities	(225,124)	(205,960)	(230,464)
Cash flows from financing activities:
Distributions to unitholders	0	0	0
Borrowings under Revolving Credit Facility	0	0	0
Repayments under Revolving Credit Facility	0	0	0
Repayments under term loan			0
Debt issuance costs	0	0	0
Payment of redemption and call premiums on senior notes	0
Proceeds from ATM Program common unit issuances, net of costs	0
Proceeds from underwritten issuance of common units, net of costs		0	0
Proceeds from issuance of Series A Preferred Units, net of costs	0
Contribution from General Partner	0	0	0
Issuance of senior notes	0
Tender and redemption of senior notes	0
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		3,223	18,811
Expenses paid by Summit Investments on behalf of contributed subsidiaries		0	3,864
Other, net	(621)	(121)	(192)
Advances from affiliates	0	0	0
Net cash (used in) provided by financing activities	(621)	3,102	22,483
Net change in cash and cash equivalents	(4,630)	(1,342)	(297)
Cash and cash equivalents, beginning of period	5,768	7,110	7,407
Cash and cash equivalents, end of period	1,138	5,768	7,110
Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	78,510	77,542	29,998
Cash flows from investing activities:
Capital expenditures	(22,248)	(85,866)	(151,012)
Proceeds from asset sale	2,300
Contributions to equity method investees	(25,513)	(31,582)	(86,200)
Acquisitions of gathering systems from affiliate		0	0
Purchase of noncontrolling interest	0
Other, net	0	0	0
Advances to affiliates	(22,031)	0	0
Net cash provided by (used in) investing activities	(67,492)	(117,448)	(237,212)
Cash flows from financing activities:
Distributions to unitholders	0	0	0
Borrowings under Revolving Credit Facility	0	0	0
Repayments under Revolving Credit Facility	0	0	0
Repayments under term loan			0
Debt issuance costs	0	0	0
Payment of redemption and call premiums on senior notes	0
Proceeds from ATM Program common unit issuances, net of costs	0
Proceeds from underwritten issuance of common units, net of costs		0	0
Proceeds from issuance of Series A Preferred Units, net of costs	0
Contribution from General Partner	0	0	0
Issuance of senior notes	0
Tender and redemption of senior notes	0
Cash advance (to) from Summit Investments (from) to contributed subsidiaries, net		20,991	199,216
Expenses paid by Summit Investments on behalf of contributed subsidiaries		1,791	6,360
Other, net	(70)	135	0
Advances from affiliates	(11,768)	15,697	2,589
Net cash (used in) provided by financing activities	(11,838)	38,614	208,165
Net change in cash and cash equivalents	(820)	(1,292)	951
Cash and cash equivalents, beginning of period	911	2,203	1,252
Cash and cash equivalents, end of period	$ 91	$ 911	$ 2,203

UNAUDITED QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Limited Partners' Capital Account [Line Items]
Revenues	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 127,083	$ 95,073	$ 89,635	$ 90,571	$ 488,741	$ 402,362	$ 400,557
Net (loss) income attributable to SMLP	(18,331)	93,546	11,157	(685)	13,901	1,922	(50,287)	(6,454)	85,687	(40,918)	(192,212)
Less net (loss) income and IDRs attributable to General Partner	1,760	3,999	2,351	2,092	2,379	2,137	935	1,810	10,202	7,261	3,398
Net income (loss) attributable to limited partners	(23,654)	$ 89,547	$ 8,806	$ (2,777)	$ 11,522	$ (215)	$ (51,222)	$ (8,264)	75,485	(48,179)	(195,610)
Series A Preferred Units
Limited Partners' Capital Account [Line Items]
Net income (loss) attributable to limited partners	$ 3,563								3,563	0	0
Common units
Limited Partners' Capital Account [Line Items]
Net income (loss) attributable to limited partners									$ 71,922	$ (48,179)	$ (195,610)
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ (0.32)	$ 1.23	$ 0.12	$ (0.04)	$ 0.16	$ 0.00	$ (0.77)	$ (0.12)	$ 0.99	$ (0.71)	$ (3.20)
Diluted (in dollars per share)	$ (0.32)	$ 1.22	$ 0.12	$ (0.04)	$ 0.16	$ 0.00	$ (0.77)	$ (0.12)	$ 0.98	$ (0.71)	$ (3.20)