Summit Midstream Partners (SMLP) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 13, 2019	Jun. 30, 2018
Entity Information [Line Items]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	1,549,922
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 633,243,457
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	SMLP
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Common units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		73,462,254
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,490,999

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 4,345	$ 1,430
Accounts receivable	97,936	72,301
Other current assets	3,971	4,327
Total current assets	106,252	78,058
Property, plant and equipment, net	1,963,713	1,795,129
Intangible assets, net	273,416	301,345
Goodwill	16,211	16,211
Investment in equity method investees	649,250	690,485
Other noncurrent assets	11,720	13,565
Total assets	3,020,562	2,894,793
Current liabilities:
Trade accounts payable	38,414	16,375
Accrued expenses	21,963	12,499
Due to affiliate	240	1,088
Deferred revenue	11,467	4,000
Ad valorem taxes payable	10,550	8,329
Accrued interest	12,286	12,310
Accrued environmental remediation	2,487	3,130
Other current liabilities	12,645	11,258
Total current liabilities	110,052	68,989
Long-term debt	1,257,731	1,051,192
Deferred Purchase Price Obligation	383,934	362,959
Noncurrent deferred revenue	39,504	12,707
Noncurrent accrued environmental remediation	3,149	2,214
Other noncurrent liabilities	4,968	7,063
Total liabilities	1,799,338	1,505,124
Commitments and contingencies (Note 16)
Series A Preferred Units (300,000 units issued and outstanding at December 31, 2018 and December 31, 2017)	293,616	294,426
Common limited partner capital (73,390,853 units issued and outstanding at December 31, 2018 and 73,085,996 units issued and outstanding at December 31, 2017)	902,358	1,056,510
General Partner interests (1,490,999 units issued and outstanding at December 31, 2018 and December 31, 2017)	25,250	27,920
Noncontrolling interest		10,813
Total partners' capital	1,221,224	1,389,669
Total liabilities and partners' capital	$ 3,020,562	$ 2,894,793

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Series A preferred unitholders, issued	300,000	300,000
Series A preferred unitholders, outstanding	300,000	300,000
Common limited partner capital (in shares), issued	73,390,853	73,085,996
Common limited partner capital (in shares), outstanding	73,390,853	73,085,996
General partner interests (in shares), issued	1,490,999	1,490,999
General partner interests (in shares), outstanding	1,490,999	1,490,999

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Total revenues	$ 506,653	$ 488,741	$ 402,362
Costs and expenses:
Cost of natural gas and NGLs	$ 107,661	$ 57,237	$ 27,421
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 96,878	$ 93,882	$ 95,334
General and administrative	52,877	54,681	52,410
Depreciation and amortization	107,100	115,475	112,239
Transaction costs	0	73	1,321
Loss on asset sales, net	0	527	93
Long-lived asset impairment	7,186	188,702	1,764
Total costs and expenses	371,702	510,577	290,582
Other (expense) income	(169)	298	116
Interest expense	(60,535)	(68,131)	(63,810)
Early extinguishment of debt	0	(22,039)	0
Deferred Purchase Price Obligation	(20,975)	200,322	(55,854)
Income (loss) before income taxes and loss from equity method investees	53,272	88,614	(7,768)
Income tax expense	(33)	(341)	(75)
Loss from equity method investees	(10,888)	(2,223)	(30,344)
Net income (loss)	42,351	86,050	(38,187)
Net income attributable to Summit Investments	0	0	2,745
Net income (loss) attributable to noncontrolling interest	168	363	(14)
Net income (loss) attributable to SMLP	42,183	85,687	(40,918)
Net income attributable to General Partner, including IDRs	9,384	10,202	7,261
Net income (loss) attributable to limited partners	32,799	75,485	(48,179)
Series A Preferred Units
Costs and expenses:
Net income (loss) attributable to limited partners	28,500	3,563	0
Common units
Costs and expenses:
Net income (loss) attributable to limited partners	$ 4,299	$ 71,922	$ (48,179)
Earnings (loss) per limited partner unit:
Basic (in dollars per share)	$ 0.06	$ 0.99	$ (0.71)
Diluted (in dollars per share)	$ 0.06	$ 0.98	$ (0.71)
Weighted-average limited partner units outstanding:
Basic (shares)	73,304	72,705	68,264
Diluted (shares)	73,615	73,047	68,264
Gathering Services and Related Fees
Revenues:
Total revenues	$ 344,616	$ 394,427	$ 345,961
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	134,834	68,459	35,833
Other Revenues
Revenues:
Total revenues	$ 27,203	$ 25,855	$ 20,568

CONSOLIDATED STATEMENTS OF PART

CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) $ in Thousands	Total	At Market Program	Class B membership interest	Adjustment	Noncontrolling interest	Noncontrolling interestAt Market Program	Noncontrolling interestClass B membership interest	Noncontrolling interestAdjustment	Summit Investments' equity in contributed subsidiaries	Summit Investments' equity in contributed subsidiariesClass B membership interest	Common	CommonAt Market Program	CommonClass B membership interest	CommonAdjustment	Limited partners, Subordinated	Limited partners, SubordinatedClass B membership interest	General Partner	General PartnerAt Market Program	General PartnerClass B membership interest	General PartnerAdjustment	Limited partners, Series A Preferred Units	Limited partners, Series A Preferred UnitsAt Market Program	Limited partners, Series A Preferred UnitsAdjustment
Beginning 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 from Summit Investments to contributed subsidiaries	223				0				223		0				0		0
Ending balance at Dec. 31, 2016	1,169,673				11,247				$ 0		1,129,132				$ 0		29,294				$ 0
Net (loss) income	86,050				363						71,922						10,202				3,563
Distributions to unitholders	(181,478)				0						(167,062)						(12,041)				(2,375)
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	$ 17,078			0	$ 0					0	$ 17,078					0	$ 0			293,238	$ 0
Contribution from General Partner	465				0						0						465				0
Purchase of noncontrolling interest	(797)				(797)						0						0				0
Other	(202)				0						(202)						0				0
Ending balance at Dec. 31, 2017	1,389,669			$ 1,393,883	10,813			$ 10,813			1,056,510			$ 1,060,640			27,920			$ 28,004	294,426		$ 294,426
January 1, 2018 impact of Topic 606day 1 adoption	4,214				0						4,130						84				0
Net (loss) income	42,351				168						4,299						9,384				28,500
Distributions to unitholders	(209,205)				0						(168,567)						(12,138)				(28,500)
Unit-based compensation	8,088				0						8,088						0				0
Tax withholdings on vested SMLP LTIP awards	(1,974)				0						(1,974)						0				0
Purchase of noncontrolling interest	(10,981)				(10,981)						0						0				0
Other	(938)				0						(128)						0				(810)
Ending balance at Dec. 31, 2018	$ 1,221,224				$ 0						$ 902,358						$ 25,250				$ 293,616

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income (loss)	$ 42,351	$ 86,050	$ (38,187)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	106,767	114,872	112,661
Amortization of debt issuance costs	4,285	4,158	3,976
Deferred Purchase Price Obligation	20,975	(200,322)	55,854
Unit-based and noncash compensation	8,328	7,951	7,985
Loss from equity method investees	10,888	2,223	30,344
Distributions from equity method investees	35,271	40,220	44,991
Loss on asset sales, net	0	527	93
Long-lived asset impairment	7,186	188,702	1,764
Early extinguishment of debt	0	22,039	0
Write-off of debt issuance costs	0	302	0
Changes in operating assets and liabilities:
Accounts receivable	(21,535)	25,063	(7,783)
Trade accounts payable	81	(3,246)	2,001
Accrued expenses	9,464	1,110	4,613
Due (to) from affiliate	(848)	830	(891)
Deferred revenue, net	5,355	(40,758)	11,302
Ad valorem taxes payable	2,221	(2,259)	317
Accrued interest	(24)	(5,173)	0
Accrued environmental remediation, net	(3,808)	(4,109)	(4,211)
Other, net	972	(348)	5,666
Net cash provided by operating activities	227,929	237,832	230,495
Cash flows from investing activities:
Capital expenditures	(200,586)	(124,215)	(142,719)
Proceeds from asset sale	496	2,300	0
Contributions to equity method investees	(4,924)	(25,513)	(31,582)
Acquisitions of gathering systems from affiliate, net of acquired cash	0	0	(359,431)
Purchase of noncontrolling interest	(10,981)	(797)	0
Other, net	(284)	(458)	(394)
Net cash provided by (used in) investing activities	(216,279)	(148,683)	(534,126)
Cash flows from financing activities:
Borrowings under Revolving Credit Facility	289,000	247,500	520,300
Repayments under Revolving Credit Facility	(84,000)	(634,500)	(204,300)
Debt issuance costs	(344)	(16,390)	(3,032)
Payment of redemption and call premiums on senior notes	0	(17,932)	0
Proceeds from ATM Program common unit issuances, net of costs	0	17,078	0
Proceeds from underwritten issuance of common units, net of costs	0	0	125,233
Proceeds from issuance of Series A Preferred Units, net of costs	0	293,238	0
Contribution from General Partner	0	465	2,702
Cash advance from Summit Investments to contributed subsidiaries, net	0	0	12,214
Expenses paid by Summit Investments on behalf of contributed subsidiaries	0	0	4,821
Issuance of senior notes	0	500,000	0
Tender and redemption of senior notes	0	(300,000)	0
Other, net	(4,186)	(3,128)	(1,168)
Net cash (used in) provided by financing activities	(8,735)	(95,147)	289,266
Net change in cash and cash equivalents	2,915	(5,998)	(14,365)
Cash and cash equivalents, beginning of period	1,430	7,428	21,793
Cash and cash equivalents, end of period	4,345	1,430	7,428
Supplemental cash flow disclosures:
Cash interest paid	64,678	71,488	63,000
Less capitalized interest	8,497	2,579	3,709
Interest paid (net of capitalized interest)	56,181	68,909	59,291
Cash paid for taxes	175	0	0
Noncash investing and financing activities
Capital expenditures in trade accounts payable (period-end accruals)	33,750	11,792	8,422
Issuance of Deferred Purchase Price Obligation to affiliate to partially fund the 2016 Drop Down	0	0	507,427
Excess of acquired carrying value over consideration paid and recognized for 2016 Drop Down Assets	0	0	247,997
Distribution of debt related to Carve-Out Financial Statements of Summit Investments	0	0	342,926
Capitalized interest allocated to contributed subsidiaries from Summit Investments	0	0	223
Capital expenditures relating to contributions in aid of construction for Topic 606 day 1 adoption	33,123	0	0
Common units
Cash flows from financing activities:
Distributions to unitholders	(180,705)	(179,103)	(167,504)
Series A Preferred Units
Cash flows from financing activities:
Distributions to unitholders	$ (28,500)	$ (2,375)	$ 0

ORGANIZATION, BUSINESS OPERATIO

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION	1. ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION Organization. SMLP, a Delaware limited partnership, was formed in May 2012 and began operations in October 2012 in connection with its IPO of common limited partner units. SMLP is a growth-oriented limited partnership focused on developing, owning and operating midstream energy infrastructure assets that are strategically located in the core producing areas of unconventional resource basins, primarily shale formations, in the continental United States. Our business activities are conducted through various operating subsidiaries, each of which is owned or controlled by our wholly owned subsidiary holding company, Summit Holdings, a Delaware limited liability company. References to the "Partnership," "we," or "our" refer collectively to SMLP and its subsidiaries. The General Partner, a Delaware limited liability company, manages our operations and activities. Summit Investments, a Delaware limited liability company, is the ultimate owner of our General Partner and has the right to appoint the entire Board of Directors. Summit Investments is controlled by Energy Capital Partners. In addition to its approximate Neither SMLP nor its subsidiaries have any employees. All of the personnel that conduct our business are employed by Summit Investments, but these individuals are sometimes referred to as our employees. Business Operations. We provide natural gas gathering, treating and processing services as well as crude oil and produced water gathering services pursuant to primarily long-term, fee-based agreements with our customers. Our results are driven primarily by the volumes of natural gas that we gather, compress, treat and/or 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, a crude oil, produced water and associated natural gas gathering system 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; • 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; • Summit Permian, an associated natural gas gathering and processing system in the northern Delaware Basin, which includes the Wolfcamp and Bone Spring formations, in southeastern New Mexico; • 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; • DFW Midstream, a natural gas gathering system operating in the Fort Worth Basin, which includes the Barnett Shale formation in north-central Texas; and • Mountaineer Midstream, a natural gas gathering system operating in the Appalachian Basin, which includes the Marcellus Shale formation in northern West Virginia. Summit Marketing provides natural gas and crude oil marketing services in and around our gathering systems. 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) 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. 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 ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2018
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. 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. 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) (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, 2018 or 2017. Amortizing Intangibles. Upon the acquisition of DFW Midstream, certain of its gas gathering contracts were deemed to have above-market pricing structures. We have recognized the above-market contracts as favorable gas gathering contracts. We amortize the favorable 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 (vii) 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 the amortization of the 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 the amortization of the 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 present value of the estimated 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 Notes 17 and 19 for additional information). 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, an income-based approach or a combination of the two approaches. Derivative Contracts. We have commodity price exposure related to our sale of the physical natural gas we retain from certain DFW Midstream 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. We sell retainage natural gas at prices that are based on the Waha Hub Index and/or the Atmos Zone 3 Index. By basing the power prices on an index and basin-relevant market, we are able to closely associate the relationship between the 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 through October 2018, the 1% ownership interest in OpCo was reflected as noncontrolling interest in partners' capital. We reflected changes in our ownership of OpCo as adjustments to noncontrolling interest. See Note 12 for additional information. Revenue Recognition. The majority of our revenue is derived from long-term, fee-based contracts with original terms of up to 25 years. We account for revenue in accordance with Topic 606, which we adopted on January 1, 2018, using the modified retrospective method. See below for further discussion of the adoption. We recognize revenue earned from fee-based gathering, treating and processing services in gathering services and related fees. We also earn revenue in the Williston Basin reporting segment from the sale of physical natural gas purchased from our customers under certain percent-of-proceeds arrangements; under Topic 606, fees from these arrangements are presented net within cost of natural gas and NGLs. We sell natural gas that we retain from certain DFW Midstream customers to offset the power expenses of the electric-driven compression on the DFW Midstream system. We also sell condensate retained from our gathering services at Grand River. Revenues from the sale of natural gas and condensate are recognized in natural gas, NGLs and condensate sales; the associated expense is included in operation and maintenance expense. Certain customers reimburse us for costs we incur on their behalf. We record costs incurred and reimbursed by our customers on a gross basis, with the revenue component recognized in other revenues. We provide gathering and/or processing services principally under contracts that contain one or more of the following arrangements: • Fee-based arrangements. • Percent-of-proceeds arrangements. Certain of our gathering and/or processing agreements provide for monthly, annual or multi-year MVCs. Under these MVCs, our customers agree to ship and/or process a minimum volume of production on our gathering systems or to pay a minimum monetary amount over certain periods during the term of the MVC. A customer must make a shortfall payment to us following 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 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 customer obligations under their MVCs as revenue and contract assets when (i) we consider it remote that the customer will utilize shortfall payments to offset gathering or processing fees in excess of its MVCs in subsequent periods; (ii) the customer incurs a shortfall in a contract with no banking mechanism or claw back provision; (iii) the customer’s banking mechanism has expired; or (iv) it is remote that the customer will use its unexercised right. 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. The aggregate difference in the basis of the Partnership’s net assets for financial and income tax purposes cannot be readily determined as the Partnership does not have access to the information about each partner’s tax attributes related to the Partnership. 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) any payment of IDRs, by the weighted-average number of limited partner units outstanding, (ii) the General Partner's approximate 2% interest in net income or loss, (iii) any net income or loss of contributed subsidiaries that is attributable to Summit Investments, (iv) the 1% noncontrolling interest in OpCo (for periods subsequent to the 2016 Drop Down and prior to 2018) and (v) net income attributable to Series A Preferred Units. 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. Pursuant to the closing of the Equity Restructuring Agreement, the IDRs and 2% general partner interest will be converted into 8,750,000 common units. 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. 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 • ASU No. 2014-09 Revenue from Contracts with Customers (“Topic 606”). We adopted Topic 606 with a date of initial application of January 1, 2018. We applied Topic 606 by recognizing the cumulative effect of initially applying Topic 606 as an adjustment to the opening balance of partners’ capital at January 1, 2018. The comparative information has not been adjusted and is reported under the accounting standards in effect for those periods. For contracts where we perform gathering services and earn a per-unit fee which is recognized at a point in time, revenue is recognized over time as the service is performed and results in revenue recognition materially consistent with historical GAAP. In addition, our contracts generally contain forms of variable consideration, which will likely be constrained as the volumes are susceptible to factors outside of our control and influence. As a result of applying the constraint guidance, timing of revenue recognition will be materially consistent with historical GAAP. Prior to the adoption of Topic 606, contributions in aid of construction were recognized as a reduction to our cost basis of property, plant and equipment and facility fees were recognized as revenue when the amounts were billed. Upon adoption of Topic 606, the contributions in aid of construction amounts previously received were capitalized to property, plant and equipment, net of any accumulated depreciation, and will be depreciated over the remaining useful lives. Any future contributions in aid of construction will be recognized as revenue over the then remaining term of the respective contract in accordance with Topic 606. Additionally, facility fees will be deferred and recognized over the remaining contract term. There are certain percent-of-proceeds contracts within our Williston Basin reportable segment where we previously recognized revenue for services provided to producers in gathering services and related fees. Such amounts which were previously presented gross in gathering services and related fees are presented net within cost of natural gas and NGLs. This change did not have any impact on our net income (loss), cash flows, or the amount we present as segment adjusted EBITDA. For contracts containing MVC arrangements with banking mechanisms we previously deferred revenue. Under Topic 606, the recognition of revenue was accelerated. This acceleration totaled $16.7 million and is included in the Topic 606 day one adjustment amounts below in deferred revenue. The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows: Balance at December 31, 2017 Adjustments Due to Topic 606 Balance at January 1, 2018 (In thousands) Assets Property, plant and equipment, net $ 1,795,129 $ 33,123 $ 1,828,252 Liabilities Deferred revenue, current 4,000 6,088 10,088 Deferred revenue, noncurrent 12,707 22,821 35,528 Partners' Capital (1) 1,084,430 4,214 1,088,644 ________ (1) Includes common limited partner capital and general partner interests . Impact on financial statements The following tables summarize the impact of Topic 606 adoption on our consolidated financial statements. Consolidated balance sheet December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Assets Accounts receivable $ 97,936 $ 91,936 $ 6,000 Property, plant and equipment, net 1,963,713 1,926,215 37,498 Liabilities Deferred revenue, current 11,467 4,071 7,396 Deferred revenue, noncurrent 39,504 8,938 30,566 Partners' Capital (1) 927,608 922,072 5,536 (1) Includes common limited partner capital and general partner interests. Consolidated statement of operations Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Revenues Gathering services and related fees $ 344,616 $ 351,589 $ (6,973 ) Costs and expenses Cost of natural gas and NGLs 107,661 120,976 (13,315 ) Depreciation and amortization 107,100 105,765 1,335 Consolidated statement of cash flows Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Cash flows from operating activities: Net income $ 42,351 $ 37,344 $ 5,007 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 106,767 105,432 1,335 Changes in operating assets and liabilities: Accounts receivable (21,535 ) (15,535 ) (6,000 ) Deferred revenue, net 5,355 5,697 (342 ) • ASU No. 2017-04 Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"). ASU 2017-04 simplifies the subsequent measurement of goodwill by, among other things, eliminating step two from the goodwill impairment test. ASU 2017-04 is effective for public companies for fiscal years beginning after December 15, 2019 and it specifies the amendments in ASU 2017-04 should be applied on a prospective basis. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. We adopted the provisions of ASU 2017-04 effective January 1, 2018. The adoption of this standard had no impact on our consolidated financial statements . Accounting Pronouncements Pending Adoption • 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 (“ROU”) asset will be recorded which represents the lessee’s right to use, or to control the use of, a specified asset for a lease term. ASU 2016-02 is effective for public companies for fiscal years beginning after December 15, 2018, and requires the modified retrospective approach for transition. We expect to utilize certain practical expedients including (i) not being required to reassess whether any expired or existing contracts are or contain leases; (ii) not being required to reassess the lease classification for any expired or existing leases (that is, all existing leases that were classified as operating leases in accordance with Topic 840 will be classified as operating leases, and all existing leases that were classified as capital leases in accordance with Topic 840 will be classified as finance leases); and (iii) not being required to reassess initial direct costs for any existing leases. We adopted ASU 2016-02 on January 1, 2019. We will recognize a ROU asset and a corresponding lease liability based on the present value of such obligations. Based on current estimates, the total ROU assets we will recognize under ASU 2016-02 will account for less than 0.5% of total consolidated assets and the corresponding lease liabilities w ill a ccount for less than 0.5 % of total consolidated liabilities . We will also provide additional disclosures around the nature of the leasing activities beginning in our Form 10-Q for the three months ended March 31, 2019. These include additional qualitative disclosures, such as a general description of leases, and quantitative disclosures, such as lease costs, weighted average remaining lease term and weighted average discount rate. • ASU No. 2018-01 Leases: Land Easement Practical Expedient for Transition to Topic 842 (“ASU 2018-01”). ASU 2018-01 provides an optional transition practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance in Topic 840. Upon adoption of Topic 842, an entity that elects this practical expedient should evaluate new or modified land easements under Topic 842 beginning at the date the entity adopts Topic 842. We expect to adopt the optional transition practical expedient of ASU 2018-01 effective January 1, 2019. • ASU No. 2018-13 Fair Value Measurement (“ASU 2018-13”). ASU 2018-13 updates the disclosure requirements on fair value measurements including new disclosures for the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. ASU 2018-13 modifies existing disclosures including clarifying the measurement uncertainty disclosure. ASU 2018-13 removes certain existing disclosure requirements including the amount and reasons for transfers between Level 1 and Level 2 fair value measurements and the policy for the timing of transfer between levels. We are currently evaluating the provisions of ASU 2018-13 to determine its impact on our financial statements and related disclosures and will adopt its provisions effective January 1, 2020 .

REVENUE

REVENUE	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
REVENUE	3. REVENUE The majority of our revenue is derived from long-term, fee-based contracts with original terms of up to 25 years. We account for revenue in accordance with Topic 606, which we adopted on January 1, 2018, using the modified retrospective method. See Note 2 for further discussion of the adoption, including the impact on our consolidated financial statements. The transaction price in our contracts is primarily based on the volume of natural gas, crude oil or produced water transferred by our gathering systems to the customer’s agreed upon delivery point multiplied by the contractual rate. For contracts that include MVCs, variable consideration up to the MVC will be included in the transaction price. For contracts that do not include MVCs, we do not estimate variable consideration because the performance obligations are completed and settled on a daily basis. For contracts containing noncash consideration such as fuel received in-kind, we measure the transaction price at the point of sale when the volume, mix and market price of the commodities are known. We have contracts with MVCs that are variable and constrained. Contracts with MVCs are reviewed on a quarterly basis and adjustments to those estimates are made during each respective reporting period, if necessary. The transaction price is allocated if the contract contains more than one performance obligation such as contracts that include MVCs. The transaction price allocated is based on the MVC for the applicable measurement period. Performance obligations . The majority of our contracts have a single performance obligation which is either to provide gathering services (an integrated service) or sell natural gas, NGLs and condensate, which are both satisfied when the related natural gas, crude oil and produced water are received and transferred to an agreed upon delivery point. We also have certain contracts with multiple performance obligations. They include an option for the customer to acquire additional services such as contracts containing MVCs. These performance obligations would also be satisfied when the related natural gas, crude oil and produced water are received and transferred to an agreed upon delivery point. In these instances, we allocate the contract’s transaction price to each performance obligation using our best estimate of the standalone selling price of each service in the contract. Performance obligations for gathering services are generally satisfied over time. We utilize either an output method (i.e., measure of progress) for guaranteed, stand-ready service contracts or an asset / system delivery time estimate for non-guaranteed, as-available service contracts. Performance obligations for the sale of natural gas, NGLs and condensate are satisfied at a point in time. There are no significant judgments for these transactions because the customer obtains control based on an agreed upon delivery point. Certain of our gathering and/or processing agreements provide for monthly, annual or multi-year MVCs. Under these MVCs, our customers agree to ship and/or process a minimum volume of production on our gathering systems or to pay a minimum monetary amount over certain periods during the term of the MVC. A customer must make a shortfall payment to us at the end of the contracted measurement period if its actual throughput volumes are less than its MVC for that period. Certain customers are entitled to utilize shortfall payments to offset gathering fees in one or more subsequent contracted measurement periods to the extent that such customer's throughput volumes in a subsequent contracted measurement period exceed its MVC for that contracted measurement period. We recognize customer obligations under their MVCs as revenue and contract assets when (i) we consider it remote that the customer will utilize shortfall payments to offset gathering or processing fees in excess of its MVCs in subsequent periods; (ii) the customer incurs a shortfall in a contract with no banking mechanism or claw back provision; (iii) the customer’s banking mechanism has expired; or (iv) it is remote that the customer will use its unexercised right. Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components. The following table presents estimated revenue expected to be recognized over the remaining contract period related to performance obligations that are unsatisfied and are comprised of estimated MVC shortfall payments. We applied the practical expedient in paragraph 606-10-50-14 of Topic 606 for certain arrangements that we consider optional purchases (i.e., there is no enforceable obligation for the customer to make purchases) and those amounts are excluded from the table. 2019 2020 2021 2022 2023 Thereafter (In thousands) Gathering services and related fees $ 126,006 $ 122,429 $ 100,070 $ 83,626 $ 70,923 $ 112,462 Revenue by Category . In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4 . Reportable Segments Year ended December 31, 2018 Utica Shale Williston Basin DJ Basin Permian Basin Piceance Basin Barnett Shale Marcellus Shale Total reportable segments All other segments Total (In thousands) Major products / services lines Gathering services and related fees $ 35,233 $ 79,606 $ 11,251 $ 115 $ 135,810 $ 59,030 $ 29,573 $ 350,618 $ (6,002 ) $ 344,616 Natural gas, NGLs and condensate sales — 31,840 371 843 14,800 2,523 — 50,377 84,457 134,834 Other revenues — 12,204 3,672 — 4,909 6,712 — 27,497 (294 ) 27,203 Total $ 35,233 $ 123,650 $ 15,294 $ 958 $ 155,519 $ 68,265 $ 29,573 $ 428,492 $ 78,161 $ 506,653 Contract balances . Contract assets relate to our rights to consideration for work completed but not billed at the reporting date and consist of the estimated MVC shortfall payments expected from our customers and unbilled activity associated with contributions in aid of construction. Contract assets are transferred to trade receivables when the rights become unconditional. The following table provides information about contract assets from contracts with customers : December 31, 2018 (In thousands) Contract assets, December 31, 2017 $ — Additions 26,403 Transfers out (17,648 ) Contract assets, December 31, 2018 $ 8,755 As of December 31, 2018, receivables with customers totaled $82.9 million and contract assets totaled $8.8 million which were included in the accounts receivable caption on the consolidated balance sheet. Contract liabilities (deferred revenue) relate to the advance consideration received from customers primarily for contributions in aid of construction. We recognize contract liabilities under these arrangements in revenue over the contract period. For the year ended December 31, 2018, we recognized $10.8 million of gathering services and related fees which was included in the contract liability balance as of the beginning of the period. See Note 9 for additional details.

SEGMENT INFORMATION

SEGMENT INFORMATION	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
SEGMENT INFORMATION	4. SEGMENT INFORMATION We evaluate our business operations each reporting period to determine whether any of our 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 fourth quarter of 2018, we determined that the DJ Basin natural gas gathering and processing system, previously reported within the Piceance/DJ Basins reportable segment, is expected to be a significant operating segment in future reporting periods. This determination was based on, among other things, the development of a new 60 MMcf/d processing plant that is expected to be operational in 2019, which will increase volume throughput beginning in 2019. In addition, we determined the Permian Basin natural gas gathering and processing system, which was commissioned in the fourth quarter of 2018, is expected to be a significant operating segment in future reporting periods. As such, we modified our current segments in the fourth quarter of 2018 such that the DJ Basin reportable segment includes the Niobrara G&P system and the Permian Basin reportable segment includes the Summit Permian natural gas gathering and processing system. For the year ended December 31, 2018, we have disclosed the required segment information for Niobrara G&P and Summit Permian and the periods prior have been recast to reflect this change. As of December 31, 2018, our reportable segments are: • the Utica Shale, which is served by Summit Utica; • Ohio Gathering, which includes our ownership interest in OGC and OCC; • the Williston Basin, which is served by Polar and Divide, Tioga Midstream and Bison Midstream; • the DJ Basin, which is served by Niobrara G&P; • the Permian Basin, which is served by Summit Permian; • the Piceance Basin, which is served by Grand River; • the Barnett Shale, which is served by DFW Midstream; and • the Marcellus Shale, which is served by Mountaineer Midstream. Each of our reportable segments provides midstream services in a specific geographic area. Our reportable segments reflect the way in which we internally report the financial information used to make decisions and allocate resources in connection with our operations. The Ohio Gathering reportable segment includes our investment in OGC and OCC. Income or loss from equity method investees, as reflected on the statements of operations, solely relates to Ohio Gathering and is recognized and disclosed on a one-month lag (see Note 8). 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, 2018 2017 2016 (In thousands) Assets: Utica Shale $ 207,357 $ 212,311 $ 199,392 Ohio Gathering 649,250 690,485 707,415 Williston Basin 526,819 512,860 724,084 DJ Basin 166,580 79,438 72,494 Permian Basin 145,702 57,590 — Piceance Basin 699,638 719,284 770,946 Barnett Shale 376,564 383,306 404,314 Marcellus Shale 208,790 217,362 224,709 Total reportable segment assets 2,980,700 2,872,636 3,103,354 Corporate and Other 44,181 22,406 12,294 Eliminations (4,319 ) (249 ) (469 ) Total assets $ 3,020,562 $ 2,894,793 $ 3,115,179 Revenues by reportable segment follow. Year ended December 31, 2018 2017 2016 (In thousands) Revenues (1): Utica Shale $ 35,233 $ 38,907 $ 24,263 Williston Basin 123,650 161,503 122,174 DJ Basin 15,294 11,860 8,439 Permian Basin 958 — — Piceance Basin 155,519 154,893 141,464 Barnett Shale 68,265 71,667 79,956 Marcellus Shale 29,573 30,394 26,111 Total reportable segments revenue 428,492 469,224 402,407 Corporate and Other 88,286 26,446 412 Eliminations (10,125 ) (6,929 ) (457 ) Total revenues $ 506,653 $ 488,741 $ 402,362 (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, 2018 2017 2016 Percentage of total revenues (1)(2): Counterparty A - Piceance Basin * * 14 % Counterparty B - Williston Basin * 13 % * Counterparty C - Piceance Basin 10 % * * (1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments (see Note 9). (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, 2018 2017 2016 (In thousands) Depreciation and amortization (1): Utica Shale $ 7,672 $ 7,009 $ 4,331 Williston Basin 22,642 33,772 33,676 DJ Basin 3,133 2,636 2,524 Permian Basin 243 — — Piceance Basin 46,919 46,289 46,616 Barnett Shale (2) 15,325 15,001 16,093 Marcellus Shale 9,090 9,047 8,841 Total reportable segment depreciation and amortization 105,024 113,754 112,081 Corporate and Other 1,743 1,118 580 Total depreciation and amortization $ 106,767 $ 114,872 $ 112,661 (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, 2018 2017 2016 (In thousands) Cash paid for capital expenditures (1): Utica Shale $ 5,719 $ 22,921 $ 78,708 Williston Basin 25,202 17,309 31,541 DJ Basin 64,920 7,150 5,807 Permian Basin 83,823 56,020 — Piceance Basin 7,887 16,564 19,912 Barnett Shale 1,370 569 3,910 Marcellus Shale 1,030 641 1,173 Total reportable segment capital expenditures 189,951 121,174 141,051 Corporate and Other 10,635 3,041 1,668 Total cash paid for capital expenditures $ 200,586 $ 124,215 $ 142,719 (1) Excludes cash paid for capital expenditures by Ohio Gathering due to equity method accounting. During the year ended December 31, 2018, Corporate and Other included cash paid of $3.3 million for corporate purposes; the remainder represents capital expenditures for the Double E Pipeline Project relating to the Summit Permian Transmission expansion. We assess the performance of our reportable segments based on segment adjusted EBITDA. 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, 2018 2017 2016 (In thousands) Reportable segment adjusted EBITDA Utica Shale $ 30,285 $ 34,011 $ 21,035 Ohio Gathering 39,969 41,246 45,602 Williston Basin 76,701 66,413 79,475 DJ Basin 7,558 6,624 3,681 Permian Basin (1,200 ) — — Piceance Basin 111,042 111,113 105,560 Barnett Shale 43,268 46,232 54,634 Marcellus Shale 24,267 23,888 19,203 Total of reportable segments' measures of profit or loss $ 331,890 $ 329,527 $ 329,190 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, 2018 2017 2016 (In thousands) Reconciliation of income (loss) before income taxes and loss from equity method investees to total of reportable segments' measures of profit: Income (loss) before income taxes and loss from equity method investees $ 53,272 $ 88,614 $ (7,768 ) Add: Corporate and Other 38,917 39,140 37,589 Interest expense 60,535 68,131 63,810 Early extinguishment of debt — 22,039 — Deferred Purchase Price Obligation 20,975 (200,322 ) 55,854 Depreciation and amortization 106,767 114,872 112,661 Proportional adjusted EBITDA for equity method investees 39,969 41,246 45,602 Adjustments related to MVC shortfall payments (3,632 ) (41,373 ) 11,600 Adjustments related to capital reimbursement activity (427 ) — — Unit-based and noncash compensation 8,328 7,951 7,985 Loss on asset sales, net — 527 93 Long-lived asset impairment 7,186 188,702 1,764 Total of reportable segments' measures of profit $ 331,890 $ 329,527 $ 329,190 For the year ended December 31, 2018, in accordance with Topic 606, adjustments related to MVC shortfall payments are recognized in gathering services and related fees (see Note 3). In accordance with Topic 606, contributions in aid of construction are recognized over the remaining term of the respective contract. We include adjustments related to capital reimbursement activity in our calculation of segment adjusted EBITDA to account for revenue recognized from contributions in aid of construction. For the years ended December 31, 2017 and 2016, we included adjustments related to MVC shortfall payments in our calculation of segment adjusted EBITDA to account for (i) the net increases or decreases in deferred revenue for MVC shortfall payments and (ii) our inclusion of expected annual or multi-year MVC shortfall payments. With respect to the impact of a net change in deferred revenue for MVC shortfall payments, we treated increases in deferred revenue balances as a favorable adjustment to segment adjusted EBITDA, while decreases in deferred revenue balances were treated as an unfavorable adjustment to segment adjusted EBITDA. We also included a proportional amount of any historical and expected MVC shortfall payments in each quarter prior to the quarter in which we actually recognize the shortfall payment. Adjustments related to MVC shortfall payments by reportable segment follow. Year ended December 31, 2018 Williston Basin Piceance Basin Barnett Shale Total (In thousands) Adjustments related to MVC shortfall payments: Net change in deferred revenue for MVC shortfall payments $ — $ — $ — $ — Expected MVC shortfall adjustments — 10 (3,642 ) (3,632 ) Total adjustments related to MVC shortfall payments $ — $ 10 $ (3,642 ) $ (3,632 ) Year ended December 31, 2017 Williston Basin Piceance Basin 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 Basin 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

PROPERTY, PLANT, AND EQUIPMENT,

PROPERTY, PLANT, AND EQUIPMENT, NET	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
PROPERTY, PLANT, AND EQUIPMENT, NET	5. PROPERTY, PLANT AND EQUIPMENT, NET Details on property, plant and equipment follow. December 31, 2018 December 31, 2017 (In thousands) Gathering and processing systems and related equipment $ 2,155,325 $ 1,973,722 Construction in progress 137,920 78,850 Land and line fill 11,748 11,735 Other 45,853 40,262 Total 2,350,846 2,104,569 Less accumulated depreciation 387,133 309,440 Property, plant and equipment, net $ 1,963,713 $ 1,795,129 During 2018, 2017 and 2016, 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 a market-based approach. In December 2018, in connection with certain strategic initiatives, we performed a recoverability assessment of certain assets within the Williston Basin reporting segment. Based on the results, we concluded that the carrying value of certain long-lived assets related to the Tioga Midstream system within the Williston Basin were not fully recoverable. We recorded an impairment charge of $3.9 million related to these assets after comparing the fair value of the long-lived assets to their carrying values. In addition, we reviewed the other assets that had been identified as potentially impaired and recognized the long-lived asset impairments in the table below. 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 related 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 6 for additional details. During 2018, 2017 and 2016, we recognized the following long-lived asset impairments, by segment. Year ended December 31, 2018 2017 2016 (In thousands) Long-lived asset impairment: Williston Basin $ 3,972 $ 101,961 $ 569 Piceance Basin 1,004 697 — DJ Basin 9 — — Barnett Shale — — 1,195 Utica Shale 1,440 878 — Permian Basin 761 — — 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, 2018 2017 2016 (In thousands) Depreciation expense $ 74,511 $ 75,120 $ 70,770 Capitalized interest 8,497 2,579 3,709

AMORTIZING INTANGIBLE ASSETS AN

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT	6. AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities prior to 2018), all of which are subject to amortization, follow. December 31, 2018 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (13,905 ) $ 10,290 Contract intangibles 278,448 (143,962 ) 134,486 Rights-of-way 166,209 (37,569 ) 128,640 Total intangible assets $ 468,852 $ (195,436 ) $ 273,416 Unfavorable gas gathering contract $ 10,962 $ (10,962 ) $ — 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 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 5). 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. We recognized amortization expense in other revenues as follows: Year ended December 31, 2018 2017 2016 (In thousands) Amortization expense – favorable gas gathering contracts $ (1,555 ) $ (1,555 ) $ (1,261 ) Amortization expense – unfavorable gas gathering contract 1,888 2,158 839 We recognized amortization expense in costs and expenses as follows: Year ended December 31, 2018 2017 2016 (In thousands) Amortization expense – contract intangibles $ 26,141 $ 34,202 $ 35,416 Amortization expense – rights-of-way 6,448 6,153 6,053 The estimated aggregate annual amortization expected to be recognized for as of December 31, 2018 for each of the five succeeding fiscal years follows. Intangible assets (In thousands) 2019 $ 32,422 2020 32,246 2021 28,554 2022 25,487 2023 25,433

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
GOODWILL	7. GOODWILL Goodwill for the periods presented of $16.2 million is related to the acquisition of the Mountaineer Midstream system in 2013. Accumulated goodwill impairments by reportable segment for those reporting units that have previously recognized goodwill follow. As of December 31, 2018, 2017, 2016 (In thousands) Accumulated goodwill impairment: Piceance Basin $ 45,478 Williston Basin 257,572 Total accumulated goodwill impairment $ 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, 2018 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, 2018 and 2017. 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

EQUITY METHOD INVESTMENTS	12 Months Ended
	Dec. 31, 2018
Equity Method Investments And Joint Ventures [Abstract]
EQUITY METHOD INVESTMENTS	8. EQUITY METHOD INVESTMENTS Ohio Gathering owns, operates and is currently developing midstream infrastructure consisting of a liquids-rich natural gas gathering system, a dry natural gas gathering system and a condensate stabilization facility in the Utica Shale in southeastern Ohio. Ohio Gathering provides gathering services pursuant to primarily long-term, fee-based gathering agreements, which include acreage dedications. Our initial investment in Ohio Gathering in 2014 included a $190.0 million payment to acquire a 1% interest from a third party, which included an option to increase our ownership to 40%, as well as a series of contributions directly to Ohio Gathering in 2014, which increased our ownership to 40%. Concurrent with and subsequent to the exercise of the option, the non-affiliated owners have retained their respective 60% ownership interest in Ohio Gathering (the "Non-affiliated Owners"). We account for our ownership interests in Ohio Gathering as an equity method investment because we have joint control with the Non-affiliated Owners, which gives us significant influence. We recognize the $190.0 million paid for the initial 1% interest as an investment in Ohio Gathering at inception. In addition, Ohio Gathering assigned a value of $7.5 million to the exercise option, which it ultimately attributed to our capital account. Neither of the aforementioned transactions involved a flow of funds to or from Ohio Gathering. As such, they created a basis difference between our recorded investment in equity method investees and the amount attributed to us by Ohio Gathering within its financial statements. We are amortizing these basis differences over the weighted-average remaining life of the contracts underlying Ohio Gathering's operations. In December 2018 and 2017, asset impairments were recognized by Ohio Gathering. In addition, Ohio Gathering was involved in legal proceedings relating to a dispute regarding pipeline right of way rights and associated trespass claims that took place prior to December 31, 2018. Ohio Gathering received a judgment on those proceedings in January 2019 and recorded an estimate of the legal exposure as of December 31, 2018. Although we recognize activity for Ohio Gathering on a one-month lag, we recorded the asset impairments and legal contingency in our results of operations for the year ending December 31, 2018 and asset impairments for the year ending December 31, 2017 because the information was available to us. We recorded our 40% share of the asset impairments and legal contingency amounting to $7.7 million in 2018 and asset impairments of $1.4 million in 2017 in loss from equity method investees in the consolidated statements of operations. A reconciliation of our 2018 2017 (In thousands) Investment in equity method investees, December 31 $ 649,250 $ 690,485 December cash distributions 2,736 4,032 December cash contributions — (3,932 ) Impairment loss 5,652 1,383 Legal contingency 2,040 — Basis difference (116,832 ) (130,184 ) Investment in equity method investees, net of basis difference, November 30 $ 542,846 $ 561,784 Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information). November 30, 2018 November 30, 2017 OGC OCC OGC OCC (In thousands) Current assets $ 37,403 $ 3,716 $ 34,383 $ 3,650 Noncurrent assets 1,262,253 27,203 1,319,448 29,156 Total assets $ 1,299,656 $ 30,919 $ 1,353,831 $ 32,806 Current liabilities $ 19,903 $ 3,912 $ 10,882 $ 3,382 Noncurrent liabilities 3,688 8,807 3,272 11,715 Total liabilities $ 23,591 $ 12,719 $ 14,154 $ 15,097 Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information). Twelve months ended November 30, 2018 Twelve months ended November 30, 2017 Twelve months ended November 30, 2016 OGC OCC OGC OCC OGC OCC (In thousands) Total revenues $ 142,398 $ 10,177 $ 140,679 $ 8,607 $ 148,662 $ 15,791 Total operating expenses 136,722 9,053 111,897 8,298 96,647 111,528 Net income (loss) 5,670 498 28,785 (907 ) 52,009 (94,230 )

DEFERRED REVENUE

DEFERRED REVENUE	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
DEFERRED REVENUE	9. DEFERRED REVENUE Certain of our gathering and/or processing agreements provide for monthly, annual or multi-year MVCs. The amount of the shortfall payment is based on the difference between the actual throughput volume shipped and/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. Utica Shale Williston Basin DJ Basin Piceance Basin Barnett Shale Marcellus Shale Total current (In thousands) Current deferred revenue, January 1, 2017 $ — $ — $ — $ — $ — $ — $ — Additions — — — 18,294 — — 18,294 Less revenue recognized — — — 14,294 — — 14,294 Current deferred revenue, December 31, 2017, as reported — — — 4,000 — — 4,000 Net impact of Topic 606 day 1 adoption 18 1,017 358 3,038 1,619 38 6,088 Current deferred revenue, January 1, 2018 18 1,017 358 7,038 1,619 38 10,088 Additions 18 1,744 943 21,955 1,651 96 26,407 Less revenue recognized 18 1,347 562 21,377 1,628 96 25,028 Current deferred revenue, December 31, 2018 $ 18 $ 1,414 $ 739 $ 7,616 $ 1,642 $ 38 $ 11,467 A rollforward of noncurrent deferred revenue follows. Utica Shale Williston Basin DJ Basin Piceance Basin Barnett Shale Marcellus Shale Total noncurrent (In thousands) Noncurrent deferred, revenue, January 1, 2017 $ — $ 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, as reported — — — 12,707 — — 12,707 Net impact of Topic 606 day 1 adoption 39 4,215 4,505 5,512 8,217 333 22,821 Noncurrent deferred revenue, January 1, 2018 39 4,215 4,505 18,219 8,217 333 35,528 Additions — 1,851 3,720 7,869 3,062 — 16,502 Less reclassification to current deferred revenue 18 1,673 941 8,146 1,651 97 12,526 Noncurrent deferred revenue, December 31, 2018 $ 21 $ 4,393 $ 7,284 $ 17,942 $ 9,628 $ 236 $ 39,504 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

DEBT

DEBT	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
DEBT	10. DEBT Debt consisted of the following: December 31, 2018 December 31, 2017 (In thousands) Summit Holdings' variable rate senior secured Revolving Credit Facility (5.03% at December 31, 2018 and 4.07% at December 31, 2017) due May 2022 $ 466,000 $ 261,000 Summit Holdings' 5.5% senior unsecured notes due August 2022 300,000 300,000 Less unamortized debt issuance costs (1) (2,362 ) (2,910 ) Summit Holdings' 5.75% senior unsecured notes due April 2025 500,000 500,000 Less unamortized debt issuance costs (1) (5,907 ) (6,898 ) Total long-term debt $ 1,257,731 $ 1,051,192 __________ (1) Issuance costs are being amortized over the life of the notes. The aggregate amount of debt maturing during each of the years after December 31, 2018 are as follow (in thousands): 2019 $ — 2020 — 2021 — 2022 766,000 2023 — Thereafter 500,000 Total long-term debt $ 1,266,000 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. Bison Midstream and its subsidiaries, Grand River and its subsidiary, DFW Midstream, Summit Marketing, Summit Permian, Permian Finance, Summit Niobrara, OpCo, Summit Utica, Meadowlark Midstream, Tioga Midstream and SMLP fully and unconditionally and jointly and severally guarantee, and pledge substantially all of their assets in support of, the indebtedness outstanding under the Revolving Credit Facility . 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 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 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). 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 As a result of the amendment, SMLP incurred approximately As of December 31, 2018, we were in compliance with the Revolving Credit Facility's financial covenants. There were 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 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. In 2018, we executed supplemental indentures to include OpCo, Summit Utica, Meadowlark Midstream and Tioga Midstream as guarantors concurrent with the purchase of a 1% noncontrolling interest held by a subsidiary of Summit Investments (see Note 12 for additional details). As a result, Bison Midstream and its subsidiaries, Grand River and its subsidiary, DFW Midstream, Summit Marketing, Summit Permian, Permian Finance, Summit Niobrara, OpCo, Summit Utica, Meadowlark Midstream and Tioga Midstream (collectively the "Guarantor Subsidiaries") are 100% owned. The Guarantor Subsidiaries and SMLP fully and unconditionally and jointly and severally guarantee the 5.75% Senior Notes At any time prior to April 15, 2020, the Co-Issuers may redeem up to The The 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 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 5.5% 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 December 31, 2018, we were in compliance with the financial covenants governing our Senior Notes. There were no defaults or events of default during the year ended December 31, 2018.

FINANCIAL INSTRUMENTS

FINANCIAL INSTRUMENTS	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
FINANCIAL INSTRUMENTS	11. FINANCIAL INSTRUMENTS Concentrations of Credit Risk. Financial instruments that potentially subject us to concentrations of credit risk consist of cash and cash equivalents and accounts receivable. We maintain our cash and cash equivalents in bank deposit accounts that frequently exceed federally insured limits. We have not experienced any losses in such accounts and do not believe we are exposed to any significant risk. Accounts receivable primarily comprise amounts due for the gathering, treating and processing services we provide to our customers and also the sale of natural gas liquids resulting from our processing services. This industry concentration has the potential to impact our overall exposure to credit risk, either positively or negatively, in that our customers may be similarly affected by changes in economic, industry or other conditions. We monitor the creditworthiness of our counterparties and can require letters of credit for receivables from counterparties that are judged to have substandard credit, unless the credit risk can otherwise be mitigated. Our top 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 Notes 17 and 19). 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, 2017 $ 563,281 Change in fair value (200,322 ) Level 3 liability, December 31, 2017 362,959 Change in fair value 20,975 Level 3 liability, December 31, 2018 $ 383,934 A summary of the estimated fair value of our debt financial instruments follows. December 31, 2018 December 31, 2017 Carrying value Estimated fair value (Level 2) Carrying value Estimated fair value (Level 2) (In thousands) Summit Holdings 5.5% Senior Notes ($300.0 million principal) $ 297,638 $ 286,625 $ 297,090 $ 301,750 Summit Holdings 5.75% Senior Notes ($500.0 million principal) 494,093 455,208 493,102 501,667 The carrying value on the balance sheet of the Revolving Credit Facility is its fair value due to its floating interest rate. The fair value for the Senior Notes is based on an average of nonbinding broker quotes as of December 31, 2018 and 2017. The use of different market assumptions or valuation methodologies may have a material effect on the estimated fair value of the Senior Notes.

PARTNERS' CAPITAL

PARTNERS' CAPITAL	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
PARTNERS' CAPITAL	12. 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, 2016 — 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 the 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 the 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 Net units issued under the SMLP LTIP — 304,857 — — Units, December 31, 2018 300,000 73,390,853 — 1,490,999 Unit Offerings. 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 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, 2018, there were no transactions under the ATM Program. 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, 2018, we have the capacity to issue additional common units under the ATM Program up to an aggregate $132.3 million. Series A Preferred Units. In 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 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%. Noncontrolling Interest. At December 31, 2017, we recorded Summit Investments' indirect retained ownership interest in OpCo and its subsidiaries as a noncontrolling interest in the consolidated financial statements. In November 2018, a subsidiary of SMLP purchased the remaining 1% ownership interest in OpCo held by a subsidiary of Summit Investments for approximately $10.9 million. As a result of this transaction, other than our investment in Ohio Gathering, all of our business activities are now conducted through wholly owned operating subsidiaries. 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 that have been acquired by SMLP. The balance also reflects net income attributable to Summit Investments for the 2016 Drop Down Assets 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, 2016 to March 3, 2016. 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 17). 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. Cash Distribution Policy 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. 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 . 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. If the recently announced Equity Restructuring is consummated, the 2% general partner interest will be converted into a non-economic general partner interest. Cash Distributions Paid and Declared. We paid the following per-unit distributions during the years ended December 31: Year ended December 31, 2018 2017 2016 Per-unit distributions to unitholders $ 2.300 $ 2.300 $ 2.300 On January 24, 2019, the Board of Directors of our General Partner declared a distribution of $0.575 We allocated the February 2019 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% Our distributions in 2016, 2017 and 2018 have all been within the Third target distribution level. 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, 2018 2017 2016 (In thousands) IDR payments $ 8,535 $ 8,460 $ 7,912 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

EARNINGS PER UNIT	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
EARNINGS PER UNIT	13. EARNINGS PER UNIT The following table details the components of EPU. Year ended December 31, 2018 2017 2016 (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 $ 32,799 $ 75,485 $ (48,179 ) Less net income attributable to Series A Preferred Units 28,500 3,563 — Net income (loss) attributable to common limited partners $ 4,299 $ 71,922 $ (48,179 ) Denominator for basic and diluted EPU: Weighted-average common units outstanding – basic 73,304 72,705 68,264 Effect of nonvested phantom units 311 342 — Weighted-average common units outstanding – diluted 73,615 73,047 68,264 Earnings (loss) per limited partner unit: Common unit – basic $ 0.06 $ 0.99 $ (0.71 ) Common unit – diluted $ 0.06 $ 0.98 $ (0.71 ) Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU 2 42 125

UNIT-BASED AND NONCASH COMPENSA

UNIT-BASED AND NONCASH COMPENSATION	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
UNIT-BASED AND NONCASH COMPENSATION	14. UNIT-BASED AND NONCASH COMPENSATION SMLP Long-Term Incentive Plan. The SMLP LTIP provides for equity awards to eligible officers, employees, consultants and directors of our General Partner and its affiliates, 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, 2018, approximately 3.2 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 unit activity: Units Weighted-average grant date fair value Nonvested phantom units, January 1, 2016 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 Phantom units granted 515,358 15.25 Phantom units vested (359,016 ) 22.39 Phantom units forfeited (41,492 ) 17.27 Nonvested phantom units, December 31, 2018 864,124 $ 17.11 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. P hantom 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. Forfeitures are recorded as incurred. 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 intrinsic value of phantom units that vested during the years ended December 31, follows. Year ended December 31, 2018 2017 2016 (In thousands) Intrinsic value of vested LTIP awards $ 5,393 $ 6,657 $ 2,957 As of December 31, 2018, the unrecognized unit-based compensation related to the SMLP LTIP was $4.9 million. Incremental unit-based compensation will be recorded over the remaining vesting period of approximately 2.2 years. Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP follows. Year ended December 31, 2018 2017 2016 (In thousands) SMLP LTIP unit-based compensation $ 8,328 $ 7,951 $ 7,550

RELATED-PARTY TRANSACTIONS

RELATED-PARTY TRANSACTIONS	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
RELATED-PARTY TRANSACTIONS	15. RELATED-PARTY TRANSACTIONS Acquisitions. See Notes 1, 12 and 17 for disclosure of the 2016 Drop Down and the funding of 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, 2018 2017 2016 (In thousands) Operation and maintenance expense $ 29,061 $ 27,450 $ 26,485 General and administrative expense 30,119 30,899 31,947 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

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	16. COMMITMENTS AND CONTINGENCIES Operating Leases. We and Summit Investments lease certain office space and equipment to support our operations. We have determined that our leases are operating leases. We recognize total rent expense incurred or allocated to us in general and administrative expenses. Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows: Year ended December 31, 2018 2017 2016 (In thousands) Rent expense $ 3,928 $ 3,772 $ 2,861 We lease office space and equipment under agreements that expire in various years through 2028. Future minimum lease payments due under noncancelable operating leases at December 31, 2018, were as follows (in thousands): 2019 $ 3,133 2020 1,018 2021 550 2022 506 2023 373 Thereafter 621 Total future minimum lease payments $ 6,201 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. I n 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, 2017 $ 9,453 Payments made (4,109 ) Accrued environmental remediation, December 31, 2017 $ 5,344 Payments made (3,808 ) Additional accruals 4,100 Accrued environmental remediation, December 31, 2018 $ 5,636 During 2018, we established additional environmental remediation accruals. As of December 31, 2018, we have recognized (i) a current liability for expenditures expected to be incurred within the next 12 months and (ii) a noncurrent liability for estimated expenditures expected to be incurred subsequent to December 31, 2019. Each of these amounts represent our best estimate for costs expected to be incurred. Neither of these amounts has been discounted to its present value. 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. 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.

ACQUISITIONS AND DROP DOWN TRAN

ACQUISITIONS AND DROP DOWN TRANSACTIONS	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
ACQUISITIONS AND DROP DOWN TRANSACTIONS	17. 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 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 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) borrowings under our Revolving Credit Facility, (ii) the net proceeds from the issuance of senior unsecured debt by us, (iii) net proceeds from the issuance of equity securities by us and/or (iv) other internally generated sources of cash. Ohio Gathering . For information on the acquisition and initial recognition of Ohio Gathering, see Note 8. 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 (In thousands) SMLP revenues $ 393,495 2016 Drop Down Assets revenues (1) 8,867 Combined revenues $ 402,362 SMLP net loss $ (40,932 ) 2016 Drop Down Assets net income (1) 2,745 Combined net loss $ (38,187 ) _______ (1) Results are fully reflected in SMLP's results of operations subsequent to closing the respective drop down.

UNAUDITED QUARTERLY FINANCIAL D

UNAUDITED QUARTERLY FINANCIAL DATA	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
UNAUDITED QUARTERLY FINANCIAL DATA	18. Summarized information on the consolidated results of operations for each of the quarters during the two-year period ended December 31, 2018, follows. Quarter ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 (In thousands, except per-unit amounts) Total revenues $ 133,671 $ 127,479 $ 128,183 $ 117,320 Net income (loss) attributable to SMLP $ 38,654 $ 57,430 $ (49,971 ) $ (3,930 ) Less net income and IDRs attributable to General Partner 2,907 3,279 1,140 2,058 Less net income attributable to Series A Preferred Units 7,125 7,125 7,125 7,125 Net income (loss) attributable to common limited partners $ 28,622 $ 47,026 $ (58,236 ) $ (13,113 ) Earnings (loss) per limited partner unit: Common unit - basic $ 0.39 $ 0.64 $ (0.79 ) $ (0.18 ) Common unit - diluted $ 0.39 $ 0.64 $ (0.79 ) $ (0.18 ) 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 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 )

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	19. We have evaluated subsequent events for recognition or disclosure in the consolidated financial statements and no events have occurred that require adjustment to or disclosure in the consolidated financial statements, except for the following: On February 26, 2019, SMLP announced that it had executed definitive agreements (the “Tioga PSAs”) with Hess Infrastructure related to the sale of Tioga Midstream for cash consideration of $90 million, subject to adjustment. The transaction is subject to customary closing conditions and is expected to close before the end of the first quarter of 2019. SMLP intends to use the net proceeds from the sale to repay outstanding indebtedness under its revolving credit facility. Also, on February 26, 2019, SMLP announced that it signed an amendment to the Contribution Agreement (the “Amendment”) related to the 2016 Drop Down pursuant to which the Partnership shall, as soon as reasonably practicable following the closing of the transactions under the Tioga PSAs, make a cash payment of $100 million to SMP Holdings. Following the closing of the Amendment, the Remaining Consideration will be fixed at $303.5 million, and will be payable by the Partnership in one or more payments over the period from March 1, 2020 through December 31, 2020, payable in (i) cash, (ii) the Partnership’s common units or (iii) a combination of cash and the Partnership’s common units, at the discretion of the Partnership. No less than 50 % of the Remaining Consideration shall be paid on or before June 30, 2020 and interest shall accrue at a rate of 8 % per annum on any portion of the Remaining Consideration that remains unpaid after March 31, 2020 . In addition, SMLP signed an equity restructuring agreement with the General Partner and SMP Holdings (the “Equity Restructuring Agreement”) pursuant to which the IDRs and the 2% general partner interest held by the General Partner will be converted into 8,750,000 common units and a non-economic general partner interest (the “Equity Restructuring” and collectively with the Tioga PSAs and the Amendment, the “February 2019 Transactions”). The February 2019 Transactions are expected to close before the end of the first quarter of 2019, subject to customary closing conditions. Immediately following the closing of the Equity Restructuring Agreement, SMP Holdings will directly own a 42.1% limited partner interest in SMLP and an affiliate of Energy Capital Partners II, LLC will directly own a 7.2% limited partner interest in SMLP. In connection with the February 2019 Transactions, the Partnership announced that it expects to reduce its common unit distribution to $0.2875 per quarter, beginning with the distribution to be paid in respect of the first quarter of 2019.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2018
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	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.
Property, Plant and Equipment	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. 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) (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	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, 2018 or 2017.
Amortizing Intangibles	Amortizing Intangibles. Upon the acquisition of DFW Midstream, certain of its gas gathering contracts were deemed to have above-market pricing structures. We have recognized the above-market contracts as favorable gas gathering contracts. We amortize the favorable 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. 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 (vii) 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	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. 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 the amortization of the Revolving Credit Facility debt issuance costs in interest expense.
Debt Issuance Costs	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 the amortization of the Senior Notes debt issuance costs in interest expense.
Deferred Purchase Price Obligation	Deferred Purchase Price Obligation. We recognize a liability for the Deferred Purchase Price Obligation to reflect the present value of the estimated 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 Notes 17 and 19 for additional information).
Impairment of Long-Lived Assets	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, an income-based approach or a combination of the two approaches.
Derivative Contracts	Derivative Contracts. We have commodity price exposure related to our sale of the physical natural gas we retain from certain DFW Midstream 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. We sell retainage natural gas at prices that are based on the Waha Hub Index and/or the Atmos Zone 3 Index. By basing the power prices on an index and basin-relevant market, we are able to closely associate the relationship between the 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	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	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. 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 through October 2018, the 1% ownership interest in OpCo was reflected as noncontrolling interest in partners' capital. We reflected changes in our ownership of OpCo as adjustments to noncontrolling interest. See Note 12 for additional information.
Revenue Recognition	Revenue Recognition. The majority of our revenue is derived from long-term, fee-based contracts with original terms of up to 25 years. We account for revenue in accordance with Topic 606, which we adopted on January 1, 2018, using the modified retrospective method. See below for further discussion of the adoption. We recognize revenue earned from fee-based gathering, treating and processing services in gathering services and related fees. We also earn revenue in the Williston Basin reporting segment from the sale of physical natural gas purchased from our customers under certain percent-of-proceeds arrangements; under Topic 606, fees from these arrangements are presented net within cost of natural gas and NGLs. We sell natural gas that we retain from certain DFW Midstream customers to offset the power expenses of the electric-driven compression on the DFW Midstream system. We also sell condensate retained from our gathering services at Grand River. Revenues from the sale of natural gas and condensate are recognized in natural gas, NGLs and condensate sales; the associated expense is included in operation and maintenance expense. Certain customers reimburse us for costs we incur on their behalf. We record costs incurred and reimbursed by our customers on a gross basis, with the revenue component recognized in other revenues. We provide gathering and/or processing services principally under contracts that contain one or more of the following arrangements: • Fee-based arrangements. • Percent-of-proceeds arrangements. Certain of our gathering and/or processing agreements provide for monthly, annual or multi-year MVCs. Under these MVCs, our customers agree to ship and/or process a minimum volume of production on our gathering systems or to pay a minimum monetary amount over certain periods during the term of the MVC. A customer must make a shortfall payment to us following 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 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 customer obligations under their MVCs as revenue and contract assets when (i) we consider it remote that the customer will utilize shortfall payments to offset gathering or processing fees in excess of its MVCs in subsequent periods; (ii) the customer incurs a shortfall in a contract with no banking mechanism or claw back provision; (iii) the customer’s banking mechanism has expired; or (iv) it is remote that the customer will use its unexercised right. 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	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	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. The aggregate difference in the basis of the Partnership’s net assets for financial and income tax purposes cannot be readily determined as the Partnership does not have access to the information about each partner’s tax attributes related to the Partnership. 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	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) any payment of IDRs, by the weighted-average number of limited partner units outstanding, (ii) the General Partner's approximate 2% interest in net income or loss, (iii) any net income or loss of contributed subsidiaries that is attributable to Summit Investments, (iv) the 1% noncontrolling interest in OpCo (for periods subsequent to the 2016 Drop Down and prior to 2018) and (v) net income attributable to Series A Preferred Units. 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. Pursuant to the closing of the Equity Restructuring Agreement, the IDRs and 2% general partner interest will be converted into 8,750,000 common units.
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	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.
Accounting Pronouncements	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 • ASU No. 2014-09 Revenue from Contracts with Customers (“Topic 606”). We adopted Topic 606 with a date of initial application of January 1, 2018. We applied Topic 606 by recognizing the cumulative effect of initially applying Topic 606 as an adjustment to the opening balance of partners’ capital at January 1, 2018. The comparative information has not been adjusted and is reported under the accounting standards in effect for those periods. For contracts where we perform gathering services and earn a per-unit fee which is recognized at a point in time, revenue is recognized over time as the service is performed and results in revenue recognition materially consistent with historical GAAP. In addition, our contracts generally contain forms of variable consideration, which will likely be constrained as the volumes are susceptible to factors outside of our control and influence. As a result of applying the constraint guidance, timing of revenue recognition will be materially consistent with historical GAAP. Prior to the adoption of Topic 606, contributions in aid of construction were recognized as a reduction to our cost basis of property, plant and equipment and facility fees were recognized as revenue when the amounts were billed. Upon adoption of Topic 606, the contributions in aid of construction amounts previously received were capitalized to property, plant and equipment, net of any accumulated depreciation, and will be depreciated over the remaining useful lives. Any future contributions in aid of construction will be recognized as revenue over the then remaining term of the respective contract in accordance with Topic 606. Additionally, facility fees will be deferred and recognized over the remaining contract term. There are certain percent-of-proceeds contracts within our Williston Basin reportable segment where we previously recognized revenue for services provided to producers in gathering services and related fees. Such amounts which were previously presented gross in gathering services and related fees are presented net within cost of natural gas and NGLs. This change did not have any impact on our net income (loss), cash flows, or the amount we present as segment adjusted EBITDA. For contracts containing MVC arrangements with banking mechanisms we previously deferred revenue. Under Topic 606, the recognition of revenue was accelerated. This acceleration totaled $16.7 million and is included in the Topic 606 day one adjustment amounts below in deferred revenue. The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows: Balance at December 31, 2017 Adjustments Due to Topic 606 Balance at January 1, 2018 (In thousands) Assets Property, plant and equipment, net $ 1,795,129 $ 33,123 $ 1,828,252 Liabilities Deferred revenue, current 4,000 6,088 10,088 Deferred revenue, noncurrent 12,707 22,821 35,528 Partners' Capital (1) 1,084,430 4,214 1,088,644 ________ (1) Includes common limited partner capital and general partner interests . Impact on financial statements The following tables summarize the impact of Topic 606 adoption on our consolidated financial statements. Consolidated balance sheet December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Assets Accounts receivable $ 97,936 $ 91,936 $ 6,000 Property, plant and equipment, net 1,963,713 1,926,215 37,498 Liabilities Deferred revenue, current 11,467 4,071 7,396 Deferred revenue, noncurrent 39,504 8,938 30,566 Partners' Capital (1) 927,608 922,072 5,536 (1) Includes common limited partner capital and general partner interests. Consolidated statement of operations Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Revenues Gathering services and related fees $ 344,616 $ 351,589 $ (6,973 ) Costs and expenses Cost of natural gas and NGLs 107,661 120,976 (13,315 ) Depreciation and amortization 107,100 105,765 1,335 Consolidated statement of cash flows Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Cash flows from operating activities: Net income $ 42,351 $ 37,344 $ 5,007 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 106,767 105,432 1,335 Changes in operating assets and liabilities: Accounts receivable (21,535 ) (15,535 ) (6,000 ) Deferred revenue, net 5,355 5,697 (342 ) • ASU No. 2017-04 Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"). ASU 2017-04 simplifies the subsequent measurement of goodwill by, among other things, eliminating step two from the goodwill impairment test. ASU 2017-04 is effective for public companies for fiscal years beginning after December 15, 2019 and it specifies the amendments in ASU 2017-04 should be applied on a prospective basis. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. We adopted the provisions of ASU 2017-04 effective January 1, 2018. The adoption of this standard had no impact on our consolidated financial statements . Accounting Pronouncements Pending Adoption • 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 (“ROU”) asset will be recorded which represents the lessee’s right to use, or to control the use of, a specified asset for a lease term. ASU 2016-02 is effective for public companies for fiscal years beginning after December 15, 2018, and requires the modified retrospective approach for transition. We expect to utilize certain practical expedients including (i) not being required to reassess whether any expired or existing contracts are or contain leases; (ii) not being required to reassess the lease classification for any expired or existing leases (that is, all existing leases that were classified as operating leases in accordance with Topic 840 will be classified as operating leases, and all existing leases that were classified as capital leases in accordance with Topic 840 will be classified as finance leases); and (iii) not being required to reassess initial direct costs for any existing leases. We adopted ASU 2016-02 on January 1, 2019. We will recognize a ROU asset and a corresponding lease liability based on the present value of such obligations. Based on current estimates, the total ROU assets we will recognize under ASU 2016-02 will account for less than 0.5% of total consolidated assets and the corresponding lease liabilities w ill a ccount for less than 0.5 % of total consolidated liabilities . We will also provide additional disclosures around the nature of the leasing activities beginning in our Form 10-Q for the three months ended March 31, 2019. These include additional qualitative disclosures, such as a general description of leases, and quantitative disclosures, such as lease costs, weighted average remaining lease term and weighted average discount rate. • ASU No. 2018-01 Leases: Land Easement Practical Expedient for Transition to Topic 842 (“ASU 2018-01”). ASU 2018-01 provides an optional transition practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance in Topic 840. Upon adoption of Topic 842, an entity that elects this practical expedient should evaluate new or modified land easements under Topic 842 beginning at the date the entity adopts Topic 842. We expect to adopt the optional transition practical expedient of ASU 2018-01 effective January 1, 2019. • ASU No. 2018-13 Fair Value Measurement (“ASU 2018-13”). ASU 2018-13 updates the disclosure requirements on fair value measurements including new disclosures for the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. ASU 2018-13 modifies existing disclosures including clarifying the measurement uncertainty disclosure. ASU 2018-13 removes certain existing disclosure requirements including the amount and reasons for transfers between Level 1 and Level 2 fair value measurements and the policy for the timing of transfer between levels. We are currently evaluating the provisions of ASU 2018-13 to determine its impact on our financial statements and related disclosures and will adopt its provisions effective January 1, 2020 .
Basis of Presentation	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. 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 ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Useful lives of Property, Plant and Equipment	Estimates of useful lives follow. Useful lives (In years) (In years) Gathering and processing systems and related equipment 12-30 Other 4-15
Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606	The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of Topic 606 was as follows: Balance at December 31, 2017 Adjustments Due to Topic 606 Balance at January 1, 2018 (In thousands) Assets Property, plant and equipment, net $ 1,795,129 $ 33,123 $ 1,828,252 Liabilities Deferred revenue, current 4,000 6,088 10,088 Deferred revenue, noncurrent 12,707 22,821 35,528 Partners' Capital (1) 1,084,430 4,214 1,088,644 ________ (1) Includes common limited partner capital and general partner interests .
Summary of Impact of Topic 606 Adoption on Consolidated Financial Statements	Impact on financial statements The following tables summarize the impact of Topic 606 adoption on our consolidated financial statements. Consolidated balance sheet December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Assets Accounts receivable $ 97,936 $ 91,936 $ 6,000 Property, plant and equipment, net 1,963,713 1,926,215 37,498 Liabilities Deferred revenue, current 11,467 4,071 7,396 Deferred revenue, noncurrent 39,504 8,938 30,566 Partners' Capital (1) 927,608 922,072 5,536 (1) Includes common limited partner capital and general partner interests. Consolidated statement of operations Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Revenues Gathering services and related fees $ 344,616 $ 351,589 $ (6,973 ) Costs and expenses Cost of natural gas and NGLs 107,661 120,976 (13,315 ) Depreciation and amortization 107,100 105,765 1,335 Consolidated statement of cash flows Year ended December 31, 2018 As Reported Balances Without Adoption of Topic 606 Effect of Change Increase / (Decrease) (In thousands) Cash flows from operating activities: Net income $ 42,351 $ 37,344 $ 5,007 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 106,767 105,432 1,335 Changes in operating assets and liabilities: Accounts receivable (21,535 ) (15,535 ) (6,000 ) Deferred revenue, net 5,355 5,697 (342 )

REVENUE (Tables)

REVENUE (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
Schedule of Estimated Revenue Expected to be Recognized and MVC Shortfall Payments	The following table presents estimated revenue expected to be recognized over the remaining contract period related to performance obligations that are unsatisfied and are comprised of estimated MVC shortfall payments. We applied the practical expedient in paragraph 606-10-50-14 of Topic 606 for certain arrangements that we consider optional purchases (i.e., there is no enforceable obligation for the customer to make purchases) and those amounts are excluded from the table. 2019 2020 2021 2022 2023 Thereafter (In thousands) Gathering services and related fees $ 126,006 $ 122,429 $ 100,070 $ 83,626 $ 70,923 $ 112,462
Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments	Revenue by Category . In the following table, revenue is disaggregated by geographic area and major products and services. For more detailed information about reportable segments, see Note 4 . Reportable Segments Year ended December 31, 2018 Utica Shale Williston Basin DJ Basin Permian Basin Piceance Basin Barnett Shale Marcellus Shale Total reportable segments All other segments Total (In thousands) Major products / services lines Gathering services and related fees $ 35,233 $ 79,606 $ 11,251 $ 115 $ 135,810 $ 59,030 $ 29,573 $ 350,618 $ (6,002 ) $ 344,616 Natural gas, NGLs and condensate sales — 31,840 371 843 14,800 2,523 — 50,377 84,457 134,834 Other revenues — 12,204 3,672 — 4,909 6,712 — 27,497 (294 ) 27,203 Total $ 35,233 $ 123,650 $ 15,294 $ 958 $ 155,519 $ 68,265 $ 29,573 $ 428,492 $ 78,161 $ 506,653
Schedule of Information about Contract Assets from Contracts with Customers	The following table provides information about contract assets from contracts with customers : December 31, 2018 (In thousands) Contract assets, December 31, 2017 $ — Additions 26,403 Transfers out (17,648 ) Contract assets, December 31, 2018 $ 8,755

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Schedule of assets by reportable segment	Assets by reportable segment follow. December 31, 2018 2017 2016 (In thousands) Assets: Utica Shale $ 207,357 $ 212,311 $ 199,392 Ohio Gathering 649,250 690,485 707,415 Williston Basin 526,819 512,860 724,084 DJ Basin 166,580 79,438 72,494 Permian Basin 145,702 57,590 — Piceance Basin 699,638 719,284 770,946 Barnett Shale 376,564 383,306 404,314 Marcellus Shale 208,790 217,362 224,709 Total reportable segment assets 2,980,700 2,872,636 3,103,354 Corporate and Other 44,181 22,406 12,294 Eliminations (4,319 ) (249 ) (469 ) Total assets $ 3,020,562 $ 2,894,793 $ 3,115,179
Schedule of segment reporting information	Revenues by reportable segment follow. Year ended December 31, 2018 2017 2016 (In thousands) Revenues (1): Utica Shale $ 35,233 $ 38,907 $ 24,263 Williston Basin 123,650 161,503 122,174 DJ Basin 15,294 11,860 8,439 Permian Basin 958 — — Piceance Basin 155,519 154,893 141,464 Barnett Shale 68,265 71,667 79,956 Marcellus Shale 29,573 30,394 26,111 Total reportable segments revenue 428,492 469,224 402,407 Corporate and Other 88,286 26,446 412 Eliminations (10,125 ) (6,929 ) (457 ) Total revenues $ 506,653 $ 488,741 $ 402,362 (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, 2018 2017 2016 (In thousands) Depreciation and amortization (1): Utica Shale $ 7,672 $ 7,009 $ 4,331 Williston Basin 22,642 33,772 33,676 DJ Basin 3,133 2,636 2,524 Permian Basin 243 — — Piceance Basin 46,919 46,289 46,616 Barnett Shale (2) 15,325 15,001 16,093 Marcellus Shale 9,090 9,047 8,841 Total reportable segment depreciation and amortization 105,024 113,754 112,081 Corporate and Other 1,743 1,118 580 Total depreciation and amortization $ 106,767 $ 114,872 $ 112,661 (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, 2018 2017 2016 (In thousands) Cash paid for capital expenditures (1): Utica Shale $ 5,719 $ 22,921 $ 78,708 Williston Basin 25,202 17,309 31,541 DJ Basin 64,920 7,150 5,807 Permian Basin 83,823 56,020 — Piceance Basin 7,887 16,564 19,912 Barnett Shale 1,370 569 3,910 Marcellus Shale 1,030 641 1,173 Total reportable segment capital expenditures 189,951 121,174 141,051 Corporate and Other 10,635 3,041 1,668 Total cash paid for capital expenditures $ 200,586 $ 124,215 $ 142,719 (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, 2018 2017 2016 Percentage of total revenues (1)(2): Counterparty A - Piceance Basin * * 14 % Counterparty B - Williston Basin * 13 % * Counterparty C - Piceance Basin 10 % * * (1) Includes recognition of revenue that was previously deferred in connection with minimum volume commitments (see Note 9). (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, 2018 2017 2016 (In thousands) Reportable segment adjusted EBITDA Utica Shale $ 30,285 $ 34,011 $ 21,035 Ohio Gathering 39,969 41,246 45,602 Williston Basin 76,701 66,413 79,475 DJ Basin 7,558 6,624 3,681 Permian Basin (1,200 ) — — Piceance Basin 111,042 111,113 105,560 Barnett Shale 43,268 46,232 54,634 Marcellus Shale 24,267 23,888 19,203 Total of reportable segments' measures of profit or loss $ 331,890 $ 329,527 $ 329,190 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, 2018 2017 2016 (In thousands) Reconciliation of income (loss) before income taxes and loss from equity method investees to total of reportable segments' measures of profit: Income (loss) before income taxes and loss from equity method investees $ 53,272 $ 88,614 $ (7,768 ) Add: Corporate and Other 38,917 39,140 37,589 Interest expense 60,535 68,131 63,810 Early extinguishment of debt — 22,039 — Deferred Purchase Price Obligation 20,975 (200,322 ) 55,854 Depreciation and amortization 106,767 114,872 112,661 Proportional adjusted EBITDA for equity method investees 39,969 41,246 45,602 Adjustments related to MVC shortfall payments (3,632 ) (41,373 ) 11,600 Adjustments related to capital reimbursement activity (427 ) — — Unit-based and noncash compensation 8,328 7,951 7,985 Loss on asset sales, net — 527 93 Long-lived asset impairment 7,186 188,702 1,764 Total of reportable segments' measures of profit $ 331,890 $ 329,527 $ 329,190
Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments	Adjustments related to MVC shortfall payments by reportable segment follow. Year ended December 31, 2018 Williston Basin Piceance Basin Barnett Shale Total (In thousands) Adjustments related to MVC shortfall payments: Net change in deferred revenue for MVC shortfall payments $ — $ — $ — $ — Expected MVC shortfall adjustments — 10 (3,642 ) (3,632 ) Total adjustments related to MVC shortfall payments $ — $ 10 $ (3,642 ) $ (3,632 ) Year ended December 31, 2017 Williston Basin Piceance Basin 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 Basin 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

PROPERTY, PLANT, AND EQUIPMEN_2

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of property, plant, and equipment, net	Details on property, plant and equipment follow. December 31, 2018 December 31, 2017 (In thousands) Gathering and processing systems and related equipment $ 2,155,325 $ 1,973,722 Construction in progress 137,920 78,850 Land and line fill 11,748 11,735 Other 45,853 40,262 Total 2,350,846 2,104,569 Less accumulated depreciation 387,133 309,440 Property, plant and equipment, net $ 1,963,713 $ 1,795,129
Schedule of long-lived asset impairments	During 2018, 2017 and 2016, we recognized the following long-lived asset impairments, by segment. Year ended December 31, 2018 2017 2016 (In thousands) Long-lived asset impairment: Williston Basin $ 3,972 $ 101,961 $ 569 Piceance Basin 1,004 697 — DJ Basin 9 — — Barnett Shale — — 1,195 Utica Shale 1,440 878 — Permian Basin 761 — —
Schedule of depreciation expense and capitalized interest costs	Depreciation expense and capitalized interest follow. Year ended December 31, 2018 2017 2016 (In thousands) Depreciation expense $ 74,511 $ 75,120 $ 70,770 Capitalized interest 8,497 2,579 3,709

AMORTIZING INTANGIBLE ASSETS _2

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible assets and liabilities subject to amortization	Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities prior to 2018), all of which are subject to amortization, follow. December 31, 2018 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (13,905 ) $ 10,290 Contract intangibles 278,448 (143,962 ) 134,486 Rights-of-way 166,209 (37,569 ) 128,640 Total intangible assets $ 468,852 $ (195,436 ) $ 273,416 Unfavorable gas gathering contract $ 10,962 $ (10,962 ) $ — December 31, 2017 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (12,350 ) $ 11,845 Contract intangibles 278,448 (117,821 ) 160,627 Rights-of-way 159,986 (31,113 ) 128,873 Total intangible assets $ 462,629 $ (161,284 ) $ 301,345 Unfavorable gas gathering contract $ 10,962 $ (9,074 ) $ 1,888
Recognized amortization expense in other revenues and cost and expenses	We recognized amortization expense in other revenues as follows: Year ended December 31, 2018 2017 2016 (In thousands) Amortization expense – favorable gas gathering contracts $ (1,555 ) $ (1,555 ) $ (1,261 ) Amortization expense – unfavorable gas gathering contract 1,888 2,158 839 We recognized amortization expense in costs and expenses as follows: Year ended December 31, 2018 2017 2016 (In thousands) Amortization expense – contract intangibles $ 26,141 $ 34,202 $ 35,416 Amortization expense – rights-of-way 6,448 6,153 6,053
Estimated aggregate annual amortization expected to be recognized	The estimated aggregate annual amortization expected to be recognized for as of December 31, 2018 for each of the five succeeding fiscal years follows. Intangible assets (In thousands) 2019 $ 32,422 2020 32,246 2021 28,554 2022 25,487 2023 25,433

GOODWILL (Tables)

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2018
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. As of December 31, 2018, 2017, 2016 (In thousands) Accumulated goodwill impairment: Piceance Basin $ 45,478 Williston Basin 257,572 Total accumulated goodwill impairment $ 303,050

EQUITY METHOD INVESTMENTS (Tabl

EQUITY METHOD INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2018
Equity Method Investments And Joint Ventures [Abstract]
Equity Method Investments	A reconciliation of our 2018 2017 (In thousands) Investment in equity method investees, December 31 $ 649,250 $ 690,485 December cash distributions 2,736 4,032 December cash contributions — (3,932 ) Impairment loss 5,652 1,383 Legal contingency 2,040 — Basis difference (116,832 ) (130,184 ) Investment in equity method investees, net of basis difference, November 30 $ 542,846 $ 561,784 Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information). November 30, 2018 November 30, 2017 OGC OCC OGC OCC (In thousands) Current assets $ 37,403 $ 3,716 $ 34,383 $ 3,650 Noncurrent assets 1,262,253 27,203 1,319,448 29,156 Total assets $ 1,299,656 $ 30,919 $ 1,353,831 $ 32,806 Current liabilities $ 19,903 $ 3,912 $ 10,882 $ 3,382 Noncurrent liabilities 3,688 8,807 3,272 11,715 Total liabilities $ 23,591 $ 12,719 $ 14,154 $ 15,097 Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information). Twelve months ended November 30, 2018 Twelve months ended November 30, 2017 Twelve months ended November 30, 2016 OGC OCC OGC OCC OGC OCC (In thousands) Total revenues $ 142,398 $ 10,177 $ 140,679 $ 8,607 $ 148,662 $ 15,791 Total operating expenses 136,722 9,053 111,897 8,298 96,647 111,528 Net income (loss) 5,670 498 28,785 (907 ) 52,009 (94,230 )

DEFERRED REVENUE (Tables)

DEFERRED REVENUE (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
Rollforward of deferred revenue	A rollforward of current deferred revenue follows. Utica Shale Williston Basin DJ Basin Piceance Basin Barnett Shale Marcellus Shale Total current (In thousands) Current deferred revenue, January 1, 2017 $ — $ — $ — $ — $ — $ — $ — Additions — — — 18,294 — — 18,294 Less revenue recognized — — — 14,294 — — 14,294 Current deferred revenue, December 31, 2017, as reported — — — 4,000 — — 4,000 Net impact of Topic 606 day 1 adoption 18 1,017 358 3,038 1,619 38 6,088 Current deferred revenue, January 1, 2018 18 1,017 358 7,038 1,619 38 10,088 Additions 18 1,744 943 21,955 1,651 96 26,407 Less revenue recognized 18 1,347 562 21,377 1,628 96 25,028 Current deferred revenue, December 31, 2018 $ 18 $ 1,414 $ 739 $ 7,616 $ 1,642 $ 38 $ 11,467 A rollforward of noncurrent deferred revenue follows. Utica Shale Williston Basin DJ Basin Piceance Basin Barnett Shale Marcellus Shale Total noncurrent (In thousands) Noncurrent deferred, revenue, January 1, 2017 $ — $ 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, as reported — — — 12,707 — — 12,707 Net impact of Topic 606 day 1 adoption 39 4,215 4,505 5,512 8,217 333 22,821 Noncurrent deferred revenue, January 1, 2018 39 4,215 4,505 18,219 8,217 333 35,528 Additions — 1,851 3,720 7,869 3,062 — 16,502 Less reclassification to current deferred revenue 18 1,673 941 8,146 1,651 97 12,526 Noncurrent deferred revenue, December 31, 2018 $ 21 $ 4,393 $ 7,284 $ 17,942 $ 9,628 $ 236 $ 39,504

DEBT (Tables)

DEBT (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of debt and capital leases	Debt consisted of the following: December 31, 2018 December 31, 2017 (In thousands) Summit Holdings' variable rate senior secured Revolving Credit Facility (5.03% at December 31, 2018 and 4.07% at December 31, 2017) due May 2022 $ 466,000 $ 261,000 Summit Holdings' 5.5% senior unsecured notes due August 2022 300,000 300,000 Less unamortized debt issuance costs (1) (2,362 ) (2,910 ) Summit Holdings' 5.75% senior unsecured notes due April 2025 500,000 500,000 Less unamortized debt issuance costs (1) (5,907 ) (6,898 ) Total long-term debt $ 1,257,731 $ 1,051,192 __________ (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, 2018 are as follow (in thousands): 2019 $ — 2020 — 2021 — 2022 766,000 2023 — Thereafter 500,000 Total long-term debt $ 1,266,000

FINANCIAL INSTRUMENTS (Tables)

FINANCIAL INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation	A rollforward of our Level 3 liability measured at fair value on a recurring basis follows (in thousands). Level 3 liability, January 1, 2017 $ 563,281 Change in fair value (200,322 ) Level 3 liability, December 31, 2017 362,959 Change in fair value 20,975 Level 3 liability, December 31, 2018 $ 383,934
Summary of the estimated fair value of debt instruments	A summary of the estimated fair value of our debt financial instruments follows. December 31, 2018 December 31, 2017 Carrying value Estimated fair value (Level 2) Carrying value Estimated fair value (Level 2) (In thousands) Summit Holdings 5.5% Senior Notes ($300.0 million principal) $ 297,638 $ 286,625 $ 297,090 $ 301,750 Summit Holdings 5.75% Senior Notes ($500.0 million principal) 494,093 455,208 493,102 501,667

PARTNERS' CAPITAL (Tables)

PARTNERS' CAPITAL (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 — 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 the 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 the 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 Net units issued under the SMLP LTIP — 304,857 — — Units, December 31, 2018 300,000 73,390,853 — 1,490,999
Details of cash distributions	We paid the following per-unit distributions during the years ended December 31: Year ended December 31, 2018 2017 2016 Per-unit distributions to unitholders $ 2.300 $ 2.300 $ 2.300 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, 2018 2017 2016 (In thousands) IDR payments $ 8,535 $ 8,460 $ 7,912
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)

EARNINGS PER UNIT (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of earnings per limited partner unit	The following table details the components of EPU. Year ended December 31, 2018 2017 2016 (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 $ 32,799 $ 75,485 $ (48,179 ) Less net income attributable to Series A Preferred Units 28,500 3,563 — Net income (loss) attributable to common limited partners $ 4,299 $ 71,922 $ (48,179 ) Denominator for basic and diluted EPU: Weighted-average common units outstanding – basic 73,304 72,705 68,264 Effect of nonvested phantom units 311 342 — Weighted-average common units outstanding – diluted 73,615 73,047 68,264 Earnings (loss) per limited partner unit: Common unit – basic $ 0.06 $ 0.99 $ (0.71 ) Common unit – diluted $ 0.06 $ 0.98 $ (0.71 ) Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU 2 42 125

UNIT-BASED AND NONCASH COMPEN_2

UNIT-BASED AND NONCASH COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of activity	The following table presents phantom unit activity: Units Weighted-average grant date fair value Nonvested phantom units, January 1, 2016 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 Phantom units granted 515,358 15.25 Phantom units vested (359,016 ) 22.39 Phantom units forfeited (41,492 ) 17.27 Nonvested phantom units, December 31, 2018 864,124 $ 17.11
Schedule of intrinsic values	The intrinsic value of phantom units that vested during the years ended December 31, follows. Year ended December 31, 2018 2017 2016 (In thousands) Intrinsic value of vested LTIP awards $ 5,393 $ 6,657 $ 2,957
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, 2018 2017 2016 (In thousands) SMLP LTIP unit-based compensation $ 8,328 $ 7,951 $ 7,550

RELATED-PARTY TRANSACTIONS (Tab

RELATED-PARTY TRANSACTIONS (Tables)	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
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, 2018 2017 2016 (In thousands) Operation and maintenance expense $ 29,061 $ 27,450 $ 26,485 General and administrative expense 30,119 30,899 31,947

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of total rent expense related to operating leases	Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows: Year ended December 31, 2018 2017 2016 (In thousands) Rent expense $ 3,928 $ 3,772 $ 2,861
Future minimum lease payments due under noncancelable operating leases	Future minimum lease payments due under noncancelable operating leases at December 31, 2018, were as follows (in thousands): 2019 $ 3,133 2020 1,018 2021 550 2022 506 2023 373 Thereafter 621 Total future minimum lease payments $ 6,201
Schedule of accrued environmental remediation	A rollforward of the aggregate accrued environmental remediation liabilities follows. Total (In thousands) Accrued environmental remediation, January 1, 2017 $ 9,453 Payments made (4,109 ) Accrued environmental remediation, December 31, 2017 $ 5,344 Payments made (3,808 ) Additional accruals 4,100 Accrued environmental remediation, December 31, 2018 $ 5,636

ACQUISITIONS AND DROP DOWN TR_2

ACQUISITIONS AND DROP DOWN TRANSACTIONS (Tables)	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
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 (In thousands) SMLP revenues $ 393,495 2016 Drop Down Assets revenues (1) 8,867 Combined revenues $ 402,362 SMLP net loss $ (40,932 ) 2016 Drop Down Assets net income (1) 2,745 Combined net loss $ (38,187 ) _______ (1) Results are fully reflected in SMLP's results of operations subsequent to closing the respective drop down.

UNAUDITED QUARTERLY FINANCIAL_2

UNAUDITED QUARTERLY FINANCIAL DATA (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018, follows. Quarter ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 (In thousands, except per-unit amounts) Total revenues $ 133,671 $ 127,479 $ 128,183 $ 117,320 Net income (loss) attributable to SMLP $ 38,654 $ 57,430 $ (49,971 ) $ (3,930 ) Less net income and IDRs attributable to General Partner 2,907 3,279 1,140 2,058 Less net income attributable to Series A Preferred Units 7,125 7,125 7,125 7,125 Net income (loss) attributable to common limited partners $ 28,622 $ 47,026 $ (58,236 ) $ (13,113 ) Earnings (loss) per limited partner unit: Common unit - basic $ 0.39 $ 0.64 $ (0.79 ) $ (0.18 ) Common unit - diluted $ 0.39 $ 0.64 $ (0.79 ) $ (0.18 ) 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 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 )

ORGANIZATION, BUSINESS OPERAT_2

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION - Narrative (Details) - shares	Feb. 26, 2019	Sep. 30, 2016	Feb. 29, 2016	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018	Dec. 31, 2017	Jun. 30, 2016
Organization And Business Operations [Line Items]
Common limited partner capital (in shares)				73,390,853			73,085,996
Summit Midstream OpCo, LP
Organization And Business Operations [Line Items]
Ownership interest					1.00%		1.00%
SMP Holdings | Contributed Entities
Organization And Business Operations [Line Items]
Percent of issued and outstanding membership interest			100.00%
SMP Holdings | OGC
Organization And Business Operations [Line Items]
Percentage of ownership interest			40.00%
Drop Down Assets 2016 Acquisition | Summit Midstream OpCo, LP
Organization And Business Operations [Line Items]
Ownership interest							1.00%
Subsidiary of Summit Investments | Summit Midstream OpCo, LP
Organization And Business Operations [Line Items]
Ownership interest					1.00%	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 | Subsequent Event
Organization And Business Operations [Line Items]
General partner interest	2.00%
General Partner | Summit Investments | Summit Midstream Partners, LP
Organization And Business Operations [Line Items]
General partner interest				2.00%
General Partner | Summit Investments | Summit Midstream Partners, LP | Subsequent Event
Organization And Business Operations [Line Items]
General partner interest	2.00%
General Partner | Common units | Subsequent Event
Organization And Business Operations [Line Items]
General partner units converted	8,750,000

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
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 ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Millions	Feb. 26, 2019	Sep. 30, 2016	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018	Dec. 31, 2017	Jun. 30, 2016
Calculated under Revenue Guidance in Effect before Topic 606
Summary of Significant Accounting Policies [Line Items]
Acceleration totaled, recognition of revenue			$ 16.7
General Partner
Summary of Significant Accounting Policies [Line Items]
General partner interest		2.00%	2.00%
General Partner | Subsequent Event
Summary of Significant Accounting Policies [Line Items]
General partner interest	2.00%
General Partner | Subsequent Event | Common units
Summary of Significant Accounting Policies [Line Items]
General partner units converted	8,750,000
Summit Midstream OpCo, LP
Summary of Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners				1.00%		1.00%
Drop Down Assets 2016 Acquisition | Summit Midstream OpCo, LP
Summary of Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners				1.00%	1.00%		1.00%
Drop Down Assets 2016 Acquisition | Summit Midstream OpCo, LP
Summary of Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners						1.00%
Minimum | Accounting Standard Update 2016-02
Summary of Significant Accounting Policies [Line Items]
Percentage of ROU assets to be recognized compared to total consolidated assets			0.50%
Percentage of lease liabilities to be recognized compared to total consolidated liabilites			0.50%
Favorable gas gathering contracts | Minimum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			10 years
Favorable gas gathering contracts | Maximum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			20 years
Other Gas Gathering Contract | Minimum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			3 years
Other Gas Gathering Contract | Maximum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			25 years
Rights-of-way | Minimum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			20 years
Rights-of-way | Maximum
Summary of Significant Accounting Policies [Line Items]
Useful lives (In years)			30 years

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cumulative Effect of Changes made to Balance Sheet for the Adoption of Topic 606 (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Property, plant and equipment, net	$ 1,963,713	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	11,467	10,088	4,000	$ 0
Deferred revenue, noncurrent	39,504	35,528	12,707	$ 57,465
Partners' Capital	927,608	1,088,644
Accounting Standards Update 2014-09
Liabilities
Deferred revenue			10,088
Deferred revenue, noncurrent			35,528
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Assets
Property, plant and equipment, net	1,926,215		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	8,938		12,707
Partners' Capital	922,072		1,084,430
Accounting Standards Update 2014-09 | Adjustments Due to Topic 606
Assets
Property, plant and equipment, net	37,498	33,123
Liabilities
Deferred revenue	7,396	6,088	6,088
Deferred revenue, noncurrent	30,566	22,821	$ 22,821
Partners' Capital	$ 5,536	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Accounts receivable	$ 97,936
Property, plant and equipment, net	1,963,713	$ 1,828,252	$ 1,795,129
Liabilities
Deferred revenue	11,467	10,088	4,000	$ 0
Deferred revenue, noncurrent	39,504	35,528	12,707	$ 57,465
Partners' Capital	927,608	1,088,644
Accounting Standards Update 2014-09
Liabilities
Deferred revenue			10,088
Deferred revenue, noncurrent			35,528
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Assets
Accounts receivable	91,936
Property, plant and equipment, net	1,926,215		1,795,129
Liabilities
Deferred revenue	4,071		4,000
Deferred revenue, noncurrent	8,938		12,707
Partners' Capital	922,072		1,084,430
Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Assets
Accounts receivable	6,000
Property, plant and equipment, net	37,498	33,123
Liabilities
Deferred revenue	7,396	6,088	6,088
Deferred revenue, noncurrent	30,566	22,821	$ 22,821
Partners' Capital	$ 5,536	$ 4,214

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Gathering services and related fees	$ 133,671	$ 127,479	$ 128,183	$ 117,320	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 506,653	$ 488,741	$ 402,362
Costs and expenses:
Cost of natural gas and NGLs									$ 107,661	$ 57,237	$ 27,421
Type of Cost, Good or Service [Extensible List]									us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization									$ 107,100	$ 115,475	$ 112,239
Gathering Services and Related Fees
Revenues:
Gathering services and related fees									344,616	$ 394,427	$ 345,961
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Costs and expenses:
Cost of natural gas and NGLs									$ 120,976
Type of Cost, Good or Service [Extensible List]									us-gaap:OilAndGasPurchasedMember
Depreciation and amortization									$ 105,765
Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Costs and expenses:
Cost of natural gas and NGLs									$ (13,315)
Type of Cost, Good or Service [Extensible List]									us-gaap:OilAndGasPurchasedMember
Depreciation and amortization									$ 1,335
Accounting Standards Update 2014-09 | Gathering Services and Related Fees | Calculated under Revenue Guidance in Effect before Topic 606
Revenues:
Gathering services and related fees									351,589
Accounting Standards Update 2014-09 | Gathering Services and Related Fees | Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees									$ (6,973)

SUMMARY OF SIGNIFICANT ACCOUN_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net (loss) income	$ 42,351	$ 86,050	$ (38,187)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	106,767	114,872	112,661
Changes in operating assets and liabilities:
Accounts receivable	(21,535)	25,063	(7,783)
Deferred revenue, net	5,355	$ (40,758)	$ 11,302
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Cash flows from operating activities:
Net (loss) income	37,344
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	105,432
Changes in operating assets and liabilities:
Accounts receivable	(15,535)
Deferred revenue, net	5,697
Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Cash flows from operating activities:
Net (loss) income	5,007
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	1,335
Changes in operating assets and liabilities:
Accounts receivable	(6,000)
Deferred revenue, net	$ (342)

REVENUE - Narrative1 (Details)

REVENUE - Narrative1 (Details)	Dec. 31, 2018
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Revenue From Contract With Customer [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	25 years

REVENUE - Narrative (Details)

REVENUE - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue From Contract With Customer [Line Items]
Revenue, performance obligation, description of payment terms	Our services are typically billed on a monthly basis and we do not offer extended payment terms. We do not have contracts with financing components.
Receivables with customers	$ 82,900
Contract assets included in accounts receivable	8,755	$ 0
Gathering Services and Related Fees
Revenue From Contract With Customer [Line Items]
Contract liabilities, revenue	$ 10,800

REVENUE - Schedule of Estimated

REVENUE - Schedule of Estimated Revenue Expected to be Recognized and MVC Shortfall Payments (Details) - Gathering Services and Related Fees $ in Thousands	Dec. 31, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, remaining performance obligation	$ 126,006
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, remaining performance obligation	$ 122,429
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, remaining performance obligation	$ 100,070
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, remaining performance obligation	$ 83,626
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	1 year
Revenue, remaining performance obligation	$ 70,923
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period	20 years
Revenue, remaining performance obligation	$ 112,462

REVENUE - Schedule of Disaggreg

REVENUE - Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Total revenues	$ 133,671	$ 127,479	$ 128,183	$ 117,320	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 506,653	$ 488,741	$ 402,362
All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues									78,161
Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues									428,492	469,224	402,407
Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues									35,233	38,907	24,263
Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues									123,650	161,503	122,174
Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues									15,294	11,860	8,439
Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues									155,519	154,893	141,464
Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues									958
Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues									68,265	71,667	79,956
Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues									29,573	$ 30,394	$ 26,111
Gathering Services and Related Fees
Disaggregation Of Revenue [Line Items]
Total revenues									344,616
Gathering Services and Related Fees | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues									(6,002)
Gathering Services and Related Fees | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues									350,618
Gathering Services and Related Fees | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues									35,233
Gathering Services and Related Fees | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues									79,606
Gathering Services and Related Fees | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues									11,251
Gathering Services and Related Fees | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues									135,810
Gathering Services and Related Fees | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues									115
Gathering Services and Related Fees | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues									59,030
Gathering Services and Related Fees | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues									29,573
Natural Gas, NGLs and Condensate Sales
Disaggregation Of Revenue [Line Items]
Total revenues									134,834
Natural Gas, NGLs and Condensate Sales | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues									84,457
Natural Gas, NGLs and Condensate Sales | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues									50,377
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues									0
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues									31,840
Natural Gas, NGLs and Condensate Sales | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues									371
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues									14,800
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues									843
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues									2,523
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues									0
Other Revenues
Disaggregation Of Revenue [Line Items]
Total revenues									27,203
Other Revenues | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues									(294)
Other Revenues | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues									27,497
Other Revenues | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues									0
Other Revenues | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues									12,204
Other Revenues | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues									3,672
Other Revenues | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues									4,909
Other Revenues | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues									0
Other Revenues | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues									6,712
Other Revenues | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues									$ 0

REVENUE - Schedule of Informati

REVENUE - Schedule of Information about Contract Assets from Contracts with Customers (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Contract With Customer Asset And Liability [Abstract]
Contract assets, December 31, 2017	$ 0
Additions	26,403
Transfers out	(17,648)
Contract assets, December 31, 2018	$ 8,755

SEGMENT INFORMATION - Narrative

SEGMENT INFORMATION - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)MMcf	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Processing plant expected to be commissioned | MMcf	60
Capital expenditures	$ 200,586	$ 124,215	$ 142,719
Corporate and Other
Segment Reporting Information [Line Items]
Capital expenditures	$ 3,300

SEGMENT INFORMATION - Assets, R

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 3,020,562				$ 2,894,793				$ 3,020,562	$ 2,894,793	$ 3,115,179
Revenues	133,671	$ 127,479	$ 128,183	$ 117,320	126,199	$ 124,945	$ 101,792	$ 135,805	506,653	488,741	402,362
Depreciation and amortization									106,767	114,872	112,661
Capital expenditures									200,586	124,215	142,719
Reportable Segments
Segment Reporting Information [Line Items]
Assets	2,980,700				2,872,636				2,980,700	2,872,636	3,103,354
Revenues									428,492	469,224	402,407
Depreciation and amortization									105,024	113,754	112,081
Capital expenditures									189,951	121,174	141,051
Reportable Segments | Utica Shale
Segment Reporting Information [Line Items]
Assets	207,357				212,311				207,357	212,311	199,392
Revenues									35,233	38,907	24,263
Depreciation and amortization									7,672	7,009	4,331
Capital expenditures									5,719	22,921	78,708
Reportable Segments | Ohio Gathering
Segment Reporting Information [Line Items]
Assets	649,250				690,485				649,250	690,485	707,415
Reportable Segments | Williston Basin
Segment Reporting Information [Line Items]
Assets	526,819				512,860				526,819	512,860	724,084
Revenues									123,650	161,503	122,174
Depreciation and amortization									22,642	33,772	33,676
Capital expenditures									25,202	17,309	31,541
Reportable Segments | Barnett Shale
Segment Reporting Information [Line Items]
Assets	376,564				383,306				376,564	383,306	404,314
Revenues									68,265	71,667	79,956
Depreciation and amortization									15,325	15,001	16,093
Capital expenditures									1,370	569	3,910
Reportable Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Assets	208,790				217,362				208,790	217,362	224,709
Revenues									29,573	30,394	26,111
Depreciation and amortization									9,090	9,047	8,841
Capital expenditures									1,030	641	1,173
Reportable Segments | DJ Basin
Segment Reporting Information [Line Items]
Assets	166,580				79,438				166,580	79,438	72,494
Revenues									15,294	11,860	8,439
Depreciation and amortization									3,133	2,636	2,524
Capital expenditures									64,920	7,150	5,807
Reportable Segments | Permian Basin
Segment Reporting Information [Line Items]
Assets	145,702				57,590				145,702	57,590	0
Revenues									958
Depreciation and amortization									243	0	0
Capital expenditures									83,823	56,020	0
Reportable Segments | Piceance Basin
Segment Reporting Information [Line Items]
Assets	699,638				719,284				699,638	719,284	770,946
Revenues									155,519	154,893	141,464
Depreciation and amortization									46,919	46,289	46,616
Capital expenditures									7,887	16,564	19,912
Corporate and Other
Segment Reporting Information [Line Items]
Assets	44,181				22,406				44,181	22,406	12,294
Revenues									88,286	26,446	412
Depreciation and amortization									1,743	1,118	580
Capital expenditures									10,635	3,041	1,668
Eliminations
Segment Reporting Information [Line Items]
Assets	$ (4,319)				$ (249)				(4,319)	(249)	(469)
Revenues									$ (10,125)	$ (6,929)	$ (457)

SEGMENT INFORMATION - Concentra

SEGMENT INFORMATION - Concentration Risk (Details) - Sales Revenue, Net - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Counterparty A - Piceance Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	0.00%	14.00%
Counterparty B - Williston Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	0.00%	13.00%	0.00%
Counterparty C - Piceance Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	10.00%	0.00%	0.00%

SEGMENT INFORMATION - Adjusted

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 331,890	$ 329,527	$ 329,190
Utica Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	30,285	34,011	21,035
Ohio Gathering
Segment Reporting Information [Line Items]
Adjusted EBITDA	39,969	41,246	45,602
Williston Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	76,701	66,413	79,475
Piceance Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	111,042	111,113	105,560
DJ Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	7,558	6,624	3,681
Barnett Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	43,268	46,232	54,634
Marcellus Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	24,267	23,888	19,203
Permian Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ (1,200)	$ 0	$ 0

SEGMENT INFORMATION - Reconcili

SEGMENT INFORMATION - Reconciliation of Net Income to Adjusted EBITDA (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting [Abstract]
Income (loss) before income taxes and loss from equity method investees	$ 53,272	$ 88,614	$ (7,768)
Add:
Corporate and Other	38,917	39,140	37,589
Interest expense	60,535	68,131	63,810
Early extinguishment of debt	0	22,039	0
Deferred Purchase Price Obligation	20,975	(200,322)	55,854
Depreciation and amortization	106,767	114,872	112,661
Proportional adjusted EBITDA for equity method investees	39,969	41,246	45,602
Adjustments related to MVC shortfall payments	(3,632)	(41,373)	11,600
Adjustments related to capital reimbursement activity	(427)	0	0
Unit-based and noncash compensation	8,328	7,951	7,985
Loss on asset sales, net	0	527	93
Long-lived asset impairment	7,186	188,702	1,764
Total of reportable segments' measures of profit	$ 331,890	$ 329,527	$ 329,190

SEGMENT INFORMATION - Schedule

SEGMENT INFORMATION - Schedule of Adjustments Related to Minimum Volume Commitments Shortfall Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	$ 0	$ (40,758)	$ 11,302
Expected MVC shortfall adjustments	(3,632)	(615)	298
Total adjustments related to MVC shortfall payments	(3,632)	(41,373)	11,600
Williston Basin
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	(37,693)	8,691
Expected MVC shortfall adjustments	0	0	0
Total adjustments related to MVC shortfall payments	0	(37,693)	8,691
Piceance Basin
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	(3,065)	3,288
Expected MVC shortfall adjustments	10	(3)	(317)
Total adjustments related to MVC shortfall payments	10	(3,068)	2,971
Barnett Shale
Segment Reporting Information [Line Items]
Net change in deferred revenue for MVC shortfall payments	0	0	(677)
Expected MVC shortfall adjustments	(3,642)	(612)	615
Total adjustments related to MVC shortfall payments	$ (3,642)	$ (612)	$ (62)

PROPERTY, PLANT, AND EQUIPMEN_3

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Property, Plant, and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,350,846		$ 2,104,569
Less accumulated depreciation	387,133		309,440
Property, plant and equipment, net	1,963,713	$ 1,828,252	1,795,129
Gathering and processing systems and related equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,155,325		1,973,722
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	137,920		78,850
Land and line fill
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,748		11,735
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 45,853		$ 40,262

PROPERTY, PLANT, AND EQUIPMEN_4

PROPERTY, PLANT, AND EQUIPMENT, NET - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Long-lived assets impairment charge			$ 7,186	$ 188,702	$ 1,764
Williston Basin
Property, Plant and Equipment [Line Items]
Long-lived assets impairment charge	$ 3,900	$ 101,900	$ 3,972	$ 101,961	$ 569

PROPERTY, PLANT, AND EQUIPMEN_5

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Long-Lived Asset Impairments (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Long-lived asset impairment			$ 7,186	$ 188,702	$ 1,764
Williston Basin
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	$ 3,900	$ 101,900	3,972	101,961	569
Piceance Basin
Property, Plant and Equipment [Line Items]
Long-lived asset impairment			1,004	697
DJ Basin
Property, Plant and Equipment [Line Items]
Long-lived asset impairment			9
Barnett Shale
Property, Plant and Equipment [Line Items]
Long-lived asset impairment				0	1,195
Utica Shale
Property, Plant and Equipment [Line Items]
Long-lived asset impairment			1,440	$ 878	$ 0
Permian Basin
Property, Plant and Equipment [Line Items]
Long-lived asset impairment			$ 761

PROPERTY, PLANT, AND EQUIPMEN_6

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Depreciation Expense and Capitalized Interest (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 74,511	$ 75,120	$ 70,770
Capitalized interest	$ 8,497	$ 2,579	$ 3,709

AMORTIZING INTANGIBLE ASSETS _3

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 468,852	$ 462,629
Accumulated amortization	(195,436)	(161,284)
Net	273,416	301,345
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(13,905)	(12,350)
Net	10,290	11,845
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(143,962)	(117,821)
Net	134,486	160,627
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	166,209	159,986
Accumulated amortization	(37,569)	(31,113)
Net	128,640	128,873
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	$ (10,962)	(9,074)
Net		$ 1,888

AMORTIZING INTANGIBLE ASSETS _4

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT (Narrative) (Details) $ in Millions	1 Months Ended
	Dec. 31, 2017USD ($)
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 _5

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Revenue | Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ (1,555)	$ (1,555)	$ (1,261)
Other Revenue | Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Amortization expense	1,888	2,158	839
Costs And Expenses | Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization expense	26,141	34,202	35,416
Costs And Expenses | Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 6,448	$ 6,153	$ 6,053

AMORTIZING INTANGIBLE ASSETS _6

AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Dec. 31, 2018USD ($)
Intangible assets
2,019	$ 32,422
2,020	32,246
2,021	28,554
2,022	25,487
2,023	$ 25,433

GOODWILL - Narrative (Details)

GOODWILL - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013
Segment Reporting Information [Line Items]
Goodwill	$ 16,211,000	$ 16,211,000
Goodwill impairment	$ 0	$ 0
Mountaineer Midstream
Segment Reporting Information [Line Items]
Goodwill			$ 16,200,000

GOODWILL - Schedule of goodwill

GOODWILL - Schedule of goodwill (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Accumulated goodwill impairment	$ 303,050	$ 303,050	$ 303,050
Piceance Basin
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 - Nar

EQUITY METHOD INVESTMENTS - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	11 Months Ended		12 Months Ended
	Jan. 31, 2014	Nov. 30, 2018	Nov. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Impairment loss				$ 10,888	$ 2,223	$ 30,344
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%
OGC | Summit Midstream Partners, LLC
Schedule Of Equity Method Investments [Line Items]
Equity method potential ownership interest	40.00%
Ownership interest value	$ 190,000
Cost method ownership interest	1.00%
Percentage of ownership interest	40.00%
Option value	$ 7,500
Impairment loss		$ 5,652	$ 1,383	$ 7,700	$ 1,400

EQUITY METHOD INVESTMENTS - Rol

EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	11 Months Ended		12 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity Method Investment [Roll Forward]
Investment in equity method investees, December 31	$ 690,485		$ 690,485
December cash distributions			35,271	$ 40,220	$ 44,991
Impairment loss			10,888	2,223	30,344
Investment in equity method investees, net of basis difference, November 30			649,250	690,485
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, December 31	649,250	$ 690,485	649,250	690,485
December cash distributions	2,736	4,032
December cash contributions	0	(3,932)
Impairment loss	5,652	1,383	$ 7,700	1,400
Legal contingency	2,040	0
Basis difference	(116,832)	(130,184)
Investment in equity method investees, net of basis difference, November 30	$ 542,846	$ 561,784		$ 649,250	$ 690,485

EQUITY METHOD INVESTMENTS - Bal

EQUITY METHOD INVESTMENTS - Balance Sheet Information (Details) - USD ($) $ in Thousands	Nov. 30, 2018	Nov. 30, 2017
OGC
Schedule Of Equity Method Investments [Line Items]
Current assets	$ 37,403	$ 34,383
Noncurrent assets	1,262,253	1,319,448
Total assets	1,299,656	1,353,831
Current liabilities	19,903	10,882
Noncurrent liabilities	3,688	3,272
Total liabilities	23,591	14,154
OCC
Schedule Of Equity Method Investments [Line Items]
Current assets	3,716	3,650
Noncurrent assets	27,203	29,156
Total assets	30,919	32,806
Current liabilities	3,912	3,382
Noncurrent liabilities	8,807	11,715
Total liabilities	$ 12,719	$ 15,097

EQUITY METHOD INVESTMENTS - Sta

EQUITY METHOD INVESTMENTS - Statements of Operations Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2016
OGC
Schedule Of Equity Method Investments [Line Items]
Total revenues	$ 142,398	$ 140,679	$ 148,662
Total operating expenses	136,722	111,897	96,647
Net income (loss)	5,670	28,785	52,009
OCC
Schedule Of Equity Method Investments [Line Items]
Total revenues	10,177	8,607	15,791
Total operating expenses	9,053	8,298	111,528
Net income (loss)	$ 498	$ (907)	$ (94,230)

DEFERRED REVENUE - Rollforward

DEFERRED REVENUE - Rollforward of deferred revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 4,000	$ 0
Current deferred revenue, Additions	26,407	18,294
Current deferred revenue, Less revenue recognized	25,028	14,294
Current deferred revenue, ending balance	11,467	4,000
Noncurrent deferred revenue, beginning balance	12,707	57,465
Noncurrent deferred revenue, Additions	16,502	0
Noncurrent deferred revenue, Less revenue recognized		40,758
Noncurrent deferred revenue, Less reclassification to current deferred revenue	12,526	4,000
Noncurrent deferred revenue, ending balance	39,504	12,707
Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	10,088
Current deferred revenue, ending balance		10,088
Noncurrent deferred revenue, beginning balance	35,528
Noncurrent deferred revenue, ending balance		35,528
Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	6,088
Current deferred revenue, ending balance	7,396	6,088
Noncurrent deferred revenue, beginning balance	22,821
Noncurrent deferred revenue, ending balance	30,566	22,821
Utica Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	18	0
Current deferred revenue, Less revenue recognized	18	0
Current deferred revenue, ending balance	18	0
Noncurrent deferred revenue, beginning balance	0	0
Noncurrent deferred revenue, Additions	0	0
Noncurrent deferred revenue, Less revenue recognized		0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	18	0
Noncurrent deferred revenue, ending balance	21	0
Utica Shale | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Current deferred revenue, ending balance		18
Noncurrent deferred revenue, beginning balance	39
Noncurrent deferred revenue, ending balance		39
Utica Shale | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	18
Current deferred revenue, ending balance		18
Noncurrent deferred revenue, beginning balance	39
Noncurrent deferred revenue, ending balance		39
Williston Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	1,744	0
Current deferred revenue, Less revenue recognized	1,347	0
Current deferred revenue, ending balance	1,414	0
Noncurrent deferred revenue, beginning balance	0	37,693
Noncurrent deferred revenue, Additions	1,851	0
Noncurrent deferred revenue, Less revenue recognized		37,693
Noncurrent deferred revenue, Less reclassification to current deferred revenue	1,673	0
Noncurrent deferred revenue, ending balance	4,393	0
Williston Basin | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Current deferred revenue, ending balance		1,017
Noncurrent deferred revenue, beginning balance	4,215
Noncurrent deferred revenue, ending balance		4,215
Williston Basin | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,017
Current deferred revenue, ending balance		1,017
Noncurrent deferred revenue, beginning balance	4,215
Noncurrent deferred revenue, ending balance		4,215
DJ Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	943	0
Current deferred revenue, Less revenue recognized	562	0
Current deferred revenue, ending balance	739	0
Noncurrent deferred revenue, beginning balance	0	0
Noncurrent deferred revenue, Additions	3,720	0
Noncurrent deferred revenue, Less revenue recognized		0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	941	0
Noncurrent deferred revenue, ending balance	7,284	0
DJ Basin | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	358
Current deferred revenue, ending balance		358
Noncurrent deferred revenue, beginning balance	4,505
Noncurrent deferred revenue, ending balance		4,505
DJ Basin | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	358
Current deferred revenue, ending balance		358
Noncurrent deferred revenue, beginning balance	4,505
Noncurrent deferred revenue, ending balance		4,505
Piceance Basin
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	4,000	0
Current deferred revenue, Additions	21,955	18,294
Current deferred revenue, Less revenue recognized	21,377	14,294
Current deferred revenue, ending balance	7,616	4,000
Noncurrent deferred revenue, beginning balance	12,707	19,772
Noncurrent deferred revenue, Additions	7,869	0
Noncurrent deferred revenue, Less revenue recognized		3,065
Noncurrent deferred revenue, Less reclassification to current deferred revenue	8,146	4,000
Noncurrent deferred revenue, ending balance	17,942	12,707
Piceance Basin | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	7,038
Current deferred revenue, ending balance		7,038
Noncurrent deferred revenue, beginning balance	18,219
Noncurrent deferred revenue, ending balance		18,219
Piceance Basin | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	3,038
Current deferred revenue, ending balance		3,038
Noncurrent deferred revenue, beginning balance	5,512
Noncurrent deferred revenue, ending balance		5,512
Barnett Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	1,651	0
Current deferred revenue, Less revenue recognized	1,628	0
Current deferred revenue, ending balance	1,642	0
Noncurrent deferred revenue, beginning balance	0	0
Noncurrent deferred revenue, Additions	3,062	0
Noncurrent deferred revenue, Less revenue recognized		0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	1,651	0
Noncurrent deferred revenue, ending balance	9,628	0
Barnett Shale | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Current deferred revenue, ending balance		1,619
Noncurrent deferred revenue, beginning balance	8,217
Noncurrent deferred revenue, ending balance		8,217
Barnett Shale | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,619
Current deferred revenue, ending balance		1,619
Noncurrent deferred revenue, beginning balance	8,217
Noncurrent deferred revenue, ending balance		8,217
Marcellus Shale
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0
Current deferred revenue, Additions	96	0
Current deferred revenue, Less revenue recognized	96	0
Current deferred revenue, ending balance	38	0
Noncurrent deferred revenue, beginning balance	0	0
Noncurrent deferred revenue, Additions	0	0
Noncurrent deferred revenue, Less revenue recognized		0
Noncurrent deferred revenue, Less reclassification to current deferred revenue	97	0
Noncurrent deferred revenue, ending balance	236	0
Marcellus Shale | Accounting Standards Update 2014-09
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Current deferred revenue, ending balance		38
Noncurrent deferred revenue, beginning balance	333
Noncurrent deferred revenue, ending balance		333
Marcellus Shale | Accounting Standards Update 2014-09 | Effect of Change Increase / (Decrease)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	38
Current deferred revenue, ending balance		38
Noncurrent deferred revenue, beginning balance	$ 333
Noncurrent deferred revenue, ending balance		$ 333

DEFERRED REVENUE - Narrative (D

DEFERRED REVENUE - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)
Williston Basin | Gathering Services and Related Fees
Deferred Revenue Arrangement [Line Items]
Recognized amount of deferred revenue	$ 37.7

DEBT - Components of Long-Term

DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,257,731	$ 1,051,192
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,257,731	1,051,192
SMP Holdings | Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	466,000	261,000
SMP Holdings | Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(2,362)	(2,910)
SMP Holdings | Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	500,000
Less unamortized debt issuance costs	$ (5,907)	$ (6,898)

DEBT - Components of Long-Ter_2

DEBT - Components of Long-Term Debt (Parenthetical) (Details) - SMP Holdings	Dec. 31, 2018	Dec. 31, 2017
Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate	5.03%	4.07%
Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Stated interest rate	5.50%	5.50%
Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Stated interest rate	5.75%

DEBT - Maturities (Details)

DEBT - Maturities (Details) $ in Thousands	Dec. 31, 2018USD ($)
Debt
2,019	$ 0
2,020	0
2,021	0
2,022	766,000
2,023	0
Thereafter	500,000
Total long-term debt	$ 1,266,000

DEBT - Revolving Credit Facilit

DEBT - Revolving Credit Facility (Narrative) (Details)	1 Months Ended	12 Months Ended
	May 31, 2017USD ($)	Dec. 31, 2018USD ($)	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
Senior secured leverage ratio	3.75
Commitment fee on unused portion of the facility		0.50%
Interest rate at period end		5.03%
Unused portion under the facility		$ 784,000,000
Cumulative payment obligations, maximum		50,000,000
Debt issuance costs		8,500,000
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)

DEBT - Senior Notes (Narrative) (Details) - USD ($)

Feb. 28, 2017

Feb. 16, 2017

Feb. 14, 2017

Jul. 31, 2014

Nov. 30, 2018

Feb. 28, 2017

Jul. 31, 2014

Jun. 30, 2013

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Oct. 31, 2018

Feb. 08, 2017

Jun. 30, 2016

Debt Instrument [Line Items]

Early extinguishment of debt

$ 0

$ 302,000

$ 0

Revolving credit facility

Debt Instrument [Line Items]

Extinguishment of debt

$ 172,000,000

Summit Midstream OpCo, LP

Debt Instrument [Line Items]

Ownership interest

1.00%

1.00%

Guarantor Subsidiaries

Debt Instrument [Line Items]

Cumulative percentage ownership in subsidiary

100.00%

5.75% Senior Notes

Debt Instrument [Line Items]

Stated interest rate

5.75%

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

$ 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

$ 300,000,000

Stated interest rate

5.50%

5.50%

Maturity date

Aug. 15, 2022

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%

Subsidiary of Summit Investments | Summit Midstream OpCo, LP

Debt Instrument [Line Items]

Ownership interest

1.00%

1.00%

1.00%

Finance Corp | Summit Holdings

Debt Instrument [Line Items]

Cumulative percentage ownership in subsidiary

100.00%

FINANCIAL INSTRUMENTS - Concent

FINANCIAL INSTRUMENTS - Concentration Risk - Narrative (Details) - Customer	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
CONCENTRATIONS OF RISK
Number of top customers	5
Five Largest Customers | Accounts receivable | Customer Concentration Risk
CONCENTRATIONS OF RISK
Concentration risk, percentage	39.00%	44.00%

FINANCIAL INSTRUMENTS - Fair Va

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, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 liability, beginning of period	$ 362,959	$ 563,281
Change in fair value	20,975	(200,322)
Level 3 liability, end of period	$ 383,934	$ 362,959

FINANCIAL INSTRUMENTS - Fair _2

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying value | 5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 297,638	$ 297,090
Carrying value | 5.75% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	494,093	493,102
Fair Value | 5.5% Senior Notes | Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	286,625	301,750
Fair Value | 5.75% Senior Notes | Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 455,208	$ 501,667

FINANCIAL INSTRUMENTS - Fair _3

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

PARTNERS' CAPITAL - Partners' C

PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	300,000	0	0
Subordinated units conversion			0
Units issued in connection with offering		300,000	0
Units issued under ATM program (in shares)		0
Net units issued under SMLP LTIP (in shares)	0	0	0
General Partner 2% capital contribution (in shares)		0	0
Units, ending balance (in shares)	300,000	300,000	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	73,085,996	72,111,121	42,062,644
Subordinated units conversion			24,409,850
Units issued in connection with offering		0	5,500,000
Units issued under ATM program (in shares)		763,548
Net units issued under SMLP LTIP (in shares)	304,857	211,327	138,627
General Partner 2% capital contribution (in shares)		0	0
Units, ending balance (in shares)	73,390,853	73,085,996	72,111,121
Limited partners, Subordinated
Rollforwards of the number of partner units
Units, beginning balance (in shares)	0	0	24,409,850
Subordinated units conversion			(24,409,850)
Units issued in connection with offering		0	0
Units issued under ATM program (in shares)		0
Net units issued under SMLP LTIP (in shares)	0	0	0
General Partner 2% capital contribution (in shares)		0	0
Units, ending balance (in shares)	0	0	0
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,490,999	1,471,187	1,354,700
Subordinated units conversion			0
Units issued in connection with offering		0	0
Units issued under ATM program (in shares)		0
Net units issued under SMLP LTIP (in shares)	0	0	4,242
General Partner 2% capital contribution (in shares)		19,812	112,245
Units, ending balance (in shares)	1,490,999	1,490,999	1,471,187

PARTNERS' CAPITAL - Narrative (

PARTNERS' CAPITAL - Narrative (Details)

Feb. 26, 2019$ / shares

Jan. 24, 2019USD ($)$ / shares

Mar. 03, 2016USD ($)

Nov. 30, 2018USD ($)

Nov. 30, 2017USD ($)$ / sharesshares

Feb. 28, 2017USD ($)shares

Sep. 30, 2016$ / sharesshares

Jun. 30, 2016USD ($)

Dec. 31, 2015

Dec. 31, 2018USD ($)Transaction$ / sharesshares

Dec. 31, 2017USD ($)$ / sharesshares

Dec. 31, 2016USD ($)$ / sharesshares

Oct. 31, 2018

Partners Capital [Line Items]

Aggregate gross proceeds

$ 0

$ 17,078,000

$ 0

Preferred units issued | shares

300,000

300,000

Repayment of outstanding revolving credit facility

$ 84,000,000

$ 634,500,000

204,300,000

Purchase of noncontrolling interest

$ 10,981,000

$ 797,000

$ 0

Maximum period following end of quarter to distribute all available cash

45 days

Per-unit annual distributions to unitholders (in dollars per unit) | $ / shares

$ 2.300

$ 2.300

$ 2.300

Subsequent Event

Partners Capital [Line Items]

Per-unit annual distributions to unitholders (in dollars per unit) | $ / shares

$ 0.575

Cash paid to unit holders

$ 45,300,000

Cash distribution date

Feb. 14, 2019

Cash distribution date of record

Feb. 7, 2019

Reducing distribution on common units | $ / shares

$ 0.2875

Drop Down Assets 2016 Acquisition

Partners Capital [Line Items]

Initial payment

$ 360,000,000

Deferred purchase price obligation

507,400,000

Net investment

$ 1,110,000,000

Working capital adjustment

$ 600,000

Summit Midstream OpCo, LP

Partners Capital [Line Items]

Ownership interest

1.00%

1.00%

Subsidiary of Summit Investments

Partners Capital [Line Items]

Purchase of noncontrolling interest

$ 10,900,000

Subsidiary of Summit Investments | Summit Midstream OpCo, LP

Partners Capital [Line Items]

Ownership interest

1.00%

1.00%

1.00%

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%

Series A Preferred Units | LIBOR

Partners Capital [Line Items]

Distribution percentage of liquidation preference rate

7.43%

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%

General Partner | Subsequent Event

Partners Capital [Line Items]

General partner interest

2.00%

Limited Partner | Public Offering | 2016 SRS

Partners Capital [Line Items]

Units issued under ATM program (in shares) | shares

4,000,000

Proceeds from Issuance of common stock

$ 0

Common

Partners Capital [Line Items]

Units issued under ATM program (in shares) | shares

763,548

Units issued in connection with offering | shares

0

5,500,000

Common | Public Offering

Partners Capital [Line Items]

Aggregate offering price

$ 150,000,000

Transactions under ATM Program | Transaction

0

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

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

PARTNERS' CAPITAL - Cash Distri

PARTNERS' CAPITAL - Cash Distributions Paid and Declared (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 2.300	$ 2.300	$ 2.300

PARTNERS' CAPITAL - Percentage

PARTNERS' CAPITAL - Percentage Allocations of Available Cash (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Partnership Target Distributions [Line Items]
IDR payments	$ 8,535	$ 8,460	$ 7,912
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)

EARNINGS PER UNIT (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners	$ 28,622	$ 47,026	$ (58,236)	$ (13,113)	$ (23,654)	$ 89,547	$ 8,806	$ (2,777)	$ 32,799	$ 75,485	$ (48,179)
Phantom Units
Earnings (loss) per limited partner unit:
Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU (in shares)									2	42	125
Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners									$ 4,299	$ 71,922	$ (48,179)
Weighted-average units outstanding (in shares)									73,304	72,705	68,264
Effect of nonvested phantom units (in shares)									311	342	0
Weighted-average units outstanding – diluted (shares)									73,615	73,047	68,264
Earnings (loss) per limited partner unit:
Basic (in dollars per share)	$ 0.39	$ 0.64	$ (0.79)	$ (0.18)	$ (0.32)	$ 1.23	$ 0.12	$ (0.04)	$ 0.06	$ 0.99	$ (0.71)
Diluted (in dollars per share)	$ 0.39	$ 0.64	$ (0.79)	$ (0.18)	$ (0.32)	$ 1.22	$ 0.12	$ (0.04)	$ 0.06	$ 0.98	$ (0.71)
Series A Preferred Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners	$ 7,125	$ 7,125	$ 7,125	$ 7,125	$ 3,563				$ 28,500	$ 3,563	$ 0

UNIT-BASED AND NONCASH COMPEN_3

UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)shares
Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 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	3,200,000
Unrecognized unit-based compensation | $	$ 4.9
Remaining vesting period	2 years 2 months 12 days

UNIT-BASED AND NONCASH COMPEN_4

UNIT-BASED AND NONCASH COMPENSATION - SMLP Long-Term Incentive Plan (Details) - SMLP LTIP - Phantom Units - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Units
Nonvested units(in shares)	749,274	691,955	379,911
Units granted (in shares)	515,358	371,972	495,535
Unit vested (in shares)	(359,016)	(293,222)	(178,953)
Units forfeited (in shares)	(41,492)	(21,431)	(4,538)
Nonvested units(in shares)	864,124	749,274	691,955
Weighted-average grant date fair value
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 20.07	$ 19.59	$ 31.13
Units granted, Weighted-average grant date fair value (in dollars per unit)	15.25	22.50	14.91
Units vested, Weighted-average grant date fair value (in dollars per unit)	22.39	24.76	33.80
Units forfeited, Weighted-average grant date fair value (in dollars per unit)	17.27	20.07	16.89
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 17.11	$ 20.07	$ 19.59

UNIT-BASED AND NONCASH COMPEN_5

UNIT-BASED AND NONCASH COMPENSATION - Schedule of Intrinsic Values (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SMLP LTIP | Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of vested LTIP awards	$ 5,393	$ 6,657	$ 2,957

UNIT-BASED AND NONCASH COMPEN_6

UNIT-BASED AND NONCASH COMPENSATION - SMP Net Profits Interests (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 8,088	$ 7,878	$ 7,550
SMLP LTIP | General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 8,328	$ 7,951	$ 7,550

RELATED-PARTY TRANSACTIONS - Na

RELATED-PARTY TRANSACTIONS - Narrative (Details) - SMP Holdings	1 Months Ended
	Feb. 28, 2017$ / sharesshares
RELATED-PARTY TRANSACTIONS
Shares sold (in shares) | shares	4,000,000
Sale price (in dollars per share) | $ / shares	$ 24

RELATED-PARTY TRANSACTIONS - Ex

RELATED-PARTY TRANSACTIONS - Expenses Incurred by the General Partner and Reimbursed (Details) - General Partner - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operation and maintenance expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 29,061	$ 27,450	$ 26,485
General and administrative expense
RELATED-PARTY TRANSACTIONS
Expenses from transactions with related party	$ 30,119	$ 30,899	$ 31,947

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Rent Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 3,928	$ 3,772	$ 2,861

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - Schedule of Future Minimum Rental Payments for Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 3,133
2,020	1,018
2,021	550
2,022	506
2,023	373
Thereafter	621
Total future minimum lease payments	$ 6,201

COMMITMENTS AND CONTINGENCIES_3

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2018	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_4

COMMITMENTS AND CONTINGENCIES - Environmental Matters (Details) - Meadowlark Midstream Gathering System - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 5,344	$ 9,453
Payments made	(3,808)	(4,109)
Additional accruals	4,100
Accrued environmental remediation, ending balance	$ 5,636	$ 5,344

ACQUISITIONS AND DROP DOWN TR_3

ACQUISITIONS AND DROP DOWN TRANSACTIONS - 2016 Drop Down (Details) $ in Thousands	Mar. 03, 2016USD ($)	Jun. 30, 2016USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Deferred purchase price obligation			$ 383,934	$ 362,959
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Initial payment	$ 360,000
Working capital adjustment		$ 600
Deferred Purchase price obligation, multiple	6.5
Estimated future payment obligation	$ 860,300		423,900
Deferred purchase price obligation	$ 507,400		$ 383,900
Discount rate	13.00%		8.25%

ACQUISITIONS AND DROP DOWN TR_4

ACQUISITIONS AND DROP DOWN TRANSACTIONS - Revenue and Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Revenues	$ 133,671	$ 127,479	$ 128,183	$ 117,320	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 506,653	$ 488,741	$ 402,362
Net (loss) income									$ 42,351	$ 86,050	(38,187)
Summit Midstream Partners, LP
Business Acquisition [Line Items]
Revenues											393,495
Net (loss) income											(40,932)
Drop Down Assets 2016 Acquisition
Business Acquisition [Line Items]
Revenues											8,867
Net (loss) income											$ 2,745

UNAUDITED QUARTERLY FINANCIAL_3

UNAUDITED QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Limited Partners' Capital Account [Line Items]
Revenues	$ 133,671	$ 127,479	$ 128,183	$ 117,320	$ 126,199	$ 124,945	$ 101,792	$ 135,805	$ 506,653	$ 488,741	$ 402,362
Net income (loss) attributable to SMLP	38,654	57,430	(49,971)	(3,930)	(18,331)	93,546	11,157	(685)	42,183	85,687	(40,918)
Less net income and IDRs attributable to General Partner	2,907	3,279	1,140	2,058	1,760	3,999	2,351	2,092	9,384	10,202	7,261
Net income (loss) attributable to limited partners	28,622	47,026	(58,236)	(13,113)	(23,654)	$ 89,547	$ 8,806	$ (2,777)	32,799	75,485	(48,179)
Series A Preferred Units
Limited Partners' Capital Account [Line Items]
Net income (loss) attributable to limited partners	$ 7,125	$ 7,125	$ 7,125	$ 7,125	$ 3,563				28,500	3,563	0
Common units
Limited Partners' Capital Account [Line Items]
Net income (loss) attributable to limited partners									$ 4,299	$ 71,922	$ (48,179)
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	$ 0.39	$ 0.64	$ (0.79)	$ (0.18)	$ (0.32)	$ 1.23	$ 0.12	$ (0.04)	$ 0.06	$ 0.99	$ (0.71)
Diluted (in dollars per share)	$ 0.39	$ 0.64	$ (0.79)	$ (0.18)	$ (0.32)	$ 1.22	$ 0.12	$ (0.04)	$ 0.06	$ 0.98	$ (0.71)

SUBSEQUENT EVENTS - Narrative (

SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Mar. 31, 2020	Feb. 26, 2019	Mar. 03, 2016	Sep. 30, 2016	Dec. 31, 2018	Jun. 30, 2020
General Partner
Subsequent Event [Line Items]
General partner interest				2.00%	2.00%
Drop Down Assets 2016 Acquisition
Subsequent Event [Line Items]
Initial payment			$ 360
Drop Down Assets 2016 Acquisition | SMP Holdings
Subsequent Event [Line Items]
Remaining consideration description					Remaining Consideration will be fixed at $303.5 million, and will be payable by the Partnership in one or more payments over the period from March 1, 2020 through % per annum on any portion of the Remaining Consideration that remains unpaid after March 31, 2020
Drop Down Assets 2016 Acquisition | SMP Holdings | Scenario Forecast
Subsequent Event [Line Items]
Minimum remaining consideration to be paid percentage						50.00%
Annual accrued Interest rate on unpaid consideration	8.00%
Subsequent Event
Subsequent Event [Line Items]
Partnership expects to reduce its common unit distribution per quarter		$ 0.2875
Subsequent Event | General Partner
Subsequent Event [Line Items]
General partner interest		2.00%
Subsequent Event | General Partner | Common units
Subsequent Event [Line Items]
General partner units converted		8,750,000
Subsequent Event | Limited Partner | Summit Midstream Partners, LP | Energy Capital Partners
Subsequent Event [Line Items]
Limited partner interest ownership percentage		7.20%
Subsequent Event | SMP Holdings | Limited Partner | Summit Midstream Partners, LP
Subsequent Event [Line Items]
Limited partner interest ownership percentage		42.10%
Subsequent Event | Drop Down Assets 2016 Acquisition | SMP Holdings
Subsequent Event [Line Items]
Initial payment		$ 100
Remaining cash consideration payable		303.5
Subsequent Event | Tioga Midstream, LLC
Subsequent Event [Line Items]
Sale of midstream asset for cash consideration		$ 90
Subsequent Event | Tioga Midstream, LLC | General Partner | Common units
Subsequent Event [Line Items]
General partner units converted		8,750,000
Subsequent Event | Tioga Midstream, LLC | SMP Holdings | General Partner
Subsequent Event [Line Items]
General partner interest		2.00%