Summit Midstream Partners (SMLP) 10-K 4 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 26, 2021	Jun. 30, 2020
Cover [Abstract]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	0001549922
Document Type	10-K
Document Period End Date	Dec. 31, 2020
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 45,998,000
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	SMLP
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		6,110,092
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Units
Security Exchange Name	NYSE
Entity File Number	001-35666
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	45-5200503
Entity Address, Address Line One	910 Louisiana Street
Entity Address, Address Line Two	Suite 4200
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77002
City Area Code	832
Local Phone Number	413-4770
Document Annual Report	true
Document Transition Report	false
ICFR Auditor Attestation Flag	true

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Cash and cash equivalents	$ 15,544	$ 9,530
Restricted cash		27,392
Accounts receivable	61,932	97,418
Other current assets	4,623	5,521
Total current assets	82,099	139,861
Property, plant and equipment, net	1,817,546	1,882,489
Intangible assets, net	199,566	232,278
Investment in equity method investees	392,740	309,728
Other noncurrent assets	7,866	9,742
TOTAL ASSETS	2,499,817	2,574,098
LIABILITIES AND CAPITAL
Trade accounts payable	11,878	24,415
Accrued expenses	13,036	11,339
Deferred revenue	9,988	13,493
Ad valorem taxes payable	9,086	8,477
Accrued compensation and employee benefits	9,658	8,719
Accrued interest	8,007	12,346
Accrued environmental remediation	1,392	1,725
Other current liabilities	5,363	3,487
Current portion of SMPH term loan		5,546
Total current liabilities	68,408	89,547
Long-term debt	1,347,326	1,622,279
Noncurrent deferred revenue	48,250	38,709
Noncurrent accrued environmental remediation	1,537	2,926
Other noncurrent liabilities	21,747	7,951
Total liabilities	1,487,268	1,761,412
Commitments and contingencies (Note 11)
Mezzanine Capital
Subsidiary Series A Preferred Units (55,251 and 30,058 units issued and outstanding at December 31, 2020 and December 31, 2019, respectively)	89,658	27,450
Partners' Capital
Series A Preferred Units (162,109 and 300,000 units issued and outstanding at December 31, 2020 and December 31, 2019, respectively)	174,425	293,616
Common limited partner capital (6,110,092 and 3,021,258 units issued and outstanding at December 31, 2020 and December 31, 2019, respectively)	748,466	305,550
Noncontrolling interest		186,070
Total partners' capital	922,891	785,236
TOTAL LIABILITIES AND CAPITAL	$ 2,499,817	$ 2,574,098

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Dec. 31, 2020	Dec. 31, 2019
Subsidiary Series A preferred unitholders, issued	162,109	300,000
Subsidiary Series A preferred unitholders, outstanding	162,109	300,000
Common limited partner capital (in shares), issued	6,110,092	3,021,258
Common limited partner capital (in shares), outstanding	6,110,092	3,021,258
Mezzanine Capital
Subsidiary Series A preferred unitholders, issued	55,251	30,058
Subsidiary Series A preferred unitholders, outstanding	55,251	30,058

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Revenues:
Total revenues		$ 383,473	$ 443,528
Costs and expenses:
Cost of natural gas and NGLs		$ 36,653	$ 63,438
Type of Cost, Good or Service [Extensible List]		us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance		$ 86,030	$ 98,719
General and administrative		73,438	55,947
Depreciation and amortization		118,132	110,354
Transaction costs		2,993	3,017
Gain on asset sales, net		(307)	(1,536)
Long-lived asset impairment		13,089	60,507
Goodwill impairment		0	16,211
Total costs and expenses		330,028	406,657
Other income		48	451
Interest expense		(78,894)	(91,966)
Gain on early extinguishment of debt		203,062	0
Income (loss) before income taxes and equity method investment income (loss)		177,661	(54,644)
Income tax benefit (expense)		146	(1,231)
Income (loss) from equity method investees		11,271	(337,851)
Net income (loss)		189,078	(393,726)
Net income (loss) attributable to noncontrolling interest		(3,274)	(209,275)
Net income (loss) attributable to limited partners		192,352	(184,451)
Deemed contribution from preferred unit exchange and purchase		110,669	0
Series A Preferred Units
Costs and expenses:
Net income (loss) attributable to limited partners		26,529	28,500
Deemed contribution from preferred unit exchange and purchase		54,945
Series A Preferred Units | Subsidiary
Costs and expenses:
Net income (loss) attributable to limited partners		13,498	58
Common units
Costs and expenses:
Net income (loss) attributable to limited partners		$ 262,994	$ (213,009)
Net income (Loss) per limited partner unit:
Basic (in dollars per share)		$ 73.22	$ (70.50)
Diluted (in dollars per share)		$ 71.19	$ (70.50)
Weighted-average limited partner units outstanding:
Basic (shares)	[1]	3,592	3,021
Diluted (shares)		3,694	3,021
Gathering Services and Related Fees
Revenues:
Total revenues		$ 302,792	$ 326,747
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues		49,319	86,994
Other Revenues
Revenues:
Total revenues		$ 31,362	$ 29,787
[1]	As a result of the GP Buy-In Transaction, our historical results are those of Summit Investments. The number of common units of 3.0 million as of December 31, 2019 represents those of Summit Investments and has been used for the earnings per unit calculation presented herein

CONSOLIDATED STATEMENTS OF PART

CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) $ in Thousands		Total	Partners' Capital	Partners' CapitalSeries A Preferred Units	Noncontrolling InterestSeries A Preferred Units	Noncontrolling InterestCommon Noncontrolling Interest	[1]
Beginning balance at Dec. 31, 2018		$ 1,391,567	$ 543,479		$ 293,616	$ 554,472
Net income (loss)		(393,784)	(213,009)		28,500	(209,275)
Net cash distribution to SMLP unitholders		(97,374)			(28,500)	(68,874)
Net cash distributions to Energy Capital Partners		(120,730)	(120,730)
Unit-based compensation		8,171				8,171
Effect of common unit issuances under SMLP LTIP			(2,664)			2,664
Tax withholdings and associated payments on vested SMLP LTIP awards		(2,614)				(2,614)
Conversion of noncontrolling interest related to cancellation of subsidiary incentive distribution rights		0	48,203			(48,203)
Conversion of noncontrolling interest related to partial cancellation of subsidiary of payable			50,271			(50,271)
Ending balance at Dec. 31, 2019		785,236	305,550		293,616	186,070
Net income (loss)		175,580	152,325	$ 14,654	11,875	(3,274)
Net cash distribution to SMLP unitholders		(6,037)				(6,037)
Unit-based compensation		8,111	4,057			4,054
Effect of common unit issuances under SMLP LTIP			(2,322)			2,322
Tax withholdings and associated payments on vested SMLP LTIP awards		(1,456)	(438)			(1,018)
Conversion of noncontrolling interest related to cancellation of subsidiary incentive distribution rights		110,669
Tax withholdings on Series A Preferred Unit Exchange		(237)	(237)
GP-Buy-In Transaction assumption of noncontrolling interest in SMLP			182,117	305,491	$ (305,491)	$ (182,117)
Repurchase of common units under GP Buy-In Transaction		(44,078)	(44,078)
Common unit issuance due to TL Restructuring		30,521	30,521
Effect of Exchange Offer and PreferredTender(2)	[2]	(25,000)	120,720	(145,720)
Other		251	251
Ending balance at Dec. 31, 2020		$ 922,891	$ 748,466	$ 174,425
[1]	Prior to the GP Buy-in Transaction, common noncontrolling interests reported by Summit Investments included equity interests in SMLP that were not owned by Summit Investments.
[2]	(2) Includes deemed contributions of $54.9 million and $55.7 million related to the Exchange Offer and Tender Offer, respectively.

CONSOLIDATED STATEMENTS OF PA_2

CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Deemed contribution from preferred unit exchange and purchase	$ 110,669
Exchanger Offer [Member]
Deemed contribution from preferred unit exchange and purchase	54,900
Tender Offer [Member]
Deemed contribution from preferred unit exchange and purchase	$ 55,700

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income (loss)	$ 189,078	$ (393,726)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	119,070	111,574
Noncash lease expense	3,242	3,086
Amortization of debt issuance costs	6,608	6,313
Unit-based and noncash compensation	8,111	8,171
(Income) loss from equity method investees	(11,271)	337,851
Distributions from equity method investees	28,185	37,300
Gain on asset sales, net	(307)	(1,536)
Gain on early extinguishment of debt	(206,975)	0
Gain on fair value of warrants	(393)	0
Settlement of interest rate derivative	134	0
Long-lived asset impairment	13,089	60,507
Goodwill impairment	0	16,211
Changes in operating assets and liabilities:
Accounts receivable	35,352	(5,466)
Trade accounts payable	(4,063)	(96)
Accrued expenses	2,841	(10,572)
Deferred revenue, net	6,036	1,683
Ad valorem taxes payable	609	(1,525)
Accrued interest	(3,343)	7
Accrued environmental remediation, net	(1,996)	(1,152)
Other, net	14,582	(6,889)
Net cash provided by operating activities	198,589	161,741
Cash flows from investing activities:
Capital expenditures	(43,128)	(182,291)
Proceeds from asset sale (net of cash of $1,475 for the year ended December 31, 2019)	0	102,111
Distribution from equity method investment	0	7,313
Other, net	2,486	313
Net cash used in investing activities	(140,569)	(90,870)
Cash flows from financing activities:
Net cash distributions to noncontrolling interest SMLP unitholders	(6,037)	(68,874)
Series A Preferred Unit distributions	0	(28,500)
Net cash distributions to Energy Capital Partners	0	(120,730)
Borrowings under Revolving Credit Facility	249,300	369,000
Repayments on Revolving Credit Facility	(69,300)	(158,000)
Transaction costs	(4,221)	0
Repayments on SMPH Term Loan before TL Restructuring	(6,300)	(65,250)
Proceeds from issuance of Subsidiary Series A preferred units, net of issuance costs	48,710	27,392
Preferred Tender	(25,000)	0
Borrowings under ECP Loans	35,000	0
Repayment of ECP Loans	(35,000)	0
Purchase of common units in GP Buy-In Transaction	(41,778)	0
Debt issuance costs	(2,558)	(673)
Proceeds from asset sale	288	0
Other, net	(2,905)	(4,487)
Net cash used in financing activities	(79,398)	(50,122)
Net change in cash, cash equivalents and restricted cash	(21,378)	20,749
Cash, cash equivalents and restricted cash, beginning of period	36,922	16,173
Cash, cash equivalents and restricted cash, end of period	15,544	36,922
Senior Secured Term Loan Due May 2022
Cash flows from financing activities:
TL Restructuring	(26,500)	0
Double E
Cash flows from investing activities:
Investment in Double E equity method investee	(99,927)	(18,316)
Open Market Repurchases
Cash flows from financing activities:
Repurchases of 2022 and 2025 Senior Notes (Note 10)	(145,567)	0
Tender Offers
Cash flows from financing activities:
Repurchases of 2022 and 2025 Senior Notes (Note 10)	$ (47,530)	$ 0

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Statement Of Cash Flows [Abstract]
Net of cash	$ 1,475

ORGANIZATION, BUSINESS OPERATIO

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION	1. ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION Organization. Summit Midstream Partners, LP (including its subsidiaries, collectively “SMLP” or the “Partnership”) is a Delaware limited partnership that was formed in May 2012 and began operations in October 2012. SMLP is a value-oriented limited partnership focused on developing, owning and operating midstream energy infrastructure assets that are strategically located in unconventional resource basins, primarily shale formations, in the continental United States. The Partnership’s business activities are conducted through various operating subsidiaries, each of which is owned or controlled by its wholly owned subsidiary holding company, Summit Holdings, a Delaware limited liability company. As of December 31, 2020, the Partnership indirectly owns its General Partner, and the General Partner is a wholly owned, indirect subsidiary of the Partnership. The General Partner has no assets or liabilities and holds the non-economic general partner interest in the Partnership. GP Buy-In Transaction. On May 28, 2020, the Partnership closed the transactions contemplated by the Purchase Agreement (the “Purchase Agreement”), dated May 3, 2020, with affiliates of its then private equity sponsor, Energy Capital Partners II, LLC (“ECP”), to acquire Summit Investments. The acquisition of Summit Investments resulted in the Partnership acquiring (a) 2.3 million SMLP common units that were pledged as collateral under the SMPH Term Loan, (b) 0.7 million SMLP common units that were not pledged as collateral under the SMPH Term Loan and (c) a deferred purchase price obligation receivable owed by the Partnership. In addition, the Partnership acquired 0.4 million SMLP common units held by an affiliate of ECP. The total purchase price was $35.0 million in cash and warrants giving ECP the right to purchase up to 0.7 million SMLP common units (refer to Note 13 – Partners’ Capital and Mezzanine Capital for additional details). Pursuant to the Purchase Agreement, the Partnership retained any liabilities stemming from the release of produced water from a produced water pipeline operated by Meadowlark Midstream, a subsidiary of the Partnership, that occurred near Williston, North Dakota and was discovered on January 6, 2015. These transactions are collectively referred to as the “GP Buy-In Transaction.” As a result of the GP Buy-In Transaction, the Partnership indirectly owns its General Partner. Following the closing of the GP Buy-In Transaction, the Partnership retired 1.1 million SMLP common units it acquired that were not pledged as collateral under the SMPH Term Loan. The 2.3 million SMLP common units that were pledged as collateral under the SMPH Term Loan were not considered outstanding with respect to voting and distributions under the Partnership Agreement until the closing of the TL Restructuring on November 17, 2020 (refer to Note 10). Under GAAP, the GP Buy-In Transaction was deemed a transaction among entities under common control with a change in reporting entity. Although SMLP is the surviving entity for legal purposes, Summit Investments is the surviving entity for accounting purposes; therefore, the historical financial results included herein, prior to the GP Buy-In Transaction are those of Summit Investments. Prior to the GP Buy-In Transaction, Summit Investments controlled SMLP and SMLP’s financial statements were consolidated into Summit Investments. Reverse Unit Split. On November 9, 2020, after the close of trading on the NYSE, the Partnership effected a 1-for-15 reverse unit split (the “Reverse Unit Split”) of its common units. The common units began trading on a split-adjusted basis on November 10, 2020. Pursuant to the Reverse Unit Split, common unitholders received one common unit for every 15 common units owned at the close of business on November 9, 2020. Immediately prior to the Reverse Unit Split, there were 56,624,887 common units issued and outstanding and immediately after the Reverse Unit Split, the number of issued and outstanding common units decreased to 3,774,992. Business Operations. The Partnership provides natural gas gathering, compression, treating and processing services as well as crude oil and produced water gathering services pursuant to primarily long-term, fee-based agreements with its customers. The Partnership’s results are primarily driven by the volumes of natural gas that it gathers, compresses, treats and/or processes as well as by the volumes of crude oil and produced water that it gathers. Other than the Partnership’s investments in Double E and Ohio Gathering, all of its business activities are conducted through wholly owned operating subsidiaries Presentation and Consolidation. The Partnership prepares its consolidated financial statements in accordance with GAAP as established by the FASB. The Partnership makes 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 revenues and expenses and the disclosure of commitments and 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. Risks and Uncertainties. The Partnership is closely monitoring the impact of the outbreak of COVID-19 on all aspects of its business, including how it has impacted and will impact its customers, employees, supply chain and distribution network. The Partnership is unable to predict the ultimate impact that COVID-19 may have on its business, future results of operations, financial position or cash flows. Given the dynamic nature of the COVID-19 pandemic and related market conditions, the Partnership cannot reasonably estimate the period of time that these events will persist or the full extent of the impact they will have on its business. The full extent to which the Partnership’s operations may be impacted by the COVID-19 pandemic will depend largely on future developments, which are highly uncertain and cannot be accurately predicted, including changes in the severity of the pandemic, countermeasures taken by governments, businesses and individuals to slow the spread of the pandemic, and the ability of pharmaceutical companies to develop effective and safe vaccines and therapeutic drugs. Furthermore, the impacts of a potential worsening of global economic conditions and the continued disruptions to and volatility in the financial markets remain unknown.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Cash, Cash Equivalents and Restricted Cash. The Partnership considers 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 the Partnership’s customers and other counterparties. The Partnership evaluates the collectability of accounts receivable and the need for an allowance for doubtful accounts based on customer-specific facts and circumstances. To the extent the collectability of a specific customer or counterparty receivable is doubtful, the Partnership recognizes 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. The Partnership records property, plant and equipment at historical cost of construction or fair value of the assets at acquisition. The Partnership capitalizes 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, the Partnership recognizes expenditures as an expense as incurred. The Partnership capitalizes project costs incurred during construction, including interest on funds borrowed to finance the construction of facilities, as construction in progress. Accrued capital expenditures are reflected in trade accounts payable. The Partnership records depreciation on a straight-line basis over an asset’s estimated useful life and bases its 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. The Partnership bases an asset’s carrying value on estimates, assumptions and judgments for useful life and salvage value. Upon sale, retirement or other disposal, the Partnership removes the carrying value of an asset and its accumulated depreciation from its balance sheet and recognizes the related gain or loss, if any. Asset Retirement Obligations. The Partnership records 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, the Partnership evaluates whether the expected retirement date and related costs of retirement can be estimated. The Partnership ha s concluded that its gathering and processing assets have an indeterminate life because they are owned and will operate for an indeterminate period when properly maintained. Because the Partnership d oes not have sufficient information to reasonably estimate the amount or timing of such obligations and does not have any current plan to discontinue use of any significant assets, the Partnership did no t provide for any asset retirement obligations as of December 31, 20 20 or 201 9 . Amortizing Intangibles. The Partnership has certain acquired gas gathering contracts that had above-market pricing structures at the acquisition date and the Partnership amortizes these favorable contracts using a straight-line method over the contract’s estimated useful life. The Partnership defines useful life as the period over which the contract is expected to contribute to the Partnership’s future cash flows. These favorable contracts have original terms ranging from 10 years to 20 years and the Partnership recognizes the amortization expense associated with these contracts in Other revenues. The Partnership amortizes 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. The Partnership recognizes the amortization expense associated with these contracts in Depreciation and amortization expense. The Partnership also has rights-of-way associated with municipal easements and easements granted within existing rights-of-way. The Partnership amortizes 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 and the Partnership recognizes the amortization expense associated with these rights-of-way assets in Depreciation and amortization expense. Equity Method Investments. The Partnership accounts for investments in which it exercises significant influence using the equity method so long as it (i) does not control the investee and (ii) is not the primary beneficiary. The Partnership reflects these investments in its consolidated balance sheets under the caption titled “investment in equity method investees.” The Partnership recognizes 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 is not limited to, absence of an ability to recover the carrying amount of the investment or an 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. The Partnership evaluates its equity method investments whenever a triggering event exists that would indicate a need to assess the investment for potential impairment. Impairment of Long-Lived Assets. The Partnership tests assets for impairment when events or circumstances indicate 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 the Partnership concludes that an asset's carrying value will not be recovered through future cash flows, the Partnership recognizes an impairment loss on the long-lived asset equal to the amount by which the carrying value exceeds its fair value. The Partnership determines fair value using a combination of market-based and income-based approaches. Environmental Matters. The Partnership is 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. The Partnership accrues 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. Commitments and Contingencies. When required, the Partnership records accruals for loss contingencies in accordance with FASB ASC 450, Contingencies . Such determinations are subject to interpretations of current facts and circumstances, forecasts of future events and estimates of the financial impacts of such events. Noncontrolling Interest and Mezzanine Capital . A noncontrolling interest represents the portion of a consolidated subsidiary that is owned by a third party. Amounts are adjusted by the noncontrolling interest holder’s proportionate share of the subsidiary’s earnings or losses each period and any distributions that are paid. A noncontrolling interest is reported as a component of equity unless the noncontrolling interest is considered redeemable, in which case the noncontrolling interest is recorded between liabilities and equity (mezzanine or temporary equity) in the Partnership’s consolidated balance sheet. Revenue Recognition. The Partnership provides gathering and/or processing services principally under contracts that contain one or more of the following arrangements described below: • Fee-based arrangements. • Percent-of-proceeds arrangements. For the contracts described above, the Partnership reflects its revenues in the financial statement captions described below. Financial statement caption: Revenue description: Revenues: Gathering services and related fees • Revenue earned from fee-based gathering, compression, treating and processing services; Natural gas, NGLs and condensate sales • Revenue from the sale of physical natural gas purchased from customers percent-of-proceeds arrangements (Costs are presented within cost of natural gas and NGLs); • Revenue from sale of condensate and NGLs retained from gathering services (Costs are presented within operation and maintenance expense); and Other revenues • Customer reimbursements to the Partnership for costs incurred by the Partnership on customer’s behalf (Recorded on a gross basis). Certain of the Partnership’s gathering and/or processing agreements provide for monthly or annual MVCs. Under these MVCs, customers agree to ship and/or process a minimum volume of production on its 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 the Partnership at the end of the contracted measurement period if its actual throughput volumes are less than its contractual 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. The Partnership recognizes customer obligations under their MVCs as revenue and contract assets when (i) it considers 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, the Partnership considers 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. Unit-Based Compensation. For awards of unit-based compensation, the Partnership determines a grant date fair value and recognizes the related compensation expense in the statements of operations over the vesting period for each respective award. Income Taxes. The Partnership is generally not subject to federal and state income taxes, except as noted below. However, its unitholders are individually responsible for paying federal and state income taxes on their share of its taxable income. Net income or loss for GAAP purposes may differ significantly from taxable income reportable to its unitholders as a result of differences between the tax basis and the GAAP basis of assets and liabilities and the taxable income allocation requirements of the Partnership’s governing documents. 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. The Partnership’s financial statements reflect provisions for these tax obligations. New accounting standards implemented in this Annual Report. 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. The adoption of ASU 2018-13 on January 1, 2020 did not have a material impact on the Partnership’s consolidated financial statements or disclosures. ASU No. 2016-13 Financial Instruments – Credit Losses (“ASU 2016-13”). ASU 2016-13 requires the use of a current expected loss model for financial assets measured at amortized cost and certain off-balance sheet credit exposures. Under this model, entities will be required to estimate the lifetime expected credit losses on such instruments based on historical experience, current conditions, and reasonable and supportable forecasts. This amended guidance also expands the disclosure requirements to enable users of financial statements to understand an entity’s assumptions, models and methods for estimating expected credit losses. The changes are effective for annual and interim periods beginning after December 15, 2019, and amendments should be applied using a modified retrospective approach. The adoption of ASU 2016-13 on January 1, 2020 did not have a material impact on the Partnership’s consolidated financial statements or disclosures. New accounting standards not yet implemented in this Annual Report. ASU No. 2020-06 Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40) (“ASU 2020-06”). ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. The ASU is part of the FASB’s simplification initiative, which aims to reduce unnecessary complexity in U.S. GAAP. The ASU’s amendments are effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years. The Partnership is currently evaluating the provisions of ASU 2020-06 to determine its impact on the Partnership’s consolidated financial statements and disclosures. ASU No. 2020-04 Reference Rate Reform (“ASU 2020-04”). ASU 2020-04 provides optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform on financial reporting. The amendments in ASU 2020-04 are effective as of March 12, 2020 through December 31, 2022. The Partnership is currently evaluating the provisions of ASU 2020-04 to determine its impact on the Partnership’s consolidated financial statements and disclosures.

REVENUE

REVENUE	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
REVENUE	3. REVENUE The majority of the Partnership’s revenue is derived from long-term, fee-based contracts with its customers, which include original terms of up to 25 years. The Partnership recognizes revenue earned from fee-based gathering, compression, treating and processing services in gathering services and related fees. The Partnership also earns revenue in the Williston Basin and Permian Basin reporting segments from the sale of physical natural gas purchased from its customers under certain percent-of-proceeds arrangements. Under ASC Topic 606, these gathering contracts are presented net within cost of natural gas and NGLs. The Partnership sell s natural gas that it retain s from certain customers in the Barnett Shale reporting segment to offset the power expenses of the electric-driven compression on the DFW Midstream system. The Partnership also sell s condensate and NGLs retained from certain of its gathering services in the Piceance Basin and Permian Basin reporting segments. Revenues from the sale of natural gas and condensate are recognized in N atural gas, NGLs and condensate sales; the associated expense is included in O peration and maintenance expense. Certain customers reimburse the Partnership for costs incur red on their behalf. The Partnership record s costs incurred and reimbursed by its customers on a gross basis, with the revenue component recognized in Other revenues . The transaction price in the Partnership’s contracts is primarily based on the volume of natural gas, crude oil or produced water transferred by its 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, the Partnership does 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, the Partnership measures the transaction price at the point of sale when the volume, mix and market price of the commodities are known. The Partnership has contracts with MVCs that are variable and constrained. Contracts with longer than monthly 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 the Partnership’s 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. The Partnership also has 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, the Partnership allocates the contract’s transaction price to each performance obligation using its best estimate of the standalone selling price of each service in the contract. Performance obligations for gathering services are generally satisfied over time. The Partnership utilizes 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 the Partnership’s gathering and/or processing agreements provide for monthly or annual MVCs. Under these MVCs, the Partnership’s customers agree to ship and/or process a minimum volume of production on its 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 the Partnership at the end of the contracted measurement period if its actual throughput volumes are less than its contractual 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. The Partnership recognizes customer obligations under their MVCs as revenue and contract assets when (i) it is considered 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. S ervices are typically billed on a monthly basis and the Partnership does not offer extended payment terms. The Partnership does 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. The Partnership applies the practical expedient in paragraph 606-10-50-14 of Topic 606 for certain arrangements that are considered optional purchases (i.e., there is no enforceable obligation for the customer to make purchases) and those amounts are therefore excluded from the table. 2021 2022 2023 2024 2025 Thereafter Gathering services and related fees $ 100,182 $ 84,215 $ 66,330 $ 49,872 $ 32,996 $ 22,706 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 20. Year ended December 31, 2020 Gathering services and related fees Natural gas, NGLs and condensate sales Other revenues Total (in thousands) Reportable Segments: Utica Shale $ 36,509 $ — $ — $ 36,509 Williston Basin 59,239 20,018 13,438 92,695 DJ Basin 23,868 245 3,957 28,070 Permian Basin 10,091 18,857 585 29,533 Piceance Basin 106,657 2,612 4,621 113,890 Barnett Shale 40,687 7,587 6,185 54,459 Marcellus Shale 25,741 — — 25,741 Total reportable segments 302,792 49,319 28,786 380,897 All other segments — — 2,576 2,576 Total $ 302,792 $ 49,319 $ 31,362 $ 383,473 Year ended December 31, 2019 Gathering services and related fees Natural gas, NGLs and condensate sales Other revenues Total (in thousands) Reportable Segments: Utica Shale $ 31,926 $ — $ 2,065 $ 33,991 Williston Basin 77,626 16,461 11,564 105,651 DJ Basin 21,940 389 3,721 26,050 Permian Basin 3,610 16,383 310 20,303 Piceance Basin 121,357 7,954 4,327 133,638 Barnett Shale 47,862 17,147 6,793 71,802 Marcellus Shale 24,471 — — 24,471 Total reportable segments 328,792 58,334 28,780 415,906 All other segments (2,045 ) 28,660 1,007 27,622 Total $ 326,747 $ 86,994 $ 29,787 $ 443,528 Contract balances . Contract assets relate to the Partnership’s rights to consideration for work completed but not billed at the reporting date and consist of the estimated MVC shortfall payments expected from its 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, 2020 December 31, 2019 (In thousands) Contract assets, January 1, 2020 $ 3,902 $ 8,755 Additions 18,834 18,077 Transfers out (20,710 ) (22,930 ) Contract assets, December 31, 2020 $ 2,026 $ 3,902 As of December 31, 2020, receivables with customers totaled $57.5 million and contract assets totaled $2.0 million which were included in the Accounts receivable caption on the consolidated balance sheet. As of December 31, 2019, receivables with customers totaled $90.4 million and contract assets totaled $3.9 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. The Partnership recognizes contract liabilities under these arrangements in revenue over the contract period. For the years ended December 31, 2020 and 2019, the Partnership recognized $4.0 million and $10.1 million, respectively, of gathering services and related fees which was included in the contract liability balance as of the beginning of the period. See Note 8 for additional details.

PROPERTY, PLANT, AND EQUIPMENT,

PROPERTY, PLANT, AND EQUIPMENT, NET	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
PROPERTY, PLANT, AND EQUIPMENT, NET	4. PROPERTY, PLANT AND EQUIPMENT, NET Details on the Partnership’s property, plant and equipment follow. December 31, 2020 December 31, 2019 (In thousands) Gathering and processing systems and related equipment $ 2,213,501 $ 2,182,950 Construction in progress 60,443 78,716 Land and line fill 10,440 10,137 Other 61,340 54,595 Total 2,345,724 2,326,398 Less accumulated depreciation (528,178 ) (443,909 ) Property, plant and equipment, net $ 1,817,546 $ 1,882,489 When the carrying amount of a long-lived asset is not recoverable, an impairment is recognized equal to the excess of the asset’s carrying value over its fair value, which is based on inputs that are not observable in the market, and thus represent Level 3 inputs under GAAP’s fair value hierarchy. The Partnership recognized $13.1 million and $59.4 million of impairments during the fiscal years ended December 31, 2020 and 2019, respectively. Due to the volatility of the inputs used, the Partnership cannot predict the likelihood of any future impairment. The details on significant impairments recognized during the years ended December 31, 2020 and 2019 are provided below. Significant 2020 Impairments. The Partnership recognized a $5.1 million impairment related to the 2021 sale of compressor equipment in which the carrying value of the equipment exceeded the sale price, and a $3.6 million impairment related to the cancellation of a DJ Basin compressor station construction project due to delays in customer drilling plans. Significant 2019 Impairments. The Partnership recognized a $34.7 million impairment resulting from the expansion of a processing plant in the DJ Basin that caused a significant reduction in the utilization of the original processing plant’s infrastructure, a $14.2 million impairment resulting from the sale of gathering system in the Piceance Basin in which the carrying value of the system exceeded the sale price, and a $9.7 million impairment of compressor equipment in the Barnett Shale. Depreciation expense and capitalized interest for the Partnership follow. Year ended December 31, 2020 2019 (In thousands) Depreciation expense $ 86,263 $ 78,489 Capitalized interest 3,878 $ 6,974

AMORTIZING INTANGIBLE ASSETS

AMORTIZING INTANGIBLE ASSETS	12 Months Ended
	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
AMORTIZING INTANGIBLE ASSETS	5. AMORTIZING INTANGIBLE ASSETS Details regarding the Partnership’s intangible assets, all of which are subject to amortization, follow. December 31, 2020 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (16,064 ) $ 8,131 Contract intangibles 278,448 (195,243 ) 83,205 Rights-of-way 157,271 (49,041 ) 108,230 Total intangible assets $ 459,914 $ (260,348 ) $ 199,566 December 31, 2019 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (15,125 ) $ 9,070 Contract intangibles 278,448 (169,549 ) 108,899 Rights-of-way 157,175 (42,866 ) 114,309 Total intangible assets $ 459,818 $ (227,540 ) $ 232,278 The Partnership recognized amortization expense of its favorable gas gathering contracts in Other revenues as follows: Year ended December 31, 2020 2019 (In thousands) Amortization expense – favorable gas gathering contracts $ (938 ) $ (1,220 ) The Partnership recognized amortization expense of its contract and right of way intangibles in costs and expenses as follows: Year ended December 31, 2020 2019 (In thousands) Amortization expense – contract intangibles $ 25,694 $ 25,587 Amortization expense – rights-of-way 6,175 6,278 The Partnership’s estimated aggregate annual amortization expected to be recognized as of December 31, 2020 for each of the five succeeding fiscal years follows. Intangible assets (In thousands) 2021 $ 28,212 2022 25,145 2023 25,091 2024 14,920 2025 14,898

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
GOODWILL	6. GOODWILL The Partnership evaluates goodwill whenever events or circumstances indicate that it is more likely than not that the fair value of a reporting unit is less than its carrying value, including goodwill. If the reporting unit’s fair value exceeds its carrying value, including goodwill, the Partnership concludes that the goodwill of the reporting unit has not been impaired and no further work is performed. If it is determined that the reporting unit’s carrying value, including goodwill, exceeds its fair value, an impairment loss is recognized in the amount of the excess of the carrying value over the fair value. The Partnership’s impairment determinations, in the context of (i) its annual impairment evaluations and (ii) 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. As of September 30, 2019, the Partnership performed its annual goodwill impairment testing for the Mountaineer Midstream reporting unit, a reporting unit included in the Marcellus Shale reportable segment, using a combination of the income and market approaches. It was determined that the fair value of the Mountaineer Midstream reporting unit did not exceed its carrying value, including goodwill. As a result, a goodwill impairment charge of $16.2 million was recorded for the year ended December 31, 2019 that reduced the goodwill balance on the Partnership’s consolidated balance sheet to zero. The Partnership did not have a transaction in 2020 that resulted in the establishment of a goodwill balance.

EQUITY METHOD INVESTMENTS

EQUITY METHOD INVESTMENTS	12 Months Ended
	Dec. 31, 2020
Equity Method Investments And Joint Ventures [Abstract]
EQUITY METHOD INVESTMENTS	7. EQUITY METHOD INVESTMENTS Double E. In June 2019, the Partnership formed Double E in connection with (the “Double E Project”). Effective June 26, 2019, Summit Permian Transmission, a wholly owned, indirect and consolidated subsidiary of the Partnership, and an affiliate of Double E’s foundation shipper (the “JV Partner”), executed a definitive joint venture agreement (“Double E Agreement”) to fund the capital expenditures associated with the development of the Double E Project. In connection with the Double E Agreement and the related Double E Project, the Partnership contributed total assets of approximately $23.6 million in exchange for a 70% ownership interest in Double E and the JV Partner contributed $7.3 million of cash in exchange for a 30% ownership interest in Double E. Concurrent with these contributions, and in accordance with the Double E Agreement, Double E distributed $7.3 million to the Partnership. Subsequent to the formation of Double E, the Partnership made additional cash investments to Double E of $99.9 million, which includes $2.7 million in capitalized interest, and $18.3 million, which includes $1.1 million in capitalized interest and $0.7 million in capitalized legal costs, in 2020 and 2019, respectively. Double E is deemed to be a variable interest entity as defined in GAAP. As of the date of the Double E Agreement, Summit Permian Transmission was not deemed to be the primary beneficiary of Double E due to the JV Partner’s voting rights on significant matters. The Partnership accounts for its ownership interest in Double E as an equity method investment because it has significant influence over Double E. The Partnership’s portion of Double E’s net assets, which was $132.9 million and $34.7 million at December 31, 2020 and 2019, respectively, is reported under the caption Investment in equity method investees on the consolidated balance sheet. For the years ended December 31, 2020 and 2019, other than the investment activity noted above, Double E did not have any results of operations given that the Double E Project is currently under development. Ohio Gathering. The Partnership has investments in OGC and OCC that it collectively refers to as Ohio Gathering. Ohio Gathering is accounted for as an equity method investment because it has joint control with non-affiliated owners, which gives the Partnership significant influence. 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. The Partnership made its initial investment in Ohio Gathering in 2014 and owned 38.2% and 38.5% of Ohio Gathering at December 31, 2020 and 2019, respectively. In December 2019, the Partnership identified certain triggering events which indicated that its equity method investment in Ohio Gathering could be impaired. In accordance with ASC Topic 323, an equity method impairment analysis was performed and determined the equity method impairment charge to be recorded on its consolidated financial statements resulting from an other-than-temporary impairment. As a result of the analysis, a n impairment charge of $ 329.7 million was recorded in 2019 in L oss from equity method investments on the accompanying consolidated statements of operations . The fair value of the Partnership’s investment in Ohio Gathering was determined based upon applying the discounted cash flow method, which is an income approach, and the guideline public company method, which is a market approach. The discounted cash flow fair value estimate is based on known or knowable information at the measurement date. The significant assumptions that were used to develop the estimate of the fair value under the discounted cash flow method include management’s best estimates of the expected future results using a probability weighted average set of cash flow forecasts and a discount rate of approximately 9.0%. Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As such, the fair value of the Ohio Gathering equity method investment represents a Level 3 measurement. As a result, actual results may differ from the estimates and assumptions made for purposes of this impairment analysis. Also in December 2019, an impairment loss of long-lived assets was recognized by OCC. Although the Partnership recognized activity for Ohio Gathering on a one-month lag, an impairment loss of $6.3 million was recorded in Loss from equity method investees in the consolidated statements of operations because the information was available to the Partnership. A reconciliation of the difference between the carrying amount of the Partnership’s interest in Ohio Gathering and the Partnership’s underlying investment in Ohio Gathering, per Ohio Gathering’s books and records, is shown below. 2020 2019 (In thousands) Investment in Ohio Gathering, December 31, $ 259,888 $ 275,000 December cash distributions 2,748 2,700 Impairment loss (1) — 232,521 Basis difference 216,591 — Investment in Ohio Gathering (Books and records), November 30, $ 479,227 $ 510,221 (1) Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information). November 30, 2020 November 30, 2019 OGC OCC OGC OCC (In thousands) Current assets $ 32,404 $ 4,902 $ 41,972 $ 2,187 Noncurrent assets 1,240,090 290 1,281,171 28,323 Total assets $ 1,272,494 $ 5,192 $ 1,323,143 $ 30,510 Current liabilities $ 8,424 $ 2,982 $ 21,798 $ 4,016 Noncurrent liabilities 8,441 5,146 4,113 6,683 Total liabilities $ 16,865 $ 8,128 $ 25,911 $ 10,699 Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information). Twelve months ended November 30, 2020 Twelve months ended November 30, 2019 OGC OCC OGC OCC (In thousands) Total revenues $ 114,524 $ 12,931 $ 142,138 $ 8,601 Total operating expenses 103,020 38,502 108,234 38,815 Net income (loss) 11,496 (25,571 ) 33,897 (30,214 )

DEFERRED REVENUE

DEFERRED REVENUE	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
DEFERRED REVENUE	8. DEFERRED REVENUE Certain of the Partnership’s gathering and/or processing agreements provide for monthly or annual 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 the Partnership’s gas gathering agreements contain provisions that can reduce or delay the cash flows that it expect s to receive from 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, the Partnership 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 the Partnership’s MVCs apply will be less than the weighted-average of the originally stated MVC contractual terms. • To the extent that certain of the Partnership’s 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. Total (In thousands) Current deferred revenue, January 1, 2020 $ 13,493 Additions 2,731 Less: revenue recognized 6,236 Current deferred revenue, December 31, 2020 $ 9,988 A rollforward of noncurrent deferred revenue follows. Total (In thousands) Noncurrent deferred revenue, January 1, 2020 $ 38,709 Additions 19,384 Less: reclassification to current deferred revenue 9,843 Noncurrent deferred revenue, December 31, 2020 $ 48,250

DEBT

DEBT	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
DEBT	9. DEBT Debt for the Partnership at December 31, 2020 and 2019 follows. December 31, 2020 December 31, 2019 (In thousands) Revolving Credit Facility : Summit Holdings' variable rate senior secured Revolving Credit Facility (2.90% at December 31, 2020 and 4.55% at December 31, 2019) due May 2022 $ 857,000 $ 677,000 ECP Loans : Summit Holdings' 8.00% senior secured term loan obtained in May 2020 and fully repaid in August 2020 — — 2022 Senior Notes : Summit Holdings' 5.5% senior unsecured notes due August 2022 234,047 300,000 Less: unamortized debt issuance costs (1) (859 ) (1,686 ) 2025 Senior Notes : Summit Holdings' 5.75% senior unsecured notes due April 2025 259,463 500,000 Less: unamortized debt issuance costs (1) (2,325 ) (5,015 ) SMPH Term Loan : SMP Holdings' variable rate senior secured term loan (7.80% at December 31, 2019) due May 2022 and extinguished in November 2020 — 161,500 Less: unamortized debt issuance costs (1) — (3,974 ) Total debt 1,347,326 1,627,825 Less: current portion — (5,546 ) Total long-term debt $ 1,347,326 $ 1,622,279 (1) The aggregate amount of Partnership’s debt maturing during each of the years after December 31, 2020 are as follows (in thousands): 2021 $ — 2022 1,091,047 2023 — 2024 — 2025 259,463 Thereafter — Total long-term debt $ 1,350,510 Revolving Credit Facility. The Partnership’s subsidiary, Summit Holdings, has a senior secured revolving credit facility (the “Revolving Credit Facility”) which allows for revolving loans, letters of credit and swingline loans. SMLP and the Guarantor Subsidiaries 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. At December 31, 2020, the Revolving Credit Facility has a $1.1 billion borrowing capacity and matures in May 2022. On December 18, 2020, Summit Holdings entered into the Fourth Amendment to the Third Amended and Restated Credit Agreement which amended the Revolving Credit Facility to: (i) reduce the commitments from $1.25 billion to $1.1 billion; (ii) eliminate the $250.0 million accordion feature; (iii) add a basket for the potential issuance of up to $400.0 million of junior lien indebtedness; (iv) revise restrictions on the Partnership’s ability to use operating cash flow to repurchase junior debt and equity securities; (v) increase the total leverage covenant from 5.50x to 5.75x at all times going forward; (vi) replace the 3.75x senior secured leverage covenant with a new 3.50x first lien leverage covenant; (vii) add a new pricing tier of L + 325 bps if the total leverage ratio is greater than 5.00x; and (viii) restrict the Partnership’s ability to resume distributions on preferred and common units, subject to achieving certain financial and liquidity thresholds. Borrowings under the Revolving Credit Facility bear interest, at the election of Summit Holdings, at a rate based on the alternate base rate (as defined in the credit agreement) plus an applicable margin ranging from 0.75% to 2.25% or the adjusted Eurodollar rate (as defined in the credit agreement) plus an applicable margin ranging from 1.75% to 3.25%, with the commitment fee ranging from 0.30% to 0.50% in each case based on the Partnership’s relative leverage at the time of determination. At December 31, 2020, the applicable margin under LIBOR borrowings was 2.75%, the interest rate was 2.9% and the unused portion of the Revolving Credit Facility totaled $ 238.9 million, subject to a commitment fee of 0.50 %, after giving effect to the issuance thereunder of a $ million outstanding but undrawn irrevocable standby letter of credit. Based on covenant limits, the available borrowing capacity under the Revolving Credit Facility as of December 31, 2020 was approximately $ million. At December 31, 2020, the Revolving Credit Facility is secured by the membership interests of Summit Holdings and the membership interests of the Guarantor Subsidiaries of Summit Holdings and by substantially all of the assets of Summit Holdings and its Guarantor 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 $50.0 million over any 12 -month period; and (vi) of Summit Holdings to make distributions, with certain exceptions, including the distribution of Available Cash (as defined in the SMLP Partnership Agreement) if no default or event of default then exists or would result therefrom and Summit Holdings is in pro forma compliance with its financial covenants. In addition, the Revolving Credit Facility requires Summit Holdings to maintain (i) a ratio of consolidated trailing 12 -month earnings before interest, income taxes, depreciation and amortization ("EBITDA") to net interest expense of not less than 2.5 to 1.0 as defined in the credit agreement, (ii) a ratio of total net indebtedness to consolidated trailing 12 -month EBITDA of not more than 5.75 to 1.00 and, (iii) a ratio of first lien net indebtedness to consolidated trailing 12 -month EBITDA of not more than 3.75 to 1.00. As of December 31, 2020, the Partnership was in compliance with the Revolving Credit Facility's financial covenants and there were no defaults or events of default during the year ended December 31, 2020. As of December 31, 2020, the Partnership had $7.4 million of debt issuance costs attributable to its Revolving Credit Facility and related amendments, which are included in Other noncurrent assets on the consolidated balance sheet. ECP Loans . On May 28, 2020, in connection with the closing of the GP Buy-In Transaction, Summit Holdings, entered into (i) a Term Loan Credit Agreement (the “ECP NewCo Term Loan Credit Agreement”), with SMP TopCo, LLC, a Delaware limited liability company and affiliate of ECP (“ECP NewCo”), as lender and administrative agent, and Mizuho Bank (USA) (“Mizuho”), as collateral agent, in a principal amount of $28.2 million (the “ECP NewCo Loan”), and (ii) a Term Loan Credit Agreement (the “ECP Holdings Term Loan Credit Agreement” and together with the ECP NewCo Term Loan Credit Agreement, the “ECP Term Loan Credit Agreements”), with ECP Holdings, as lender, and ECP NewCo, as administrative agent and Mizuho, as collateral agent, in a principal amount of $6.8 million (the “ECP Holdings Loan” and together with the ECP NewCo Loan, the “ECP Loans”). The ECP Loans were set to mature on March 31, 2021 and bore interest at a fixed rate of 8.00% per annum, with the interest payment due at maturity of the ECP Loans. With borrowings under the Partnership’s Revolving Credit Facility, the Partnership fully repaid all amounts outstanding under the ECP Loans ($35 million of principal and $0.6 million of accrued interest) on August 7, 2020. 2022 Senior Notes. In July 2014, the Co-Issuers co-issued the 2022 Senior Notes. The Partnership pays interest on the 2022 Senior Notes semi-annually in cash in arrears on February 15 and August 15 of each year. The 2022 Senior Notes are senior, unsecured obligations and rank equally in right of payment with all of our existing and future senior obligations. The 2022 Senior Notes are effectively subordinated in right of payment to all secured indebtedness, to the extent of the collateral securing such indebtedness. The Co-Issuers may redeem all or part of the 2022 Senior Notes at a redemption price of 100.000%, plus accrued and unpaid interest, if any. Debt issuance costs of $5.1 million are being amortized over the life of the 2022 Senior Notes. As of and during the year ended December 31, 2020, we were in compliance with the financial covenants governing its 2022 Senior Notes. 2025 Senior Notes. In February 2017, the Co-Issuers co-issued the 2025 Senior Notes. The Partnership pays interest on the 2025 Senior Notes semi-annually in cash in arrears on April 15 and October 15 of each year. The 2025 Senior Notes are senior, unsecured obligations and rank equally in right of payment with all of the Partnership’s existing and future senior obligations. The 2025 Senior Notes are effectively subordinated in right of payment to all of the Partnership’s secured indebtedness, to the extent of the collateral securing such indebtedness. The Co-Issuers have the right to redeem all or part of the 2025 Senior Notes at a redemption price of 104.313%. On and after April 15, 2021, the Co-Issuers may redeem all or part of the 2025 Senior Notes at a redemption price of 102.875% (with the redemption price declining ratably each year to 100.000% on April 15, 2023), plus accrued and unpaid interest, if any, to, but not including, the redemption date. Debt issuance costs of $7.7 million are being amortized over the life of the 2025 Senior Notes. As of and during the year ended December 31, 20 20 , that Partnership was in compliance with the financial covenants governing its 2025 Senior Notes. SMPH Term Loan and TL Restructuring. Until November 17, 2020, a subsidiary of the Partnership, SMP Holdings, had a senior secured term loan facility with $155.2 million of principal outstanding and a maturity date of May 15, 2022. Borrowings under the SMPH Term Loan bore interest at LIBOR plus 6.00% or ABR plus 5.00%, as defined in the SMPH Term Loan, and were secured by the following collateral: (i) a perfected first-priority lien on, and pledge of (a) all of the capital stock issued by SMP Holdings, (b) 2.3 million SMLP units owned by SMP Holdings, (c) all of the equity interests owned by SMP Holdings in the General Partner, and (ii) substantially all other personal property of SMP Holdings. On September 29, 2020, SMP Holdings, Summit Investments and, for limited purposes, the Partnership, entered into the TL Restructuring. At the closing of the TL Restructuring on November 17, 2020, the Term Loan Agent executed a Strict Foreclosure on behalf of the Term Loan Lenders on the 2,306,972 common units held by SMP Holdings and pledged as collateral under the SMPH Term Loan, which was then distributed to all SMPH Term Loan Lenders on a pro rata basis. In addition to the Strict Foreclosure, SMP Holdings paid to each of the SMPH Term Loan Lenders its pro rata share of $26.5 million in cash that the Partnership paid to SMP Holdings to fully satisfy its obligation with respect to the deferred purchased price obligation.

LIABILITY MANAGEMENT TRANSACTIO

LIABILITY MANAGEMENT TRANSACTIONS	12 Months Ended
	Dec. 31, 2020
Liability Management Transactions [Abstract]
Liability Management Transactions	10. LIABILITY MANAGEMENT TRANSACTIONS During the year ended December 31, 2020, the Partnership and its subsidiaries completed several liability management transactions, described below, that resulted in the early extinguishment of an aggregate $306.5 million face value of the Partnership’s indebtedness. Open Market Repurchases. During the year ended December 31, 2020, the Partnership made a number of open market repurchases of the 2022 Senior Notes and 2025 Senior Notes that resulted in the extinguishment of $32.4 million of face value of the 2022 Senior Notes and $201.8 million of face value of the 2025 Senior Notes (the “Open Market Repurchases”). Total cash consideration paid to repurchase the principal amounts outstanding of the 2022 Senior Notes and 2025 Senior Notes, plus accrued interest totaled $150.3 million and the Partnership recognized a $86.4 million gain on the early extinguishment of debt during the year ended December 31, 2020. Debt Tender Offers. In September 2020, the Co-Issuers completed the Debt Tender Offers to purchase a portion of the 2022 Senior Notes and 2025 Senior Notes. Upon concluding the Debt Tender Offers, the Co-Issuers repurchased $33.5 million principal amount of the 2022 Senior Notes and $38.7 million principal amount of the 2025 Senior Notes. Total cash consideration paid to repurchase the principal amounts outstanding of the 2022 and 2025 Senior Notes, plus accrued interest, totaled $48.7 million, and the Partnership recognized a $23.3 million gain on the early extinguishment of debt during the year ended December 31, 2020. SMPH Term Loan Restructuring . On November 17, 2020, the Partnership completed the TL Restructuring and recognized a gain of $94.0 million equal to the difference between the face value of the cancelled SMPH Term Loan and the fair value of the total consideration transferred, including unamortized debt issuance costs, and certain direct transaction costs related to the restructuring. The transaction was accounted for under ASC Topic 470-60 “Troubled Debt Restructuring with Debtors” and had a total impact of $26.15 per share. Summary of gain on extinguishment of debt. The Partnership recognized a $203.1 million gain on extinguishment of debt during the year ended December 31, 2020, the components of which are summarized in the table below. ECP Loan Repayment Open Market Repurchases Tender Offers TL Restructuring Total 2022 2025 2022 2025 (in thousands) Senior Notes Senior Notes Senior Notes Senior Notes Gain on Repurchases of Senior Notes and TL Restructuring $ — $ 11,554 $ 76,789 $ 9,223 $ 15,479 $ 99,175 $ 212,220 Debt issue costs (361 ) (143 ) (1,541 ) (125 ) (351 ) (2,724 ) (5,245 ) Transaction costs (249 ) (105 ) (105 ) (467 ) (467 ) (2,520 ) (3,913 ) Gain (loss) on debt extinguishment $ (610 ) $ 11,306 $ 75,143 $ 8,631 $ 14,661 $ 93,931 $ 203,062

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	11. COMMITMENTS AND CONTINGENCIES Environmental Matters. Although the Partnership believes that it is in material compliance with applicable environmental regulations, the risk of environmental remediation costs and liabilities are inherent in pipeline ownership and operation. Furthermore, the Partnership can provide no assurances that significant environmental remediation costs and liabilities will not be incurred in the future. The Partnership is currently not aware of any material contingent liabilities that exist with respect to environmental matters, except as noted below. In 2015, the Partnership learned of the rupture of a four-inch produced water gathering pipeline on the Meadowlark Midstream system near Williston, North Dakota (“Meadowlark Rupture”). The Meadowlark Rupture, 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. The pollution liability policy was exhausted in 2015. Prior to the GP Buy-In Transaction, Summit Investments and SMP Holdings indemnified the Partnership for certain obligations and liabilities related to the incident. As a result of the GP Buy-In Transaction, the Partnership is no longer indemnified for these obligations. A rollforward of the Partnership’s undiscounted accrued environmental remediation liabilities related to the Meadowlark Rupture follows. Total (In thousands) Accrued environmental remediation, January 1, 2019 $ 5,636 Payments made (2,284 ) Additional accruals 1,299 Accrued environmental remediation, December 31, 2019 $ 4,651 Payments made (1,722 ) Accrued environmental remediation, December 31, 2020 $ 2,929 As of December 31, 2020, the Partnership has recognized (i) a current liability for remediation effort expenditures expected to be incurred within the next 12 months and (ii) a noncurrent liability for estimated remediation expenditures expected to be incurred subsequent to December 31, 2021. Each of these amounts represent the Partnership’s best estimate for costs expected to be incurred. Neither of these amounts have been discounted to its present value. Beginning in 2015, the U.S. Department of Justice (“DOJ”) issued grand jury subpoenas to Summit Investments, the Partnership, our General Partner and Meadowlark Midstream requesting certain materials related to the Meadowlark Rupture. On June 19, 2015, Meadowlark Midstream and Summit Investments received a complaint from the North Dakota Industrial Commission seeking approximately $2.5 million in fines and other fees related to the incident. This matter is also under investigation by the U.S. Environmental Protection Agency, the North Dakota Office of the Attorney General, the North Dakota Department of Environmental Quality, and the North Dakota Game and Fish Department. The government’s investigation is still ongoing. During this time, the Partnership has entered ‎into tolling agreements with both the DOJ and the North Dakota attorney general, which ‎were most recently extended to May 7, 2021. There can be no assurance that these tolling agreements will be extended. Discussions with the DOJ and other agencies regarding a resolution of this matter are ongoing. Liability for this incident could arise under civil and misdemeanor and felony criminal statutes, including the Clean Water Act. In accordance with GAAP, the Partnership has accrued a $17.0 million loss contingency for this matter as of December 31, 2020. While the Partnership believes a loss for claims and/or actions arising from the Meadowlark Rupture, whether in a negotiated settlement or as a result of litigation, is probable, due to the complexity of resolving the numerous issues surrounding this matter, at this time we cannot reasonably predict whether any actual loss, if incurred, would be materially higher or lower than the accrued amount. 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.

FINANCIAL INSTRUMENTS

FINANCIAL INSTRUMENTS	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
FINANCIAL INSTRUMENTS	12. FINANCIAL INSTRUMENTS Concentrations of Credit Risk. Financial instruments that potentially subject the Partnership to concentrations of credit risk consist of cash and cash equivalents, restricted cash and accounts receivable. The Partnership maintains its cash and cash equivalents and restricted cash in bank deposit accounts that frequently exceed federally insured limits. The Partnership has not experienced any losses in such accounts and does not believe it is exposed to any significant risk. Accounts receivable primarily comprise amounts due for the gathering, compression, treating and processing services the Partnership provides to its customers and also the sale of natural gas liquids resulting from its processing services. This industry concentration has the potential to impact its overall exposure to credit risk, either positively or negatively, in that the Partnerships customers may be similarly affected by changes in economic, industry or other conditions. The Partnership monitors the creditworthiness of its counterparties and can require letters of credit or other forms of credit assurance for receivables from counterparties that are judged to have substandard credit, unless the credit risk can otherwise be mitigated. The Partnerships top five customers or counterparties accounted for 51% of its total accounts receivable at December 31, 2020, compared to 46% as of December 31, 2019. Fair Value. The carrying amount of cash and cash equivalents, restricted cash, accounts receivable and trade accounts payable reported on the consolidated balance sheet approximates fair value due to their short-term maturities. A summary of the estimated fair value of the Partnerships debt financial instruments follows. December 31, 2020 December 31, 2019 Carrying Value, Net Estimated fair value (Level 2) Carrying Value, Net Estimated fair value (Level 2) (in thousands) 2022 Senior Notes $ 233,188 $ 215,713 $ 298,314 $ 266,750 2025 Senior Notes 257,138 168,002 494,985 382,708 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, 2020 and 2019. 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 AND MEZZANINE

PARTNERS' CAPITAL AND MEZZANINE CAPITAL	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
PARTNERS' CAPITAL AND MEZZANINE CAPITAL	13. PARTNERS' CAPITAL AND MEZZANINE CAPITAL Noncontrolling Interest. In March 2019, and prior to the GP Buy-In Transaction, a subsidiary of Summit Investments cancelled its incentive distribution rights agreement with SMLP and converted its 2% economic general partner interest to a non-economic general partner interest in exchange for 0.6 million SMLP common units. This exchange is reflected in the Consolidated Statements of Partners’ Capital as a reduction to noncontrolling interest and an increase to Partners’ Capital. Common Units. A rollforward of the number of common limited partner units follows for the period from December 31, 2018 to December 31, 2020. Common Units (2) Units, December 31, 2018 3,021,258 2019 activity — Units, December 31, 2019 3,021,258 GP Buy-In Transaction (1) (132,687 ) Common units issued for SMLP LTIP, net 95,987 TL Restructuring (Note 9) 2,306,972 Series A Preferred Unit Exchange Offer, net of shares withheld for taxes 817,845 Other 717 Units, December 31, 2020 6,110,092 (1) (2) As adjusted for reverse unit split. Series A Preferred Units. In November 2017, the Partnership 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. The Partnership used the net proceeds of $293.2 million (after deducting underwriting discounts and offering expenses) to repay outstanding borrowings under the Revolving Credit Facility. The Series A Preferred Units rank senior to (i) common units 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%. In connection with the GP Buy-In Transaction, A rollforward of the number of Series A Preferred Units follows for the period from December 31, 2018 to December 31, 2020. Series A Preferred Units Series A Preferred Units, December 31, 2018 300,000 2019 activity — Series A Preferred Units, December 31, 2019 300,000 Series A Preferred Unit Exchange Offer (62,816 ) Series A Preferred Unit Tender (75,075 ) Series A Preferred Units, December 31, 2020 162,109 Series A Preferred Unit Exchange Offer . In July 2020, the Partnership completed an offer to exchange its Series A Preferred Units for newly issued common units, whereby it issued 837,547 SMLP common units in exchange for 62,816 Series A Preferred Units. Series A Preferred Tender Offer. In December 2020, the Partnership completed a cash tender offer for its Series A Preferred Units whereby it accepted 75,075 Series A Preferred Units for a purchase price of $333.00 per Series A Preferred Unit and an aggregate purchase price of $25.0 million. Subsidiary Series A Preferred Units. The Partnership has Subsidiary Series A Preferred Units that ranks senior to each other class or series of limited partner interests or other equity securities in Permian Holdco that may be established in the future that expressly ranks junior to the Subsidiary Series A Preferred Units as to the payment of distributions and amounts payable upon a liquidation event (the “Junior Securities”). The Subsidiary Series A Preferred Units rank equal in all respects with each class or series of limited partner interests or other equity securities in Permian Holdco that may be established in the future that is not expressly made senior or subordinated to the Subsidiary Series A Preferred Units as to the payment of distributions and amounts payable on a liquidation event (the “Parity Securities”). The Subsidiary Series A Preferred Units rank junior to (i) all of Permian Holdco’s or a subsidiary of Permian Holdco’s future indebtedness and other liabilities with respect to assets available to satisfy claims against Permian Holdco and (ii) each other class or series of limited partner interests or other equity securities in Permian Holdco established in the future that is expressly made senior to the Subsidiary Series A Preferred Units as to the payment of distributions and amounts payable upon a liquidation event. Income is allocated to the Subsidiary Series A Preferred Units in an amount equal to the earned distributions for the respective reporting period. Distributions on the Subsidiary Series A Preferred Units are cumulative and compounding and are payable 21 days following the quarterly period ended March, June, September and December of each year (each, a “Series 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 Series A Distribution Payment Date, in each case, when, as, and if declared by Permian Holdco out of legally available funds for such purpose. The distribution rate for the Subsidiary Series A Preferred Units is 7.00% per annum of the $1,000 liquidation preference per Subsidiary Series A Preferred Unit. A pro-rated initial distribution on the Subsidiary Series A Preferred Units was Paid-in-kind (“PIK”) on January 21, 2020 in an amount equal to 7.00% per Subsidiary Series A Preferred Unit plus 1.00% per annum of the undrawn commitment units. In 2020, the Partnership issued 55,251 Subsidiary Series A Preferred Units at a price of $1,000 per unit for net proceeds of $48.7 million (after deducting underwriting discounts and offering expenses). All proceeds were used to fund capital expenses associated with the Double E Project. The proceeds associated with the issuance of Subsidiary Series A Preferred Units are classified as restricted cash on the accompanying consolidated balance sheets in accordance with the underlying agreement These Subsidiary Series A Preferred Units are considered redeemable securities under GAAP due to the existence of certain redemption provisions that are outside of the Partnership’s control. Therefore, the securities are classified as temporary equity in the mezzanine section of the consolidated balance sheets. The Partnership records its Subsidiary Series A Preferred Units at fair value upon issuance, net of issuance costs, and subsequently records an effective interest method accretion amount each reporting period to accrete the carrying value to a most probable redemption value that is based on a predetermined internal rate of return measure. The Partnership also elected to make payment-in-kind (“PIK”) distributions to holders of the Subsidiary Series A Preferred Units during the year ended December 31, 2020 and these PIK distributions increase the liquidation preference on each Subsidiary Series A Preferred Unit. Ultimately, Net Income (Loss) Attributable to common limited partners includes adjustments for PIK distributions and redemption accretion. If the Subsidiary Series A Preferred Units were redeemed on December 31, 2020, the redemption amount would be $ million, when considering the applicable multiple of invested capital metric and make-whole amount provisions contained in the Subsidiary Series A Preferred Unit agreement. December 31, 2020 (in thousands) Balance at December 31, 2019 $ 27,450 New issuances 50,000 PIK distributions 5,251 Issuance costs (1,290 ) Redemption accretion 8,247 Balance at December 31, 2020 (1) $ 89,658 (1) Warrants. On May 28, 2020 and in connection with the GP Buy-In Transaction, the Partnership issued (i) a warrant to purchase up to 0.5 million SMLP common units to ECP NewCo (the “ECP NewCo Warrant”) and (ii) a warrant to purchase up to 0.1 million SMLP common units to ECP Holdings (the “ECP Holdings Warrant” and together with the ECP NewCo Warrant, the “ECP Warrants”). The exercise price under the ECP Warrants is $15.35 per SMLP common unit and the Partnership may issue a maximum of 0.7 million SMLP common units under the ECP Warrants. Upon exercise of the ECP Warrants, each of ECP NewCo and ECP Holdings may receive, at its election: (i) a number of SMLP common units equal to the number of SMLP common units for which the ECP Warrants are being exercised, if exercising the ECP Warrants by cash payment of the exercise price; (ii) a number of SMLP common units equal to the product of the number of common units being exercised multiplied by (a) the difference between the average of the daily volume-weighted average price (“VWAP”) of the SMLP common units on the New York Stock Exchange (the “NYSE”) on each of the three trading days prior to the delivery of the notice of exercise (the “VWAP Average”) and the exercise price (the “VWAP Difference”), divided by (b) the VWAP Average; and/or (iii) an amount in cash, to the extent that the Partnership’s leverage ratio would be at least 0.5x less than the maximum applicable ratio set forth in the Revolving Credit Facility, equal to the product of (a) the number of SMLP common units exercised and (b) the VWAP Difference, subject to certain adjustments under the ECP Warrants. The ECP Warrants are subject to standard anti-dilution adjustments for stock dividends, stock splits (including reverse splits) and recapitalizations and are exercisable at any time on or before May 28, 2023. Upon exercise of the ECP Warrants, the proceeds to the holders of the ECP Warrants, whether in the form of cash or common units, will be capped at $30.00 per SMLP common unit above the exercise price. At issuance the ECP Warrants were valued at $2.3 million using a Black-Scholes model and accounted for as a liability instrument. At December 31, 2020, the ECP Warrants were valued at $1.9 million. Cash Distribution Policy. In connection with the GP Buy-In Transaction, the Partnership suspended its cash distributions to holders of its common units, commencing with respect to the quarter ending March 31, 2020. Upon the resumption of distributions, the Partnership Agreement requires that it distribute all available cash, subject to reserves established by its General Partner, within 45 days after the end of each quarter to unitholders of record on the applicable record date. The amount of distributions paid under this policy is subject to fluctuations based on the amount of cash the Partnership generates from its business and the decision to make any distribution is determined by the General Partner, taking into consideration the terms of the Partnership Agreement. Cash Distributions Paid and Declared. Prior to the GP Buy-In Transaction, SMLP paid the following per-unit cash distributions to its unitholders during the years ended December 31: Year ended December 31, 2020 2019 Per-unit distributions to unitholders $ 0.125 $ 1.4375 On January 29, 2020, the Board of Directors declared a distribution of $0.125 per unit for the quarterly period ended December 31, 2019. This distribution, which totaled $11.7 million, was paid on February 14, 2020 to unitholders of record at the close of business on February 7, 2020.

EARNINGS PER UNIT

EARNINGS PER UNIT	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
EARNINGS PER UNIT	14. EARNINGS PER UNIT The following table details the components of EPU. Year ended December 31, 2020 2019 (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 $ 192,352 $ (184,451 ) Less: Net income attributable to Series A Preferred Units 26,529 28,500 Net income attributable to Subsidiary Series A Preferred Units 13,498 58 Add: Deemed capital contribution from Series A Preferred Unit Exchange Offer 54,945 — Deemed capital contribution from Series A Preferred Tender Offer 55,724 Net income (loss) attributable to common limited partners $ 262,994 $ (213,009 ) Denominator for basic and diluted EPU: Weighted-average common units outstanding – basic (1) 3,592 3,021 Effect of nonvested phantom units 102 — Weighted-average common units outstanding – diluted 3,694 3,021 Net Income (Loss) per limited partner unit: Common unit – basic $ 73.22 $ (70.50 ) Common unit – diluted $ 71.19 $ (70.50 ) Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU 240 12 (1) As a result of the GP Buy-In Transaction, our historical results are those of Summit Investments. The number of common units of 3.0 million as of December 31, 2019 represents those of Summit Investments and has been used for the earnings per unit calculation presented herein

SUPPLEMENTAL CASH FLOW INFORMAT

SUPPLEMENTAL CASH FLOW INFORMATION	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
SUPPLEMENTAL CASH FLOW INFORMATION	15. SUPPLEMENTAL CASH FLOW INFORMATION Year ended December 31, 2020 2019 (In thousands) Supplemental cash flow information: Cash interest paid $ 79,450 $ 92,536 Less capitalized interest 3,878 6,974 Interest paid (net of capitalized interest) $ 75,572 $ 85,562 Cash paid for taxes $ 190 $ 150 Noncash investing and financing activities: Capital expenditures in trade accounts payable (period-end accruals) $ 6,154 $ 19,846 Warrant issuance for GP Buy-In Transaction 2,300 — Asset contribution to an equity method investment — 23,643 Right-of-use assets relating to ASC Topic 842 3,685 5,448 Fair value of SMLP equity for TL Restructuring 30,521 — Accretion of Subsidiary Series A Preferred Units 8,247 —

UNIT-BASED AND NONCASH COMPENSA

UNIT-BASED AND NONCASH COMPENSATION	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
UNIT-BASED AND NONCASH COMPENSATION	16. UNIT-BASED AND NONCASH COMPENSATION SMLP Long-Term Incentive Plan. The Partnership’s Long-Term Incentive Plan (“SMLP LTIP”) provides for equity awards to eligible officers, employees, consultants and directors of the Partnership, thereby linking the recipients' compensation directly to SMLP’s performance. 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. A total of 1.0 million common units was reserved for issuance pursuant to and in accordance with the SMLP LTIP. As of December 31, 2020, approximately 0.4 million common units remained available for future issuance. The following table presents phantom unit activity for the periods presented: Units Weighted-average grant date fair value Nonvested phantom units, December 31, 2018 57,608 $ 256.65 Phantom units granted 127,540 97.20 Phantom units vested (40,174 ) 251.70 Phantom units forfeited (4,574 ) 193.05 Nonvested phantom units, December 31, 2019 140,400 115.35 Phantom units granted 345,997 9.20 Phantom units vested (193,349 ) 54.07 Phantom units forfeited (1,357 ) 119.24 Nonvested phantom units, December 31, 2020 291,691 $ 26.57 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 generally vest ratably over a three-year The intrinsic value of phantom units that vested during the years ended December 31, follows. Year ended December 31, 2020 2019 (In thousands) Intrinsic value of vested LTIP awards $ 2,602 $ 5,940 As of December 31, 2020, the unrecognized unit-based compensation related to the SMLP LTIP was $3.4 million. Incremental unit-based compensation will be recorded over the remaining weighted-average vesting period of approximately 1.4 years. Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP follows. Year ended December 31, 2020 2019 (In thousands) SMLP LTIP unit-based compensation $ 8,111 $ 8,171

LEASES

LEASES	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
LEASES	17. LEASES Leases. The Partnership leases certain office space and equipment under operating leases. The Partnership leases office space for terms of between 3 and 10 years. Office space leases limit exposure to risks related to ownership, such as fluctuations in real estate prices. The Partnership leases equipment primarily to support its operations in response to the needs of its gathering systems for terms of between 3 and 4 years. The Partnership also leases vehicles under finance leases to support its operations in response to the needs of its gathering systems for a term of 3 years. Some of the Partnerships leases are subject to annual escalations relating to the Consumer Price Index (“CPI”). While lease liabilities are not remeasured as a result of changes to the CPI, changes to the CPI are treated as variable lease payments and recognized in the period in which the obligation for those payments was incurred. In accordance with the provisions in the Revolving Credit Facility, the Partnership’s aggregate finance lease obligations cannot exceed $50.0 million in any period of twelve consecutive calendar months during the life of such leases. Significant assumptions or judgments include the determination of whether a contract contains a lease and the discount rate used in the lease liabilities. The rate implicit in the lease contracts are not readily determinable. In determining the discount rate used in for lease liabilities, the Partnership analyzed certain factors in its incremental borrowing rate, including collateral assumptions and the term used. The incremental borrowing rate on the Revolving Credit Facility was 2.90% at December 31, 2020, which reflects the fixed rate at which the Partnership could borrow a similar amount, for a similar term and with similar collateral as in the lease contracts at the commencement date. In connection with the adoption of Topic 842, the Partnership elected the package of practical expedients which permits them not to reassess under the new standard its prior conclusions about lease identification, lease classification and initial direct costs. The Company also elected the practical expedient to not separate the nonlease components from the associated lease components for all classes of underlying assets, as well as the short-term lease recognition exemption. ROU assets (included in the Property, plant and equipment, net caption on the Partnership’s consolidated balance sheet) and lease liabilities (included in the Other current liabilities and Other noncurrent liabilities captions on the Partnership’s consolidated balance sheet) follow: December 31, 2020 December 31, 2019 (In thousands) ROU assets Operating $ 3,736 $ 3,580 Finance 1,748 3,159 $ 5,484 $ 6,739 Lease liabilities, current Operating $ 2,298 $ 1,221 Finance 618 1,246 $ 2,916 $ 2,467 Lease liabilities, noncurrent Operating $ 3,182 $ 2,513 Finance 154 676 $ 3,336 $ 3,189 Lease cost and Other information follow: Year ended December 31, 2020 2019 (In thousands) Lease cost Finance lease cost: Amortization of ROU assets (included in depreciation and amortization) $ 1,274 $ 1,559 Interest on lease liabilities (included in interest expense) 52 102 Operating lease cost (included in general and administrative expense) 2,644 3,345 $ 3,970 $ 5,006 Year ended December 31, 2020 2019 (In thousands) Other information Cash paid for amounts included in the measurement of lease liabilities Operating cash outflows from operating leases $ 1,472 $ 3,396 Operating cash outflows from finance leases 52 102 Financing cash outflows from finance leases 1,150 1,873 ROU assets obtained in exchange for new operating lease liabilities 3,552 1,218 ROU assets obtained in exchange for new finance lease liabilities 133 1,350 Weighted-average remaining lease term (years) - operating leases 4.9 5.8 Weighted-average remaining lease term (years) - finance leases 1.4 2.0 Weighted-average discount rate - operating leases 5 % 5 % Weighted-average discount rate - finance leases 4 % 4 % The Partnership recognizes total lease expense incurred or allocated to us in general and administrative expenses. Lease expense related to operating leases, including lease expense incurred on the Partnership’s behalf and allocated to us, was as follows: Year ended December 31, 2020 2019 (In thousands) Lease expense $ 3,436 $ 4,038 Future minimum lease payments due under noncancelable leases at December 31, 2020, were as follows: December 31, 2020 (In thousands) Operating Finance 2021 $ 1,750 $ 655 2022 1,308 126 2023 854 42 2024 573 — 2025 464 — 2026 156 — Thereafter 579 — Total future minimum lease payments $ 5,684 $ 823

DISPOSITIONS

DISPOSITIONS	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
DISPOSITIONS	18. DISPOSITIONS In 2019, the Partnership completed two divestitures described in further detail below. Red Rock Gathering Asset Disposition. In December 2019, the Partnership completed the sale of certain assets contained in its Piceance Basin reportable segment for a cash purchase price of $12.0 million. Prior to the sale, the Partnership recorded an impairment charge of $14.2 million based on the expected consideration and the carrying value for the disposed assets. The financial contribution of these assets in 2019 (a component of the Piceance Basin reportable segment) are included in the Partnership’s consolidated financial statements and footnotes for the period from January 1, 2019 through December 1, 2019. Tioga Midstream Disposition. In March 2019, the Partnership closed on the sale of certain assets contained in the Williston Basin reportable segment for a cash purchase price of $90.0 million. Upon closing the sale in March 2019, the Partnership recorded a gain on sale of $0.9 million based on the difference between the consideration received and the carrying value for disposed assets. The gain is included in the Gain on asset sales, net caption on the consolidated statement of operations. The financial contribution of these assets in 2019 (a component of the Williston Basin reportable segment) are included in the Partnership’s consolidated financial statements and footnotes for the period from January 1, 2019 through March 22, 2019.

RESTRUCTURING

RESTRUCTURING	12 Months Ended
	Dec. 31, 2020
Restructuring And Related Activities [Abstract]
RESTRUCTURING	19. RESTRUCTURING 2020 Restructuring Activities. In 2020, management approved and initiated a plan to restructure its operations (“2020 Restructuring Plan”), resulting in certain management, facility and organizational changes. Under the 2020 Restructuring Plan, the Partnership expensed approximately $5.6 million in 2020 of costs associated with its restructuring activities. These activities consisted primarily of employee-related severance costs, and are included within the General and administrative caption on the consolidated statement of operations. 2019 Restructuring Activities. In 2019, management approved and initiated a plan to restructure its operations (“2019 Restructuring Plan”), resulting in certain management, facility and organizational changes. Under the 2019 Restructuring Plan, the Partnership expensed approximately $3.5 million and $4.9 million in 2020 and 2019, respectively, of costs associated with its restructuring activities. These activities consisted primarily of employee-related costs and consulting costs in support of the project. These costs are included within the General and administrative caption on the consolidated statement of operations Details for the 2020 Restructuring Plan and the 2019 Restructuring Plan follow. Severance Charges Other Restructuring Charges Total Severance and Other 2020 2019 2020 2019 2020 2019 (In thousands) 2020 Restructuring Plan $ 5,591 $ — $ 56 $ — $ 5,647 $ — 2019 Restructuring Plan 2,159 3,767 1,293 1,187 3,452 4,954 $ 7,750 $ 3,767 $ 1,349 $ 1,187 $ 9,099 $ 4,954 Accrued December 31, 2018 Charges Incurred Cash Payments Accrued December 31, 2019 Charges Incurred Cash Payments Accrued December 31, 2020 (In thousands) Employee-related costs $ — $ 3,767 $ (951 ) $ 2,816 $ 7,750 $ (7,018 ) $ 3,548 Other — 1,187 (1,187 ) — 1,349 (1,349 ) — Total restructuring costs $ — $ 4,954 $ (2,138 ) $ 2,816 $ 9,099 $ (8,367 ) $ 3,548

SEGMENT INFORMATION

SEGMENT INFORMATION	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
SEGMENT INFORMATION	20. SEGMENT INFORMATION As of December 31, 2020, the Partnership’s reportable segments are: • the Utica Shale, which is served by Summit Utica; • Ohio Gathering, which includes the Partnership’s ownership interest in OGC and OCC; • the Williston Basin, which is served by Polar and Divide, Meadowlark 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. Until March 22, 2019, the Partnership owned Tioga Midstream, a crude oil, produced water and associated natural gas gathering system operating in the Williston Basin. Until December 1, 2019, the Partnership owned certain assets in the Red Rock Gathering system operating in the Piceance Basin. Refer to Note 16 to the consolidated financial statements for details on the sale of Tioga Midstream and on the sale of certain assets in the Red Rock Gathering system. Each of the Partnership’s reportable segments provides midstream services in a specific geographic area. Reportable segments reflect the way in which the Partnership internally report’s the financial information used to make decisions and allocate resources in connection with its operations. The Ohio Gathering reportable segment includes the Partnership’s investment in Ohio Gathering. Income or loss from equity method investees, as reflected on the statements of operations, relates to Ohio Gathering and is recognized and disclosed on a one-month lag see Note 7. For the year ended December 31, 2020, other than the investment activity described in Note 7 , Double E did not have any results of operations given that the Double E Project is currently under development. The Double E Project is expected to be operational in the fourth quarter of 2021. Corporate and Other represents those results that: (i) are not specifically attributable to a reportable segment; (ii) are not individually reportable (such as Double E); or (iii) have not been allocated to a reportable segments for the purpose of evaluating their performance, including certain general and administrative expense items, certain natural gas and crude oil marketing services and transaction costs. Assets by reportable segment follow. December 31, 2020 2019 (in thousands) Assets (1) Utica Shale $ 209,425 $ 206,368 Ohio Gathering 259,888 275,000 Williston Basin 425,873 452,152 DJ Basin 199,920 205,308 Permian Basin 165,765 185,708 Piceance Basin 579,800 631,140 Barnett Shale 336,629 350,638 Marcellus Shale 176,441 184,631 Total reportable segment assets 2,353,741 2,490,945 Corporate and Other 146,076 83,153 Total assets $ 2,499,817 $ 2,574,098 (1) Revenues by reportable segment follow. Year ended December 31, 2020 2019 (In thousands) Revenues: Utica Shale $ 36,509 $ 33,991 Williston Basin 92,695 105,651 DJ Basin 28,070 26,050 Permian Basin 29,533 20,303 Piceance Basin 113,890 133,638 Barnett Shale 54,459 71,802 Marcellus Shale 25,741 24,471 Total reportable segments revenue 380,897 415,906 Corporate and Other 2,576 30,552 Eliminations — (2,930 ) Total revenues $ 383,473 $ 443,528 Counterparties accounting for a significant portion of total revenues were as follows: Year ended December 31, 2020 2019 Percentage of total revenues (1) Counterparty A - Piceance Basin 13 % 11 % Counterparty B - Williston Basin * 10 % Counterparty C - Utica Shale 5 % * Counterparty C - Permian Basin 5 % * Counterparty C - Barnett Shale 1 % * * Less than 10% Depreciation and amortization, including the amortization expense associated with the Partnership’s favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows. Year ended December 31, 2020 2019 (In thousands) Depreciation and amortization: Utica Shale $ 7,696 $ 7,659 Williston Basin 25,911 19,829 DJ Basin 6,146 3,732 Permian Basin 5,455 4,868 Piceance Basin 45,203 47,018 Barnett Shale (1) 16,112 16,575 Marcellus Shale 9,195 9,141 Total reportable segment depreciation and amortization 115,718 108,822 Corporate and Other 3,352 2,752 Total depreciation and amortization $ 119,070 $ 111,574 (1) Cash paid for capital expenditures by reportable segment follow. Year ended December 31, 2020 2019 (In thousands) Cash paid for capital expenditures: Utica Shale $ 6,957 $ 3,902 Williston Basin 8,767 30,861 DJ Basin 12,829 80,487 Permian Basin 7,014 44,955 Piceance Basin 1,370 1,946 Barnett Shale (1) 1,878 184 Marcellus Shale 700 693 Total reportable segment capital expenditures 39,515 163,028 Corporate and Other 3,613 19,263 Total cash paid for capital expenditures $ 43,128 $ 182,291 (1) For the year ended December 31, 2019, Corporate and Other includes cash paid of $1.6 million for corporate purposes; the remainder represents capital expenditures relating to the Double E Project. The Partnership assesses the performance of its reportable segments based on segment adjusted EBITDA. The Partnership defines segment adjusted EBITDA as total revenues less total costs and expenses; plus (i) other income excluding interest income, (ii) proportional adjusted EBITDA for equity method investees, (iii) depreciation and amortization, (iv) adjustments related to MVC shortfall payments, (v) adjustments related to capital reimbursement activity, (vi) unit-based and noncash compensation, (vii) impairments (vii) other noncash expenses or losses, less other noncash income or gains and (ix) restructuring expenses. Proportional adjusted EBITDA for the Partnership’s equity method investees is defined as the product of (i) total revenues less total expenses, excluding impairments and other noncash income or expense items, and amortization for deferred contract costs; and (ii) ownership interest in Ohio Gathering during the respective period. For the purpose of evaluating segment performance, the Partnership excludes the effect of Corporate and Other revenues and expenses, such as certain general and administrative expenses (including compensation-related expenses and professional services fees), certain natural gas and crude oil marketing services, transaction costs, interest expense and income tax expense or benefit from segment adjusted EBITDA. Segment adjusted EBITDA by reportable segment follows. Year ended December 31, 2020 2019 (In thousands) Reportable segment adjusted EBITDA Utica Shale $ 32,783 $ 29,292 Ohio Gathering 31,056 39,126 Williston Basin 52,060 69,437 DJ Basin 19,449 18,668 Permian Basin 4,426 (879 ) Piceance Basin 88,820 98,765 Barnett Shale 32,093 43,043 Marcellus Shale 22,015 20,051 Total of reportable segments' measures of profit $ 282,702 $ 317,503 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, 2020 2019 (In thousands) Reconciliation of income (loss) before income taxes and income (loss) from equity method investees to total of reportable segments' measures of profit: Income (loss) before income taxes and income (loss) from equity method investees $ 177,661 $ (54,644 ) Add: Corporate and Other expense 59,585 44,808 Interest expense 78,894 91,966 Gain on early extinguishment of debt (203,062 ) — Depreciation and amortization (1) 119,070 111,574 Proportional adjusted EBITDA for equity method investees 31,056 39,126 Adjustments related to MVC shortfall payments — 3,476 Adjustments related to capital reimbursement activity (1,395 ) (2,156 ) Unit-based and noncash compensation 8,111 8,171 Gain on asset sales, net (307 ) (1,536 ) Long-lived asset impairment 13,089 60,507 Goodwill impairment — 16,211 Total of reportable segments' measures of profit $ 282,702 $ 317,503 (1) For the years ended December 31, 2020 and 2019, adjustments related to MVC shortfall payments recognize the earnings from MVC shortfall payments ratably over the term of the associated MVC. Contributions in aid of construction are recognized over the remaining term of the respective contract. The Partnership includes adjustments related to capital reimbursement activity in its calculation of segment adjusted EBITDA to account for revenue recognized from contributions in aid of construction.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	21. In January 2021, Double E received its Notice to Proceed with construction, as well as approval of its implementation plan, from FERC. With the receipt of the 7(c) certificate and the NTP, construction on the Double E pipeline commenced in February 2021, and the pipeline is expected to be in-service by the end of 2021. In February 2021, Summit Permian Transmission, LLC, received $175.0 million of commitments to finance the development of the Double E Project. The lenders have committed to provide senior secured credit facilities consisting of a $160.0 million delayed draw term loan facility and a $15.0 million working capital facility (the “Credit Facilities”). The Credit Facilities are non-recourse to SMLP and mature seven years after the date of initial borrowing. SMLP expects to close and fund on the Credit Facilities shortly and will post a $ 15.0 million letter of credit under its corporate revolving credit facility to support back-end equity contributions, if needed, upon first funding.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash. The Partnership considers 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 the Partnership’s customers and other counterparties. The Partnership evaluates the collectability of accounts receivable and the need for an allowance for doubtful accounts based on customer-specific facts and circumstances. To the extent the collectability of a specific customer or counterparty receivable is doubtful, the Partnership recognizes 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. The Partnership records property, plant and equipment at historical cost of construction or fair value of the assets at acquisition. The Partnership capitalizes 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, the Partnership recognizes expenditures as an expense as incurred. The Partnership capitalizes project costs incurred during construction, including interest on funds borrowed to finance the construction of facilities, as construction in progress. Accrued capital expenditures are reflected in trade accounts payable. The Partnership records depreciation on a straight-line basis over an asset’s estimated useful life and bases its 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. The Partnership bases an asset’s carrying value on estimates, assumptions and judgments for useful life and salvage value. Upon sale, retirement or other disposal, the Partnership removes the carrying value of an asset and its accumulated depreciation from its balance sheet and recognizes the related gain or loss, if any.
Asset Retirement Obligations	Asset Retirement Obligations. The Partnership records 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, the Partnership evaluates whether the expected retirement date and related costs of retirement can be estimated. The Partnership ha s concluded that its gathering and processing assets have an indeterminate life because they are owned and will operate for an indeterminate period when properly maintained. Because the Partnership d oes not have sufficient information to reasonably estimate the amount or timing of such obligations and does not have any current plan to discontinue use of any significant assets, the Partnership did no t provide for any asset retirement obligations as of December 31, 20 20 or 201 9 .
Amortizing Intangibles	Amortizing Intangibles. The Partnership has certain acquired gas gathering contracts that had above-market pricing structures at the acquisition date and the Partnership amortizes these favorable contracts using a straight-line method over the contract’s estimated useful life. The Partnership defines useful life as the period over which the contract is expected to contribute to the Partnership’s future cash flows. These favorable contracts have original terms ranging from 10 years to 20 years and the Partnership recognizes the amortization expense associated with these contracts in Other revenues. The Partnership amortizes 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. The Partnership recognizes the amortization expense associated with these contracts in Depreciation and amortization expense. The Partnership also has rights-of-way associated with municipal easements and easements granted within existing rights-of-way. The Partnership amortizes 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 and the Partnership recognizes the amortization expense associated with these rights-of-way assets in Depreciation and amortization expense.
Equity Method Investments	Equity Method Investments. The Partnership accounts for investments in which it exercises significant influence using the equity method so long as it (i) does not control the investee and (ii) is not the primary beneficiary. The Partnership reflects these investments in its consolidated balance sheets under the caption titled “investment in equity method investees.” The Partnership recognizes 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 is not limited to, absence of an ability to recover the carrying amount of the investment or an 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. The Partnership evaluates its equity method investments whenever a triggering event exists that would indicate a need to assess the investment for potential impairment.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets. The Partnership tests assets for impairment when events or circumstances indicate 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 the Partnership concludes that an asset's carrying value will not be recovered through future cash flows, the Partnership recognizes an impairment loss on the long-lived asset equal to the amount by which the carrying value exceeds its fair value. The Partnership determines fair value using a combination of market-based and income-based approaches.
Environmental Matters	Environmental Matters. The Partnership is 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. The Partnership accrues 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.
Commitments and Contingencies	Commitments and Contingencies. When required, the Partnership records accruals for loss contingencies in accordance with FASB ASC 450, Contingencies . Such determinations are subject to interpretations of current facts and circumstances, forecasts of future events and estimates of the financial impacts of such events.
Noncontrolling Interest and Mezzanine Capital	Noncontrolling Interest and Mezzanine Capital . A noncontrolling interest represents the portion of a consolidated subsidiary that is owned by a third party. Amounts are adjusted by the noncontrolling interest holder’s proportionate share of the subsidiary’s earnings or losses each period and any distributions that are paid. A noncontrolling interest is reported as a component of equity unless the noncontrolling interest is considered redeemable, in which case the noncontrolling interest is recorded between liabilities and equity (mezzanine or temporary equity) in the Partnership’s consolidated balance sheet.
Revenue Recognition	Revenue Recognition. The Partnership provides gathering and/or processing services principally under contracts that contain one or more of the following arrangements described below: • Fee-based arrangements. • Percent-of-proceeds arrangements. For the contracts described above, the Partnership reflects its revenues in the financial statement captions described below. Financial statement caption: Revenue description: Revenues: Gathering services and related fees • Revenue earned from fee-based gathering, compression, treating and processing services; Natural gas, NGLs and condensate sales • Revenue from the sale of physical natural gas purchased from customers percent-of-proceeds arrangements (Costs are presented within cost of natural gas and NGLs); • Revenue from sale of condensate and NGLs retained from gathering services (Costs are presented within operation and maintenance expense); and Other revenues • Customer reimbursements to the Partnership for costs incurred by the Partnership on customer’s behalf (Recorded on a gross basis). Certain of the Partnership’s gathering and/or processing agreements provide for monthly or annual MVCs. Under these MVCs, customers agree to ship and/or process a minimum volume of production on its 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 the Partnership at the end of the contracted measurement period if its actual throughput volumes are less than its contractual 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. The Partnership recognizes customer obligations under their MVCs as revenue and contract assets when (i) it considers 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, the Partnership considers 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.
Unit-Based Compensation	Unit-Based Compensation. For awards of unit-based compensation, the Partnership determines a grant date fair value and recognizes the related compensation expense in the statements of operations over the vesting period for each respective award.
Income Taxes	Income Taxes. The Partnership is generally not subject to federal and state income taxes, except as noted below. However, its unitholders are individually responsible for paying federal and state income taxes on their share of its taxable income. Net income or loss for GAAP purposes may differ significantly from taxable income reportable to its unitholders as a result of differences between the tax basis and the GAAP basis of assets and liabilities and the taxable income allocation requirements of the Partnership’s governing documents. 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. The Partnership’s financial statements reflect provisions for these tax obligations.
New Accounting Standards Implemented in this Annual Report	New accounting standards implemented in this Annual Report. 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. The adoption of ASU 2018-13 on January 1, 2020 did not have a material impact on the Partnership’s consolidated financial statements or disclosures. ASU No. 2016-13 Financial Instruments – Credit Losses (“ASU 2016-13”). ASU 2016-13 requires the use of a current expected loss model for financial assets measured at amortized cost and certain off-balance sheet credit exposures. Under this model, entities will be required to estimate the lifetime expected credit losses on such instruments based on historical experience, current conditions, and reasonable and supportable forecasts. This amended guidance also expands the disclosure requirements to enable users of financial statements to understand an entity’s assumptions, models and methods for estimating expected credit losses. The changes are effective for annual and interim periods beginning after December 15, 2019, and amendments should be applied using a modified retrospective approach. The adoption of ASU 2016-13 on January 1, 2020 did not have a material impact on the Partnership’s consolidated financial statements or disclosures. New accounting standards not yet implemented in this Annual Report. ASU No. 2020-06 Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40) (“ASU 2020-06”). ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. The ASU is part of the FASB’s simplification initiative, which aims to reduce unnecessary complexity in U.S. GAAP. The ASU’s amendments are effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years. The Partnership is currently evaluating the provisions of ASU 2020-06 to determine its impact on the Partnership’s consolidated financial statements and disclosures. ASU No. 2020-04 Reference Rate Reform (“ASU 2020-04”). ASU 2020-04 provides optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform on financial reporting. The amendments in ASU 2020-04 are effective as of March 12, 2020 through December 31, 2022. The Partnership is currently evaluating the provisions of ASU 2020-04 to determine its impact on the Partnership’s consolidated financial statements and disclosures.
Basis of Presentation	Presentation and Consolidation. The Partnership prepares its consolidated financial statements in accordance with GAAP as established by the FASB. The Partnership makes 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 revenues and expenses and the disclosure of commitments and 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.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2020
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

REVENUE (Tables)

REVENUE (Tables)	12 Months Ended
	Dec. 31, 2020
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. The Partnership applies the practical expedient in paragraph 606-10-50-14 of Topic 606 for certain arrangements that are considered optional purchases (i.e., there is no enforceable obligation for the customer to make purchases) and those amounts are therefore excluded from the table. 2021 2022 2023 2024 2025 Thereafter Gathering services and related fees $ 100,182 $ 84,215 $ 66,330 $ 49,872 $ 32,996 $ 22,706
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 20. Year ended December 31, 2020 Gathering services and related fees Natural gas, NGLs and condensate sales Other revenues Total (in thousands) Reportable Segments: Utica Shale $ 36,509 $ — $ — $ 36,509 Williston Basin 59,239 20,018 13,438 92,695 DJ Basin 23,868 245 3,957 28,070 Permian Basin 10,091 18,857 585 29,533 Piceance Basin 106,657 2,612 4,621 113,890 Barnett Shale 40,687 7,587 6,185 54,459 Marcellus Shale 25,741 — — 25,741 Total reportable segments 302,792 49,319 28,786 380,897 All other segments — — 2,576 2,576 Total $ 302,792 $ 49,319 $ 31,362 $ 383,473 Year ended December 31, 2019 Gathering services and related fees Natural gas, NGLs and condensate sales Other revenues Total (in thousands) Reportable Segments: Utica Shale $ 31,926 $ — $ 2,065 $ 33,991 Williston Basin 77,626 16,461 11,564 105,651 DJ Basin 21,940 389 3,721 26,050 Permian Basin 3,610 16,383 310 20,303 Piceance Basin 121,357 7,954 4,327 133,638 Barnett Shale 47,862 17,147 6,793 71,802 Marcellus Shale 24,471 — — 24,471 Total reportable segments 328,792 58,334 28,780 415,906 All other segments (2,045 ) 28,660 1,007 27,622 Total $ 326,747 $ 86,994 $ 29,787 $ 443,528
Schedule of Information about Contract Assets from Contracts with Customers	The following table provides information about contract assets from contracts with customers : December 31, 2020 December 31, 2019 (In thousands) Contract assets, January 1, 2020 $ 3,902 $ 8,755 Additions 18,834 18,077 Transfers out (20,710 ) (22,930 ) Contract assets, December 31, 2020 $ 2,026 $ 3,902

PROPERTY, PLANT, AND EQUIPMEN_2

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of property, plant, and equipment, net	Details on the Partnership’s property, plant and equipment follow. December 31, 2020 December 31, 2019 (In thousands) Gathering and processing systems and related equipment $ 2,213,501 $ 2,182,950 Construction in progress 60,443 78,716 Land and line fill 10,440 10,137 Other 61,340 54,595 Total 2,345,724 2,326,398 Less accumulated depreciation (528,178 ) (443,909 ) Property, plant and equipment, net $ 1,817,546 $ 1,882,489
Schedule of depreciation expense and capitalized interest costs	Depreciation expense and capitalized interest for the Partnership follow. Year ended December 31, 2020 2019 (In thousands) Depreciation expense $ 86,263 $ 78,489 Capitalized interest 3,878 $ 6,974

AMORTIZING INTANGIBLE ASSETS (T

AMORTIZING INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible assets and liabilities subject to amortization	Details regarding the Partnership’s intangible assets, all of which are subject to amortization, follow. December 31, 2020 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (16,064 ) $ 8,131 Contract intangibles 278,448 (195,243 ) 83,205 Rights-of-way 157,271 (49,041 ) 108,230 Total intangible assets $ 459,914 $ (260,348 ) $ 199,566 December 31, 2019 Gross carrying amount Accumulated amortization Net (In thousands) Favorable gas gathering contracts $ 24,195 $ (15,125 ) $ 9,070 Contract intangibles 278,448 (169,549 ) 108,899 Rights-of-way 157,175 (42,866 ) 114,309 Total intangible assets $ 459,818 $ (227,540 ) $ 232,278
Recognized amortization expense in other revenues and cost and expenses	The Partnership recognized amortization expense of its favorable gas gathering contracts in Other revenues as follows: Year ended December 31, 2020 2019 (In thousands) Amortization expense – favorable gas gathering contracts $ (938 ) $ (1,220 ) The Partnership recognized amortization expense of its contract and right of way intangibles in costs and expenses as follows: Year ended December 31, 2020 2019 (In thousands) Amortization expense – contract intangibles $ 25,694 $ 25,587 Amortization expense – rights-of-way 6,175 6,278
Estimated aggregate annual amortization expected to be recognized	The Partnership’s estimated aggregate annual amortization expected to be recognized as of December 31, 2020 for each of the five succeeding fiscal years follows. Intangible assets (In thousands) 2021 $ 28,212 2022 25,145 2023 25,091 2024 14,920 2025 14,898

EQUITY METHOD INVESTMENTS (Tabl

EQUITY METHOD INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2020
Equity Method Investments And Joint Ventures [Abstract]
Equity Method Investments	A reconciliation of the difference between the carrying amount of the Partnership’s interest in Ohio Gathering and the Partnership’s underlying investment in Ohio Gathering, per Ohio Gathering’s books and records, is shown below. 2020 2019 (In thousands) Investment in Ohio Gathering, December 31, $ 259,888 $ 275,000 December cash distributions 2,748 2,700 Impairment loss (1) — 232,521 Basis difference 216,591 — Investment in Ohio Gathering (Books and records), November 30, $ 479,227 $ 510,221 (1) Summarized balance sheet information for OGC and OCC follows (amounts represent 100% of investee financial information). November 30, 2020 November 30, 2019 OGC OCC OGC OCC (In thousands) Current assets $ 32,404 $ 4,902 $ 41,972 $ 2,187 Noncurrent assets 1,240,090 290 1,281,171 28,323 Total assets $ 1,272,494 $ 5,192 $ 1,323,143 $ 30,510 Current liabilities $ 8,424 $ 2,982 $ 21,798 $ 4,016 Noncurrent liabilities 8,441 5,146 4,113 6,683 Total liabilities $ 16,865 $ 8,128 $ 25,911 $ 10,699 Summarized statements of operations information for OGC and OCC follow (amounts represent 100% of investee financial information). Twelve months ended November 30, 2020 Twelve months ended November 30, 2019 OGC OCC OGC OCC (In thousands) Total revenues $ 114,524 $ 12,931 $ 142,138 $ 8,601 Total operating expenses 103,020 38,502 108,234 38,815 Net income (loss) 11,496 (25,571 ) 33,897 (30,214 )

DEFERRED REVENUE (Tables)

DEFERRED REVENUE (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
Rollforward of deferred revenue	A rollforward of current deferred revenue follows. Total (In thousands) Current deferred revenue, January 1, 2020 $ 13,493 Additions 2,731 Less: revenue recognized 6,236 Current deferred revenue, December 31, 2020 $ 9,988 A rollforward of noncurrent deferred revenue follows. Total (In thousands) Noncurrent deferred revenue, January 1, 2020 $ 38,709 Additions 19,384 Less: reclassification to current deferred revenue 9,843 Noncurrent deferred revenue, December 31, 2020 $ 48,250

DEBT (Tables)

DEBT (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of debt and capital leases	Debt for the Partnership at December 31, 2020 and 2019 follows. December 31, 2020 December 31, 2019 (In thousands) Revolving Credit Facility : Summit Holdings' variable rate senior secured Revolving Credit Facility (2.90% at December 31, 2020 and 4.55% at December 31, 2019) due May 2022 $ 857,000 $ 677,000 ECP Loans : Summit Holdings' 8.00% senior secured term loan obtained in May 2020 and fully repaid in August 2020 — — 2022 Senior Notes : Summit Holdings' 5.5% senior unsecured notes due August 2022 234,047 300,000 Less: unamortized debt issuance costs (1) (859 ) (1,686 ) 2025 Senior Notes : Summit Holdings' 5.75% senior unsecured notes due April 2025 259,463 500,000 Less: unamortized debt issuance costs (1) (2,325 ) (5,015 ) SMPH Term Loan : SMP Holdings' variable rate senior secured term loan (7.80% at December 31, 2019) due May 2022 and extinguished in November 2020 — 161,500 Less: unamortized debt issuance costs (1) — (3,974 ) Total debt 1,347,326 1,627,825 Less: current portion — (5,546 ) Total long-term debt $ 1,347,326 $ 1,622,279 (1)
Schedule of maturities of long-term debt	The aggregate amount of Partnership’s debt maturing during each of the years after December 31, 2020 are as follows (in thousands): 2021 $ — 2022 1,091,047 2023 — 2024 — 2025 259,463 Thereafter — Total long-term debt $ 1,350,510

LIABILITY MANAGEMENT TRANSACT_2

LIABILITY MANAGEMENT TRANSACTIONS (Tables)	12 Months Ended
	Dec. 31, 2020
Liability Management Transactions [Abstract]
Schedule of extinguishment of debt	The Partnership recognized a $203.1 million gain on extinguishment of debt during the year ended December 31, 2020, the components of which are summarized in the table below. ECP Loan Repayment Open Market Repurchases Tender Offers TL Restructuring Total 2022 2025 2022 2025 (in thousands) Senior Notes Senior Notes Senior Notes Senior Notes Gain on Repurchases of Senior Notes and TL Restructuring $ — $ 11,554 $ 76,789 $ 9,223 $ 15,479 $ 99,175 $ 212,220 Debt issue costs (361 ) (143 ) (1,541 ) (125 ) (351 ) (2,724 ) (5,245 ) Transaction costs (249 ) (105 ) (105 ) (467 ) (467 ) (2,520 ) (3,913 ) Gain (loss) on debt extinguishment $ (610 ) $ 11,306 $ 75,143 $ 8,631 $ 14,661 $ 93,931 $ 203,062

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Schedule of accrued environmental remediation	A rollforward of the Partnership’s undiscounted accrued environmental remediation liabilities related to the Meadowlark Rupture follows. Total (In thousands) Accrued environmental remediation, January 1, 2019 $ 5,636 Payments made (2,284 ) Additional accruals 1,299 Accrued environmental remediation, December 31, 2019 $ 4,651 Payments made (1,722 ) Accrued environmental remediation, December 31, 2020 $ 2,929

FINANCIAL INSTRUMENTS (Tables)

FINANCIAL INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of the estimated fair value of debt instruments	A summary of the estimated fair value of the Partnerships debt financial instruments follows. December 31, 2020 December 31, 2019 Carrying Value, Net Estimated fair value (Level 2) Carrying Value, Net Estimated fair value (Level 2) (in thousands) 2022 Senior Notes $ 233,188 $ 215,713 $ 298,314 $ 266,750 2025 Senior Notes 257,138 168,002 494,985 382,708

PARTNERS' CAPITAL AND MEZZANI_2

PARTNERS' CAPITAL AND MEZZANINE CAPITAL (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of partner units activity	A rollforward of the number of common limited partner units follows for the period from December 31, 2018 to December 31, 2020. Common Units (2) Units, December 31, 2018 3,021,258 2019 activity — Units, December 31, 2019 3,021,258 GP Buy-In Transaction (1) (132,687 ) Common units issued for SMLP LTIP, net 95,987 TL Restructuring (Note 9) 2,306,972 Series A Preferred Unit Exchange Offer, net of shares withheld for taxes 817,845 Other 717 Units, December 31, 2020 6,110,092 (1) (2) As adjusted for reverse unit split. A rollforward of the number of Series A Preferred Units follows for the period from December 31, 2018 to December 31, 2020. Series A Preferred Units Series A Preferred Units, December 31, 2018 300,000 2019 activity — Series A Preferred Units, December 31, 2019 300,000 Series A Preferred Unit Exchange Offer (62,816 ) Series A Preferred Unit Tender (75,075 ) Series A Preferred Units, December 31, 2020 162,109
Change in subsidiary	The following table shows the change in the Partnership’s Subsidiary Series A Preferred Unit balance during the year ended December 31, 2020: December 31, 2020 (in thousands) Balance at December 31, 2019 $ 27,450 New issuances 50,000 PIK distributions 5,251 Issuance costs (1,290 ) Redemption accretion 8,247 Balance at December 31, 2020 (1) $ 89,658 (1)
Details of cash distributions	Cash Distributions Paid and Declared. Prior to the GP Buy-In Transaction, SMLP paid the following per-unit cash distributions to its unitholders during the years ended December 31: Year ended December 31, 2020 2019 Per-unit distributions to unitholders $ 0.125 $ 1.4375

EARNINGS PER UNIT (Tables)

EARNINGS PER UNIT (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Schedule of earnings per limited partner unit	The following table details the components of EPU. Year ended December 31, 2020 2019 (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 $ 192,352 $ (184,451 ) Less: Net income attributable to Series A Preferred Units 26,529 28,500 Net income attributable to Subsidiary Series A Preferred Units 13,498 58 Add: Deemed capital contribution from Series A Preferred Unit Exchange Offer 54,945 — Deemed capital contribution from Series A Preferred Tender Offer 55,724 Net income (loss) attributable to common limited partners $ 262,994 $ (213,009 ) Denominator for basic and diluted EPU: Weighted-average common units outstanding – basic (1) 3,592 3,021 Effect of nonvested phantom units 102 — Weighted-average common units outstanding – diluted 3,694 3,021 Net Income (Loss) per limited partner unit: Common unit – basic $ 73.22 $ (70.50 ) Common unit – diluted $ 71.19 $ (70.50 ) Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU 240 12 (1) As a result of the GP Buy-In Transaction, our historical results are those of Summit Investments. The number of common units of 3.0 million as of December 31, 2019 represents those of Summit Investments and has been used for the earnings per unit calculation presented herein

SUPPLEMENTAL CASH FLOW INFORM_2

SUPPLEMENTAL CASH FLOW INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Schedule of supplemental cash flow information	Year ended December 31, 2020 2019 (In thousands) Supplemental cash flow information: Cash interest paid $ 79,450 $ 92,536 Less capitalized interest 3,878 6,974 Interest paid (net of capitalized interest) $ 75,572 $ 85,562 Cash paid for taxes $ 190 $ 150 Noncash investing and financing activities: Capital expenditures in trade accounts payable (period-end accruals) $ 6,154 $ 19,846 Warrant issuance for GP Buy-In Transaction 2,300 — Asset contribution to an equity method investment — 23,643 Right-of-use assets relating to ASC Topic 842 3,685 5,448 Fair value of SMLP equity for TL Restructuring 30,521 — Accretion of Subsidiary Series A Preferred Units 8,247 —

UNIT-BASED AND NONCASH COMPEN_2

UNIT-BASED AND NONCASH COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of activity	The following table presents phantom unit activity for the periods presented: Units Weighted-average grant date fair value Nonvested phantom units, December 31, 2018 57,608 $ 256.65 Phantom units granted 127,540 97.20 Phantom units vested (40,174 ) 251.70 Phantom units forfeited (4,574 ) 193.05 Nonvested phantom units, December 31, 2019 140,400 115.35 Phantom units granted 345,997 9.20 Phantom units vested (193,349 ) 54.07 Phantom units forfeited (1,357 ) 119.24 Nonvested phantom units, December 31, 2020 291,691 $ 26.57
Schedule of intrinsic values	The intrinsic value of phantom units that vested during the years ended December 31, follows. Year ended December 31, 2020 2019 (In thousands) Intrinsic value of vested LTIP awards $ 2,602 $ 5,940
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, 2020 2019 (In thousands) SMLP LTIP unit-based compensation $ 8,111 $ 8,171

LEASES (Tables)

LEASES (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Schedule of right-of-use assets and lease liabilities	ROU assets (included in the Property, plant and equipment, net caption on the Partnership’s consolidated balance sheet) and lease liabilities (included in the Other current liabilities and Other noncurrent liabilities captions on the Partnership’s consolidated balance sheet) follow: December 31, 2020 December 31, 2019 (In thousands) ROU assets Operating $ 3,736 $ 3,580 Finance 1,748 3,159 $ 5,484 $ 6,739 Lease liabilities, current Operating $ 2,298 $ 1,221 Finance 618 1,246 $ 2,916 $ 2,467 Lease liabilities, noncurrent Operating $ 3,182 $ 2,513 Finance 154 676 $ 3,336 $ 3,189
Schedule of lease cost and other information	Lease cost and Other information follow: Year ended December 31, 2020 2019 (In thousands) Lease cost Finance lease cost: Amortization of ROU assets (included in depreciation and amortization) $ 1,274 $ 1,559 Interest on lease liabilities (included in interest expense) 52 102 Operating lease cost (included in general and administrative expense) 2,644 3,345 $ 3,970 $ 5,006 Year ended December 31, 2020 2019 (In thousands) Other information Cash paid for amounts included in the measurement of lease liabilities Operating cash outflows from operating leases $ 1,472 $ 3,396 Operating cash outflows from finance leases 52 102 Financing cash outflows from finance leases 1,150 1,873 ROU assets obtained in exchange for new operating lease liabilities 3,552 1,218 ROU assets obtained in exchange for new finance lease liabilities 133 1,350 Weighted-average remaining lease term (years) - operating leases 4.9 5.8 Weighted-average remaining lease term (years) - finance leases 1.4 2.0 Weighted-average discount rate - operating leases 5 % 5 % Weighted-average discount rate - finance leases 4 % 4 %
Schedule of total rent expense related to operating leases	The Partnership recognizes total lease expense incurred or allocated to us in general and administrative expenses. Lease expense related to operating leases, including lease expense incurred on the Partnership’s behalf and allocated to us, was as follows: Year ended December 31, 2020 2019 (In thousands) Lease expense $ 3,436 $ 4,038
Future minimum lease payments due and future minimum rentals to be received under noncancelable leases and subleases	Future minimum lease payments due under noncancelable leases at December 31, 2020, were as follows: December 31, 2020 (In thousands) Operating Finance 2021 $ 1,750 $ 655 2022 1,308 126 2023 854 42 2024 573 — 2025 464 — 2026 156 — Thereafter 579 — Total future minimum lease payments $ 5,684 $ 823

RESTRUCTURING (Tables)

RESTRUCTURING (Tables)	12 Months Ended
	Dec. 31, 2020
Restructuring And Related Activities [Abstract]
Restructuring and Related Costs	Details for the 2020 Restructuring Plan and the 2019 Restructuring Plan follow. Severance Charges Other Restructuring Charges Total Severance and Other 2020 2019 2020 2019 2020 2019 (In thousands) 2020 Restructuring Plan $ 5,591 $ — $ 56 $ — $ 5,647 $ — 2019 Restructuring Plan 2,159 3,767 1,293 1,187 3,452 4,954 $ 7,750 $ 3,767 $ 1,349 $ 1,187 $ 9,099 $ 4,954 Accrued December 31, 2018 Charges Incurred Cash Payments Accrued December 31, 2019 Charges Incurred Cash Payments Accrued December 31, 2020 (In thousands) Employee-related costs $ — $ 3,767 $ (951 ) $ 2,816 $ 7,750 $ (7,018 ) $ 3,548 Other — 1,187 (1,187 ) — 1,349 (1,349 ) — Total restructuring costs $ — $ 4,954 $ (2,138 ) $ 2,816 $ 9,099 $ (8,367 ) $ 3,548

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Schedule of assets by reportable segment	Assets by reportable segment follow. December 31, 2020 2019 (in thousands) Assets (1) Utica Shale $ 209,425 $ 206,368 Ohio Gathering 259,888 275,000 Williston Basin 425,873 452,152 DJ Basin 199,920 205,308 Permian Basin 165,765 185,708 Piceance Basin 579,800 631,140 Barnett Shale 336,629 350,638 Marcellus Shale 176,441 184,631 Total reportable segment assets 2,353,741 2,490,945 Corporate and Other 146,076 83,153 Total assets $ 2,499,817 $ 2,574,098 (1)
Schedule of segment reporting information	Revenues by reportable segment follow. Year ended December 31, 2020 2019 (In thousands) Revenues: Utica Shale $ 36,509 $ 33,991 Williston Basin 92,695 105,651 DJ Basin 28,070 26,050 Permian Basin 29,533 20,303 Piceance Basin 113,890 133,638 Barnett Shale 54,459 71,802 Marcellus Shale 25,741 24,471 Total reportable segments revenue 380,897 415,906 Corporate and Other 2,576 30,552 Eliminations — (2,930 ) Total revenues $ 383,473 $ 443,528 Depreciation and amortization, including the amortization expense associated with the Partnership’s favorable and unfavorable gas gathering contracts as reported in other revenues, by reportable segment follows. Year ended December 31, 2020 2019 (In thousands) Depreciation and amortization: Utica Shale $ 7,696 $ 7,659 Williston Basin 25,911 19,829 DJ Basin 6,146 3,732 Permian Basin 5,455 4,868 Piceance Basin 45,203 47,018 Barnett Shale (1) 16,112 16,575 Marcellus Shale 9,195 9,141 Total reportable segment depreciation and amortization 115,718 108,822 Corporate and Other 3,352 2,752 Total depreciation and amortization $ 119,070 $ 111,574 (1) Cash paid for capital expenditures by reportable segment follow. Year ended December 31, 2020 2019 (In thousands) Cash paid for capital expenditures: Utica Shale $ 6,957 $ 3,902 Williston Basin 8,767 30,861 DJ Basin 12,829 80,487 Permian Basin 7,014 44,955 Piceance Basin 1,370 1,946 Barnett Shale (1) 1,878 184 Marcellus Shale 700 693 Total reportable segment capital expenditures 39,515 163,028 Corporate and Other 3,613 19,263 Total cash paid for capital expenditures $ 43,128 $ 182,291 (1)
Schedule of counterparties accounting for more than 10% of total revenues	Counterparties accounting for a significant portion of total revenues were as follows: Year ended December 31, 2020 2019 Percentage of total revenues (1) Counterparty A - Piceance Basin 13 % 11 % Counterparty B - Williston Basin * 10 % Counterparty C - Utica Shale 5 % * Counterparty C - Permian Basin 5 % * Counterparty C - Barnett Shale 1 % * * Less than 10%
Reconciliation of net income to adjusted EBITDA	Segment adjusted EBITDA by reportable segment follows. Year ended December 31, 2020 2019 (In thousands) Reportable segment adjusted EBITDA Utica Shale $ 32,783 $ 29,292 Ohio Gathering 31,056 39,126 Williston Basin 52,060 69,437 DJ Basin 19,449 18,668 Permian Basin 4,426 (879 ) Piceance Basin 88,820 98,765 Barnett Shale 32,093 43,043 Marcellus Shale 22,015 20,051 Total of reportable segments' measures of profit $ 282,702 $ 317,503 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, 2020 2019 (In thousands) Reconciliation of income (loss) before income taxes and income (loss) from equity method investees to total of reportable segments' measures of profit: Income (loss) before income taxes and income (loss) from equity method investees $ 177,661 $ (54,644 ) Add: Corporate and Other expense 59,585 44,808 Interest expense 78,894 91,966 Gain on early extinguishment of debt (203,062 ) — Depreciation and amortization (1) 119,070 111,574 Proportional adjusted EBITDA for equity method investees 31,056 39,126 Adjustments related to MVC shortfall payments — 3,476 Adjustments related to capital reimbursement activity (1,395 ) (2,156 ) Unit-based and noncash compensation 8,111 8,171 Gain on asset sales, net (307 ) (1,536 ) Long-lived asset impairment 13,089 60,507 Goodwill impairment — 16,211 Total of reportable segments' measures of profit $ 282,702 $ 317,503 (1)

ORGANIZATION, BUSINESS OPERAT_2

ORGANIZATION, BUSINESS OPERATIONS AND PRESENTATION AND CONSOLIDATION - Narrative (Details) $ in Millions	Nov. 09, 2020shares	Dec. 31, 2020USD ($)shares	Dec. 31, 2019shares
Organization And Business Operations [Line Items]
Purchase agreement date		May 28, 2020
Common limited partner capital (shares)		6,110,092	3,021,258
Summit Midstream GP, LLC | Reverse Unit Split
Organization And Business Operations [Line Items]
Common limited partner capital (shares)	56,624,887	3,774,992
Reverse unit split ratio	0.06
Reverse unit split description	On November 9, 2020, after the close of trading on the NYSE, the Partnership effected a 1-for-15 reverse unit split (the “Reverse Unit Split”) of its common units. The common units began trading on a split-adjusted basis on November 10, 2020
Common units | Energy Capital Partners
Organization And Business Operations [Line Items]
Common limited partner capital (shares)		0.4
Summit Midstream Partners Holdings L L C
Organization And Business Operations [Line Items]
SMLP common units pledged as collateral under the SMPH Term Loan		2,300,000
SMLP common units not pledged as collateral under the SMPH Term Loan		700,000
Common units retire		1,100,000
Summit Investments
Organization And Business Operations [Line Items]
Cash paid to Acquire investment | $		$ 35
Summit Investments | Common units
Organization And Business Operations [Line Items]
Number of common units purchased.		700,000

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
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 ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items]
Asset retirement obligation	$ 0	$ 0
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

REVENUE - Narrative1 (Details)

REVENUE - Narrative1 (Details)	Dec. 31, 2020
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue From Contract With Customer [Line Items]
Revenue, performance obligation, description of payment terms	Services are typically billed on a monthly basis and the Partnership does not offer extended payment terms. The Partnership does not have contracts with financing components.
Receivables with customers	$ 57,500	$ 90,400
Contract assets included in accounts receivable	2,026	3,902	$ 8,755
Gathering Services and Related Fees
Revenue From Contract With Customer [Line Items]
Contract liabilities, revenue	$ 4,000	$ 10,100

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, 2020USD ($)
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	$ 100,182
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	$ 84,215
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	$ 66,330
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	1 year
Revenue, remaining performance obligation	$ 49,872
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
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	$ 32,996
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, Period
Revenue, remaining performance obligation	$ 22,706

REVENUE - Schedule of Disaggreg

REVENUE - Schedule of Disaggregated Revenue by Geographic Area and Major Products and Services Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Total revenues	$ 383,473	$ 443,528
All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	2,576	27,622
Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	380,897	415,906
Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	36,509	33,991
Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	92,695	105,651
Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues	28,070	26,050
Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues	29,533	20,303
Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues	113,890	133,638
Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	54,459	71,802
Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	25,741	24,471
Gathering Services and Related Fees
Disaggregation Of Revenue [Line Items]
Total revenues	302,792	326,747
Gathering Services and Related Fees | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	0	(2,045)
Gathering Services and Related Fees | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	302,792	328,792
Gathering Services and Related Fees | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	36,509	31,926
Gathering Services and Related Fees | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	59,239	77,626
Gathering Services and Related Fees | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues	23,868	21,940
Gathering Services and Related Fees | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues	10,091	3,610
Gathering Services and Related Fees | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues	106,657	121,357
Gathering Services and Related Fees | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	40,687	47,862
Gathering Services and Related Fees | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	25,741	24,471
Natural Gas, NGLs and Condensate Sales
Disaggregation Of Revenue [Line Items]
Total revenues	49,319	86,994
Natural Gas, NGLs and Condensate Sales | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	0	28,660
Natural Gas, NGLs and Condensate Sales | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	49,319	58,334
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0	0
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	20,018	16,461
Natural Gas, NGLs and Condensate Sales | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues	245	389
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues	18,857	16,383
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues	2,612	7,954
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	7,587	17,147
Natural Gas, NGLs and Condensate Sales | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0	0
Other Revenues
Disaggregation Of Revenue [Line Items]
Total revenues	31,362	29,787
Other Revenues | All Other Segments
Disaggregation Of Revenue [Line Items]
Total revenues	2,576	1,007
Other Revenues | Reportable Segments
Disaggregation Of Revenue [Line Items]
Total revenues	28,786	28,780
Other Revenues | Reportable Segments | Utica Shale
Disaggregation Of Revenue [Line Items]
Total revenues	0	2,065
Other Revenues | Reportable Segments | Williston Basin
Disaggregation Of Revenue [Line Items]
Total revenues	13,438	11,564
Other Revenues | Reportable Segments | DJ Basin
Disaggregation Of Revenue [Line Items]
Total revenues	3,957	3,721
Other Revenues | Reportable Segments | Permian Basin
Disaggregation Of Revenue [Line Items]
Total revenues	585	310
Other Revenues | Reportable Segments | Piceance Basin
Disaggregation Of Revenue [Line Items]
Total revenues	4,621	4,327
Other Revenues | Reportable Segments | Barnett Shale
Disaggregation Of Revenue [Line Items]
Total revenues	6,185	6,793
Other Revenues | Reportable Segments | Marcellus Shale
Disaggregation Of Revenue [Line Items]
Total revenues	$ 0	$ 0

REVENUE - Schedule of Informati

REVENUE - Schedule of Information about Contract Assets from Contracts with Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract With Customer Asset And Liability [Abstract]
Contract assets, beginning of period	$ 3,902	$ 8,755
Additions	18,834	18,077
Transfers out	(20,710)	(22,930)
Contract assets, end of period	$ 2,026	$ 3,902

PROPERTY, PLANT, AND EQUIPMEN_3

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Property, Plant, and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,345,724	$ 2,326,398
Less accumulated depreciation	(528,178)	(443,909)
Property, plant and equipment, net	1,817,546	1,882,489
Gathering and processing systems and related equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,213,501	2,182,950
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	60,443	78,716
Land and line fill
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	10,440	10,137
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 61,340	$ 54,595

PROPERTY, PLANT, AND EQUIPMEN_4

PROPERTY, PLANT, AND EQUIPMENT, NET - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Impairment of property, plant and equipment	$ 13.1	$ 59.4
Asset impairment charges	5.1	14.2
DJ Basin
Property, Plant and Equipment [Line Items]
Asset impairment charges	$ 3.6	34.7
Barnett Shale | Fixed Assets
Property, Plant and Equipment [Line Items]
Asset impairment charges		$ 9.7

PROPERTY, PLANT, AND EQUIPMEN_5

PROPERTY, PLANT, AND EQUIPMENT, NET - Schedule of Depreciation Expense and Capitalized Interest (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 86,263	$ 78,489
Capitalized interest	$ 3,878	$ 6,974

AMORTIZING INTANGIBLE ASSETS -

AMORTIZING INTANGIBLE ASSETS - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 459,914	$ 459,818
Accumulated amortization	(260,348)	(227,540)
Net	199,566	232,278
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(16,064)	(15,125)
Net	8,131	9,070
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(195,243)	(169,549)
Net	83,205	108,899
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	157,271	157,175
Accumulated amortization	(49,041)	(42,866)
Net	$ 108,230	$ 114,309

AMORTIZING INTANGIBLE ASSETS _2

AMORTIZING INTANGIBLE ASSETS - Recognized Amortization Expense in Other Revenues and Cost and Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Other Revenue | Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ (938)	$ (1,220)
Costs And Expenses | Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization expense	25,694	25,587
Costs And Expenses | Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 6,175	$ 6,278

AMORTIZING INTANGIBLE ASSETS _3

AMORTIZING INTANGIBLE ASSETS - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Dec. 31, 2020USD ($)
Intangible assets
2021	$ 28,212
2022	25,145
2023	25,091
2024	14,920
2025	$ 14,898

GOODWILL - Narrative (Details)

GOODWILL - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill impairment	$ 0	$ 16,211
Mountaineer Midstream
Goodwill [Line Items]
Goodwill impairment		$ 16,200
Goodwill	$ 0

EQUITY METHOD INVESTMENTS - Nar

EQUITY METHOD INVESTMENTS - Narrative (Details) - USD ($) $ in Thousands	11 Months Ended		12 Months Ended
	Nov. 30, 2019	[1]	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Equity Method Investments [Line Items]
Asset contribution to an equity method investment				$ 23,643
Cash distribution to partnership			$ 0	7,313
Capitalized interest paid			3,878	6,974
Impairment loss			(11,271)	337,851
Double E
Schedule Of Equity Method Investments [Line Items]
Additional cash investments			99,900	18,300
Capitalized interest paid			2,700	1,100
Capitalized legal costs			700	700
Net Assets			132,900	34,700
Result of operations			$ 0	0
Double E | Principal Owner
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest			70.00%
Asset contribution to an equity method investment			$ 23,600
Cash distribution to partnership			$ 7,300
Double E | JV Partner
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest			30.00%
Cash contribution			$ 7,300
OGC | Summit Midstream Partners, LLC
Schedule Of Equity Method Investments [Line Items]
Impairment loss	$ 232,521		$ 329,700	329,700
WeightedAverageDiscountRatePercent1			9.00%
OGC | Summit Midstream Partners, LLC | Long Lived Assets
Schedule Of Equity Method Investments [Line Items]
Impairment loss				$ 6,300
OGC | Principal Owner
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interest			38.20%	38.50%
[1]	(1)

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, 2020	Nov. 30, 2019		Dec. 31, 2020	Dec. 31, 2019
Equity Method Investment [Roll Forward]
Investment in Ohio Gathering, December 31,	$ 309,728			$ 309,728
December cash distributions				28,185	$ 37,300
Impairment loss				(11,271)	337,851
Investment in Ohio Gathering, November 30,				392,740	309,728
Investment in equity method investees	309,728			309,728	309,728
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in Ohio Gathering, December 31,	259,888	$ 275,000		259,888	275,000
December cash distributions	2,748	2,700
Impairment loss		232,521	[1]	329,700	329,700
Basis difference	216,591
Investment in Ohio Gathering, November 30,	479,227	510,221			259,888
Investment in equity method investees	$ 479,227	$ 510,221		$ 259,888	$ 275,000
[1]	(1)

EQUITY METHOD INVESTMENTS - R_2

EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees Parenthetical (Details) - USD ($) $ in Thousands	11 Months Ended		12 Months Ended
	Nov. 30, 2019	[1]	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Equity Method Investments [Line Items]
Impairment loss			$ (11,271)	$ 337,851
OGC | Summit Midstream Partners, LLC
Schedule Of Equity Method Investments [Line Items]
Impairment loss	$ 232,521		329,700	$ 329,700
Write-off basis difference			103,500
OCC | Summit Midstream Partners, LLC
Schedule Of Equity Method Investments [Line Items]
Impairment loss			$ 6,300
[1]	(1)

EQUITY METHOD INVESTMENTS - Bal

EQUITY METHOD INVESTMENTS - Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Nov. 30, 2020	Dec. 31, 2019	Nov. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Current assets	$ 82,099		$ 139,861
TOTAL ASSETS	2,499,817		2,574,098
Current liabilities	68,408		89,547
Total liabilities	$ 1,487,268		$ 1,761,412
OGC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule Of Equity Method Investments [Line Items]
Current assets		$ 32,404		$ 41,972
Noncurrent assets		1,240,090		1,281,171
TOTAL ASSETS		1,272,494		1,323,143
Current liabilities		8,424		21,798
Noncurrent liabilities		8,441		4,113
Total liabilities		16,865		25,911
OCC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule Of Equity Method Investments [Line Items]
Current assets		4,902		2,187
Noncurrent assets		290		28,323
TOTAL ASSETS		5,192		30,510
Current liabilities		2,982		4,016
Noncurrent liabilities		5,146		6,683
Total liabilities		$ 8,128		$ 10,699

EQUITY METHOD INVESTMENTS - Sta

EQUITY METHOD INVESTMENTS - Statements of Operations Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Nov. 30, 2020	Dec. 31, 2019	Nov. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Net income (loss)	$ 189,078		$ (393,726)
OGC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule Of Equity Method Investments [Line Items]
Result of operations		$ 114,524		$ 142,138
Total operating expenses		103,020		108,234
Net income (loss)		11,496		33,897
OCC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule Of Equity Method Investments [Line Items]
Result of operations		12,931		8,601
Total operating expenses		38,502		38,815
Net income (loss)		$ (25,571)		$ (30,214)

DEFERRED REVENUE - Rollforward

DEFERRED REVENUE - Rollforward of deferred revenue (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 13,493
Current deferred revenue, Additions	2,731
Current deferred revenue, Less revenue recognized	6,236
Current deferred revenue, ending balance	9,988
Noncurrent deferred revenue, beginning balance	38,709
Noncurrent deferred revenue, Additions	19,384
Noncurrent deferred revenue, Less reclassification to current deferred revenue	9,843
Noncurrent deferred revenue, ending balance	$ 48,250

DEBT - Components of Long-Term

DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Total debt		$ 1,350,510
Less: current portion			$ (5,546)
Long-term debt		1,347,326	1,622,279
Summit Midstream Partners Holdings L L C
Line of Credit Facility [Line Items]
Total debt		1,347,326	1,627,825
Less: current portion		0	(5,546)
Long-term debt		1,347,326	1,622,279
Summit Midstream Partners Holdings L L C | Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility		857,000	677,000
Summit Midstream Partners Holdings L L C | Senior Secured Term Loan Fully Paid August 2020
Line of Credit Facility [Line Items]
Revolving credit facility		0	0
Summit Midstream Partners Holdings L L C | 5.5% Senior Notes
Line of Credit Facility [Line Items]
Unsecured notes		259,463	500,000
Less unamortized debt issuance costs	[1]	(2,325)	(5,015)
Summit Midstream Partners Holdings L L C | Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes		234,047	300,000
Less unamortized debt issuance costs	[1]	(859)	(1,686)
Summit Midstream Partners Holdings L L C | Senior Secured Term Loan Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility		0	161,500
Less unamortized debt issuance costs	[1]	$ 0	$ (3,974)
[1]	(1)

DEBT - Components of Long-Ter_2

DEBT - Components of Long-Term Debt (Parenthetical) (Details) - Summit Midstream Partners Holdings L L C	Dec. 31, 2020	Dec. 31, 2019
Senior Secured Term Loan Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate		7.80%
Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Stated interest rate	5.50%	5.50%
Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Variable interest rate	2.90%	4.55%
5.5% Senior Notes
Line of Credit Facility [Line Items]
Stated interest rate	5.75%	5.75%
Senior Secured Term Loan Fully Paid August 2020
Line of Credit Facility [Line Items]
Stated interest rate	8.00%	8.00%

DEBT - Maturities (Details)

DEBT - Maturities (Details) $ in Thousands	Dec. 31, 2020USD ($)
Debt
2021	$ 0
2022	1,091,047
2023	0
2024	0
2025	259,463
Thereafter	0
Total long-term debt	$ 1,350,510

DEBT - Revolving Credit Facilit

DEBT - Revolving Credit Facility (Narrative) (Details) - Revolving credit facility	Dec. 18, 2020USD ($)	Dec. 31, 2020USD ($)	May 25, 2017
Line of Credit Facility [Line Items]
Borrowing capacity		$ 1,100,000,000
Revolving credit facility maturity date		2022-05
Borrowing Capacity		$ 105,000,000
Accordion feature	$ 250,000,000
Senior secured leverage ratio	3.75
First lien net leverage ratio	3.50
Commitment fee on unused portion of the facility		0.50%
Interest rate at period end		2.90%
Unused portion under the facility		$ 238,900,000
Line of credit facility, outstanding amount		4,100,000
Cumulative payment obligations, maximum		50,000,000
Debt issuance costs		$ 7,400,000
LIBOR
Line of Credit Facility [Line Items]
Pricing tier ( L+ 325 bps)	3.25%	2.75%
Maximum
Line of Credit Facility [Line Items]
Borrowing Capacity	$ 1,250,000,000
Leverage ratio	5.75
Commitment fee on unused portion of the facility		0.50%
Ratio of total net indebtedness to consolidated trailing 12-month EBITDA		5.75
Ratio of first lien net indebtedness to consolidated trailing 12-month EBITDA		3.75
Maximum | LIBOR
Line of Credit Facility [Line Items]
Pricing tier ( L+ 325 bps)		3.25%
Maximum | ABR
Line of Credit Facility [Line Items]
Pricing tier ( L+ 325 bps)		2.25%
Minimum
Line of Credit Facility [Line Items]
Borrowing Capacity	$ 1,100
Leverage ratio			5.50
Leverage ratio	5
Commitment fee on unused portion of the facility		0.30%
Ratio of consolidated trailing 12-month EBITDA to net interest expense		2.5
Minimum | LIBOR
Line of Credit Facility [Line Items]
Pricing tier ( L+ 325 bps)		1.75%
Minimum | ABR
Line of Credit Facility [Line Items]
Pricing tier ( L+ 325 bps)		0.75%
Junior Lien | Maximum
Line of Credit Facility [Line Items]
Accordion feature	$ 400,000,000

DEBT - ECP Loans (Narrative) (D

DEBT - ECP Loans (Narrative) (Details) $ in Millions	May 28, 2020USD ($)
Debt Instrument [Line Items]
Line of credit facility description	(i) a Term Loan Credit Agreement (the “ECP NewCo Term Loan Credit Agreement”), with SMP TopCo, LLC, a Delaware limited liability company and affiliate of ECP (“ECP NewCo”), as lender and administrative agent, and Mizuho Bank (USA) (“Mizuho”), as collateral agent, in a principal amount of $28.2 million (the “ECP NewCo Loan”), and (ii) a Term Loan Credit Agreement (the “ECP Holdings Term Loan Credit Agreement” and together with the ECP NewCo Term Loan Credit Agreement, the “ECP Term Loan Credit Agreements”), with ECP Holdings, as lender, and ECP NewCo, as administrative agent and Mizuho, as collateral agent, in a principal amount of $6.8 million (the “ECP Holdings Loan” and together with the ECP NewCo Loan, the “ECP Loans”). The ECP Loans were set to mature on March 31, 2021 and bore interest at a fixed rate of 8.00% per annum, with the interest payment due at maturity of the ECP Loans. With borrowings under the Partnership’s Revolving Credit Facility, the Partnership fully repaid all amounts outstanding under the ECP Loans ($35 million of principal and $0.6 million of accrued interest) on August 7, 2020.
ECP NewCo Term Loan Credit Agreement [Member]
Debt Instrument [Line Items]
Debt instrument principal amount	$ 28.2
ECP Term Loan Credit Agreements [Member]
Debt Instrument [Line Items]
Debt instrument principal amount	$ 6.8
Maturity date	Mar. 31, 2021
Stated interest rate	8.00%
Repayments of loans	$ 35
Accrued interest on loan	$ 0.6

DEBT - Senior Notes (Narrative)

DEBT - Senior Notes (Narrative) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2014	Dec. 31, 2020
2022 Senior Note
Debt Instrument [Line Items]
Debt issuance costs	$ 5,100,000
2025 Senior Notes
Debt Instrument [Line Items]
Deferred issuance costs		$ 7,700
Summit Holdings and Finance Corporation | 2022 Senior Note
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount	100.00%
Summit Holdings and Finance Corporation | 2025 Senior Notes | Redemption period one
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		104.313%
Summit Holdings and Finance Corporation | 2025 Senior Notes | Redemption period two
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		102.875%
Summit Holdings and Finance Corporation | 2025 Senior Notes | Redemption period three
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		100.00%

DEBT - Liability Management Tra

DEBT - Liability Management Transactions (Narrative) (Details) - Liability Management Transactions - SMPH Term Loan Restructuring $ in Millions	Nov. 17, 2020USD ($)shares
Debt Instrument [Line Items]
Term Loan result in consensual debt discharge of full amount owed under Term Loan exchange	$ 155.2
Capital stock issued	$ 2.3
Common stock held | shares	2,306,972
Cash payment on the deferred purchase price obligation	$ 26.5
LIBOR
Debt Instrument [Line Items]
Pricing tier ( L+ 325 bps)	6.00%
ABR
Debt Instrument [Line Items]
Pricing tier ( L+ 325 bps)	5.00%

LIABILITY MANAGEMENT TRANSACT_3

LIABILITY MANAGEMENT TRANSACTIONS (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 17, 2020	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Debt Instrument [Line Items]
Early extinguishment of debt		$ 306,500
Gain on early extinguishment of debt		203,062	$ 0
Open Market Repurchases | Liability Management Transactions
Debt Instrument [Line Items]
Cash consideration paid		150,300
Gain on early extinguishment of debt		86,400
Open Market Repurchases | Liability Management Transactions | Senior Notes Due 2022
Debt Instrument [Line Items]
Senior Notes repurchased		32,400
Open Market Repurchases | Liability Management Transactions | 5.5% Senior Notes
Debt Instrument [Line Items]
Senior Notes repurchased		201,800
Tender Offers | Liability Management Transactions
Debt Instrument [Line Items]
Cash consideration paid				$ 48,700
Gain on early extinguishment of debt		$ 23,300
Tender Offers | Liability Management Transactions | Senior Notes Due 2022
Debt Instrument [Line Items]
Senior Notes repurchased				33,500
Tender Offers | Liability Management Transactions | 5.5% Senior Notes
Debt Instrument [Line Items]
Senior Notes repurchased				$ 38,700
SMPH Term Loan Restructuring | Liability Management Transactions
Debt Instrument [Line Items]
Gain on early extinguishment of debt	$ 94,000
Extinguishment of debt, gain (loss), per share, net of tax	$ 26.15

LIABILITY MANAGEMENT TRANSACT_4

LIABILITY MANAGEMENT TRANSACTIONS - Gain on Extinguishment of Debt (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Gain on early extinguishment of debt	$ 203,062	$ 0
Liability Management Transactions | Open Market Repurchases And Tender Offer Repurchase
Debt Instrument [Line Items]
Gain on early extinguishment of debt	$ 203,100

LIABILITY MANAGEMENT TRANSACT_5

LIABILITY MANAGEMENT TRANSACTIONS - Schedule of Extinguishment of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	$ 212,220
Debt issue costs	(5,245)
Transaction costs	(3,913)
Gain (loss) on debt extinguishment	203,062	$ 0
ECP Loan Repayment
Extinguishment Of Debt [Line Items]
Debt issue costs	(361)
Transaction costs	(249)
Gain (loss) on debt extinguishment	(610)
T L Restructuring
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	99,175
Debt issue costs	(2,724)
Transaction costs	(2,520)
Gain (loss) on debt extinguishment	93,931
Senior Notes Due 2022 | Open Market Repurchases
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	11,554
Debt issue costs	(143)
Transaction costs	(105)
Gain (loss) on debt extinguishment	11,306
Senior Notes Due 2022 | Tender Offers
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	9,223
Debt issue costs	(125)
Transaction costs	(467)
Gain (loss) on debt extinguishment	8,631
Senior Notes Due 2025 | Open Market Repurchases
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	76,789
Debt issue costs	(1,541)
Transaction costs	(105)
Gain (loss) on debt extinguishment	75,143
Senior Notes Due 2025 | Tender Offers
Extinguishment Of Debt [Line Items]
Gain on Repurchases of Senior Notes and TL Restructuring	15,479
Debt issue costs	(351)
Transaction costs	(467)
Gain (loss) on debt extinguishment	$ 14,661

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - Meadowlark Midstream Gathering System	12 Months Ended
	Dec. 31, 2020USD ($)
Loss Contingencies [Line Items]
Legal fines and other fees	$ 2,500,000
Accrued loss contingency	17,000,000
Maximum | Principal Owner
Loss Contingencies [Line Items]
Coverage from pollution liability insurance policy	25,000,000
Coverage from property and business interruption insurance policy	$ 200,000,000

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - Environmental Matters (Details) - Meadowlark Midstream Gathering System - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 4,651	$ 5,636
Payments made	(1,722)	(2,284)
Additional accruals		1,299
Accrued environmental remediation, ending balance	$ 2,929	$ 4,651

FINANCIAL INSTRUMENTS - Concent

FINANCIAL INSTRUMENTS - Concentration Risk - Narrative (Details) - Customer	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
CONCENTRATIONS OF RISK
Number of top customers	5
Five Largest Customers | Accounts receivable | Customer Concentration Risk
CONCENTRATIONS OF RISK
Concentration risk, percentage	51.00%	46.00%

FINANCIAL INSTRUMENTS - Fair va

FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Value, Net | Senior Notes Due 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 233,188	$ 298,314
Carrying Value, Net | 2025 Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	257,138	494,985
Fair Value | Senior Notes Due 2022 | Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	215,713	266,750
Fair Value | 2025 Senior Notes | Estimated fair value (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 168,002	$ 382,708

PARTNERS' CAPITAL AND MEZZANI_3

PARTNERS' CAPITAL AND MEZZANINE CAPITAL - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 29, 2020	Jan. 21, 2020	Dec. 31, 2020	Jul. 31, 2020	May 28, 2020	Mar. 31, 2019	Nov. 30, 2017	Dec. 31, 2020	Dec. 31, 2019
Partners Capital [Line Items]
Preferred units issued			162,109					162,109	300,000
Repayment of outstanding revolving credit facility								$ 69,300	$ 158,000
Maximum period following end of quarter to distribute all available cash								45 days
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.125							$ 0.125	$ 1.4375
Cash paid to unit holders	$ 11,700
Cash distribution date	Feb. 14, 2020
Cash distribution date of record	Feb. 7, 2020
Revolving credit facility
Partners Capital [Line Items]
Repayment of outstanding revolving credit facility							$ 293,200
Series A Preferred Units
Partners Capital [Line Items]
Preferred units issued			55,251				300,000	55,251
Interest in partnership per unit							$ 1,000
Distribution rate							9.50%	7.00%
Liquidation preference per unit			$ 1,000				$ 1,000	$ 1,000
Distribution percentage of liquidation preference rate description								three-month LIBOR plus a spread of 7.43%
Series A preferred per unit, price			$ 1,000					$ 1,000
Proceeds from issuance of common limited partners units								$ 48,700
Redemption amount			$ 106,600					106,600
Series A Preferred Units | Permian Holdco
Partners Capital [Line Items]
Preferred units issued									30,000
Series A preferred per unit, price									$ 1,000
Series A Preferred Units | Double E Pipeline L L C | Permian Holdco
Partners Capital [Line Items]
Proceeds from issuance of common limited partners units									$ 27,400
Series A Preferred Units | Paid-in-kind
Partners Capital [Line Items]
Distribution percentage of subsidiary unit		7.00%
Distribution percentage of liquidation preference rate of undrawn commitment units		1.00%
Series A Preferred Units | LIBOR
Partners Capital [Line Items]
Distribution percentage of liquidation preference rate							7.43%
General Partner | Summit Midstream Partners, LP
Partners Capital [Line Items]
General partner interest						2.00%
Series A Preferred Units | Preferred Stock Exchange Offer
Partners Capital [Line Items]
Preferred units issued				62,816
Common units | Energy Capital Partners
Partners Capital [Line Items]
Number of common units purchased.					500,000
Exercise price of Warrants					$ 15.35
Valuation of warrants			$ 1,900		$ 2,300			$ 1,900
Warrant or right, reason for issuance, description					Upon exercise of the ECP Warrants, each of ECP NewCo and ECP Holdings may receive, at its election: (i) a number of SMLP common units equal to the number of SMLP common units for which the ECP Warrants are being exercised, if exercising the ECP Warrants by cash payment of the exercise price; (ii) a number of SMLP common units equal to the product of the number of common units being exercised multiplied by (a) the difference between the average of the daily volume-weighted average price (“VWAP”) of the SMLP common units on the New York Stock Exchange (the “NYSE”) on each of the three trading days prior to the delivery of the notice of exercise (the “VWAP Average”) and the exercise price (the “VWAP Difference”), divided by (b) the VWAP Average; and/or (iii) an amount in cash, to the extent that the Partnership’s leverage ratio would be at least 0.5x less than the maximum applicable ratio set forth in the Revolving Credit Facility, equal to the product of (a) the number of SMLP common units exercised and (b) the VWAP Difference, subject to certain adjustments under the ECP Warrants.
Warrants exercisable date					May 28, 2023
Common units | Energy Capital Partners | ECP Holdings Warrant
Partners Capital [Line Items]
Number of common units purchased.					100,000
Common units | Maximum | Energy Capital Partners
Partners Capital [Line Items]
Exercise price of Warrants					$ 30
Common units, issued					0.7
Common units | Preferred Stock Exchange Offer
Partners Capital [Line Items]
General partner units converted				837,547
Common units | Summit Midstream Partners, LP
Partners Capital [Line Items]
General partner units converted						600,000
Series A Preferred Units | Tender offer
Partners Capital [Line Items]
Number of shares purchased			75,075
Number of shares purchased, value			$ 25,000
Purchase price per share			$ 333

PARTNERS' CAPITAL AND MEZZANI_4

PARTNERS' CAPITAL AND MEZZANINE CAPITAL - Partners' Capital and Schedule of Units (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Common Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	3,021,258	3,021,258
2019 activity		0
GP Buy-In Transaction(1)	(132,687)
Common units issued for SMLP LTIP, net	95,987
TL Restructuring (Note 9)	2,306,972
Series A Preferred Unit Exchange Offer, net of shares withheld for taxes	817,845
Other	717
Units, ending balance (in shares)	6,110,092	3,021,258
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	300,000	300,000
2019 activity		0
Series A Preferred Unit Exchange Offer, net of shares withheld for taxes	(62,816)
Units, ending balance (in shares)	162,109	300,000
Series A Preferred Unit Tender	(75,075)

PARTNERS' CAPITAL AND MEZZANI_5

PARTNERS' CAPITAL AND MEZZANINE CAPITAL- Change in Subsidiary (Details)	12 Months Ended
	Dec. 31, 2020USD ($)shares
Partners Capital [Line Items]
Issuance costs	$ (3,900,000)
Series A Preferred Units
Partners Capital [Line Items]
Beginning Balance	$ 27,450,000
New issuances | shares	50,000
PIK distributions | shares	5,251
Issuance costs	$ (1,290,000)
Redemption accretion	8,247,000
Ending Balance	$ 89,658,000

PARTNERS' CAPITAL AND MEZZANI_6

PARTNERS' CAPITAL AND MEZZANINE CAPITAL - Parenthetical (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Equity [Abstract]
Issuance costs	$ 3.9

PARTNERS' CAPITAL AND MEZZANI_7

PARTNERS' CAPITAL AND MEZZANINE CAPITAL - Cash Distributions Paid and Declared (Details) - $ / shares	Jan. 29, 2020	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Per-unit annual distributions to unitholders (in dollars per unit)	$ 0.125	$ 0.125	$ 1.4375

EARNINGS PER UNIT (Details)

EARNINGS PER UNIT (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		$ 192,352	$ (184,451)
Deemed capital contribution from Series A Preferred Unit Exchange Offer		$ 110,669	$ 0
Phantom Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Nonvested anti-dilutive phantom units excluded from the calculation of diluted EPU		240	12
Limited Partner Interest Before Subordinated Units Converted To Common Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		$ 192,352	$ (184,451)
Series A Preferred Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		26,529	28,500
Deemed capital contribution from Series A Preferred Unit Exchange Offer		54,945
Deemed capital contribution from Series A Preferred Tender Offer		55,724
Series A Preferred Units | Subsidiaries
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		13,498	58
Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income (loss) attributable to limited partners		$ 262,994	$ (213,009)
Weighted-average common units outstanding – basic	[1]	3,592,000	3,021,000
Effect of nonvested phantom units		102
Weighted-average common units outstanding – diluted		3,694,000	3,021,000
Common unit – basic		$ 73.22	$ (70.50)
Common unit – diluted		$ 71.19	$ (70.50)
[1]	As a result of the GP Buy-In Transaction, our historical results are those of Summit Investments. The number of common units of 3.0 million as of December 31, 2019 represents those of Summit Investments and has been used for the earnings per unit calculation presented herein

EARNINGS PER UNIT (Parenthetica

EARNINGS PER UNIT (Parenthetical) (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common limited partner capital (in shares), outstanding	6,110,092	3,021,258
Summit Midstream Partners Holdings L L C | Common units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common limited partner capital (in shares), outstanding		3,000,000

SUPPLEMENTAL CASH FLOW INFORM_3

SUPPLEMENTAL CASH FLOW INFORMATION - Schedule of supplemental cash flow information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Supplemental cash flow information:
Cash interest paid	$ 79,450	$ 92,536
Capitalized interest paid	3,878	6,974
Interest paid (net of capitalized interest)	75,572	85,562
Cash paid for taxes	190	150
Noncash investing and financing activities:
Capital expenditures in trade accounts payable (period-end accruals)	6,154	19,846
Warrant issuance for GP Buy-In Transaction	2,300
Asset contribution to an equity method investment		23,643
Fair value of SMLP equity for TL Restructuring	30,521
ASC Topic 842
Noncash investing and financing activities:
Right-of-use assets relating to ASC Topic 842	3,685	$ 5,448
Series A Preferred Units
Noncash investing and financing activities:
Accretion of Subsidiary Series A Preferred Units	$ 8,247

UNIT-BASED AND NONCASH COMPEN_3

UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average remaining vesting period	1 year 4 months 24 days
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)	1,000,000
Units remaining available	400,000
Unrecognized unit-based compensation | $	$ 3.4

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, 2020	Dec. 31, 2019
Units
Nonvested units (in shares)	140,400	57,608
Units granted (in shares)	345,997	127,540
Unit vested (in shares)	(193,349)	(40,174)
Units forfeited (in shares)	(1,357)	(4,574)
Nonvested units (in shares)	291,691	140,400
Weighted-average grant date fair value
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 115.35	$ 256.65
Units granted, Weighted-average grant date fair value (in dollars per unit)	9.20	97.20
Units vested, Weighted-average grant date fair value (in dollars per unit)	54.07	251.70
Units forfeited, Weighted-average grant date fair value (in dollars per unit)	119.24	193.05
Nonvested, Weighted-average grant date fair value, beginning (in dollars per unit)	$ 26.57	$ 115.35

UNIT-BASED AND NONCASH COMPEN_5

UNIT-BASED AND NONCASH COMPENSATION - Schedule of Intrinsic Values (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
SMLP LTIP | Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of vested LTIP awards	$ 2,602	$ 5,940

UNIT-BASED AND NONCASH COMPEN_6

UNIT-BASED AND NONCASH COMPENSATION - SMP Net Profits Interests (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 8,111	$ 8,171
SMLP LTIP | General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SMLP LTIP unit-based compensation	$ 8,111	$ 8,171

LEASES - Narrative (Details)

LEASES - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)
Loss Contingencies [Line Items]
Finance leases, term of contract	3 years
Lessee, operating lease, assumptions and judgments, discount rate, description	The rate implicit in the lease contracts are not readily determinable. In determining the discount rate used in for lease liabilities, the Partnership analyzed certain factors in its incremental borrowing rate, including collateral assumptions and the term used. The incremental borrowing rate on the Revolving Credit Facility was 2.90% at December 31, 2020, which reflects the fixed rate at which the Partnership could borrow a similar amount, for a similar term and with similar collateral as in the lease contracts at the commencement d
Revolving credit facility
Loss Contingencies [Line Items]
Finance operating Lease obligations not exceed consolidated total assets	$ 50,000,000
Percentage of operating lease incremental borrowing rate	2.90%
Minimum | Office Space
Loss Contingencies [Line Items]
Operating leases, term of contract	3 years
Minimum | Equipment
Loss Contingencies [Line Items]
Operating leases, term of contract	3 years
Maximum | Office Space
Loss Contingencies [Line Items]
Operating leases, term of contract	10 years
Maximum | Equipment
Loss Contingencies [Line Items]
Operating leases, term of contract	4 years

LEASES - Schedule of Right of U

LEASES - Schedule of Right of Use Assets and Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating	$ 3,736	$ 3,580
Finance	1,748	3,159
Total	5,484	6,739
Lease liabilities, current
Operating	$ 2,298	$ 1,221
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherCurrentLiabilitiesMember	us-gaap:OtherCurrentLiabilitiesMember
Finance	$ 618	$ 1,246
Total	2,916	2,467
Lease liabilities, noncurrent
Operating	$ 3,182	$ 2,513
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentLiabilitiesMember	us-gaap:OtherNoncurrentLiabilitiesMember
Finance	$ 154	$ 676
Total	$ 3,336	$ 3,189

LEASES - Schedule of Lease Cost

LEASES - Schedule of Lease Cost and Other Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease cost
Amortization of ROU assets (included in depreciation and amortization)	$ 1,274	$ 1,559
Interest on lease liabilities (included in interest expense)	52	102
Operating lease cost (included in general and administrative expense)	2,644	3,345
Total lease cost	3,970	5,006
Other information
Operating cash outflows from operating leases	1,472	3,396
Operating cash outflows from finance leases	52	102
Financing cash outflows from finance leases	1,150	1,873
ROU assets obtained in exchange for new operating lease liabilities	3,552	1,218
ROU assets obtained in exchange for new finance lease liabilities	$ 133	$ 1,350
Weighted-average remaining lease term (years) - operating leases	4 years 10 months 24 days	5 years 9 months 18 days
Weighted-average remaining lease term (years) - finance leases	1 year 4 months 24 days	2 years
Weighted-average discount rate - operating leases	5.00%	5.00%
Weighted-average discount rate - finance leases	4.00%	4.00%

LEASES - Rent Expense (Details)

LEASES - Rent Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Lease expense	$ 3,436	$ 4,038

LEASES - Future Minimum Lease P

LEASES - Future Minimum Lease Payments Due and Future Minimum Rentals to be Received under Noncancelable Leases (Details) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
Operating lease, Future minimum lease payments, 2021	$ 1,750
Operating lease, Future minimum lease payments, 2022	1,308
Operating lease, Future minimum lease payments, 2023	854
Operating lease, Future minimum lease payments, 2024	573
Operating lease, Future minimum lease payments, 2025	464
Operating lease, Future minimum lease payments, 2026	156
Operating lease, Future minimum lease payments, Thereafter	579
Operating lease, Total future minimum lease payments	5,684
Finance lease, Future minimum lease payments, 2021	655
Finance lease, Future minimum lease payments, 2022	126
Finance lease, Future minimum lease payments, 2023	42
Finance lease, Total future minimum lease payments	$ 823

DISPOSITIONS - Red Rock Gatheri

DISPOSITIONS - Red Rock Gathering Asset Disposition (Details) - USD ($) $ in Thousands	Dec. 02, 2019	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Proceeds from asset sale (net of cash of $1,475 for the year ended December 31, 2019)			$ 0	$ 102,111
Long-lived asset impairment			$ 13,089	$ 60,507
Red Rock Gathering Company L L C | Disposal, not discontinued operations | Purchase and Sale Agreement
Business Acquisition [Line Items]
Proceeds from asset sale (net of cash of $1,475 for the year ended December 31, 2019)		$ 12,000
Long-lived asset impairment	$ 14,200

DISPOSITIONS - Tioga Midstream

DISPOSITIONS - Tioga Midstream Disposition (Details) - Tioga Midstream, LLC - Disposal, not discontinued operations - Purchase and Sale Agreement - USD ($) $ in Millions	Mar. 22, 2019	Mar. 31, 2019
Business Acquisition [Line Items]
Cash purchase price of disposals		$ 90
Gain on sale of business	$ 0.9

RESTRUCTURING - Additional info

RESTRUCTURING - Additional information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
2020 Restructuring Plan
Restructuring Cost And Reserve [Line Items]
Restructuring partnership expenses	$ 5.6
2019 Restructuring Plan
Restructuring Cost And Reserve [Line Items]
Restructuring partnership expenses	$ 3.5
Restructuring costs		$ 4.9

RESTRUCTURING - Restructuring a

RESTRUCTURING - Restructuring and related costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Severance Charges	$ 7,750	$ 3,767
Other Restructuring Charges	1,349	1,187
Total Severance and Other	9,099	4,954
Accrued, Beginning Balance	2,816
Charges Incurred	9,099	4,954
Cash Payments	(8,367)	(2,138)
Accrued, Ending Balance	3,548	2,816
Employee-related costs
Restructuring Cost And Reserve [Line Items]
Accrued, Beginning Balance	2,816
Charges Incurred	7,750	3,767
Cash Payments	(7,018)	(951)
Accrued, Ending Balance	3,548	2,816
Other
Restructuring Cost And Reserve [Line Items]
Charges Incurred	1,349	1,187
Cash Payments	(1,349)	(1,187)
2020 Restructuring Plan
Restructuring Cost And Reserve [Line Items]
Severance Charges	5,591
Other Restructuring Charges	56
Total Severance and Other	5,647
2019 Restructuring Plan
Restructuring Cost And Reserve [Line Items]
Severance Charges	2,159	3,767
Other Restructuring Charges	1,293	1,187
Total Severance and Other	$ 3,452	$ 4,954

SEGMENT INFORMATION - Assets, R

SEGMENT INFORMATION - Assets, Revenues, Depreciation and Amortization, and Capital Expenditures by Reportable Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 2,499,817	$ 2,574,098
Revenues	383,473	443,528
Depreciation and amortization	119,070	111,574
Capital expenditures	43,128	182,291
Reportable Segments
Segment Reporting Information [Line Items]
Assets	2,353,741	2,490,945
Revenues	380,897	415,906
Depreciation and amortization	115,718	108,822
Capital expenditures	39,515	163,028
Reportable Segments | Utica Shale
Segment Reporting Information [Line Items]
Assets	209,425	206,368
Revenues	36,509	33,991
Depreciation and amortization	7,696	7,659
Capital expenditures	6,957	3,902
Reportable Segments | Ohio Gathering
Segment Reporting Information [Line Items]
Assets	259,888	275,000
Reportable Segments | Williston Basin
Segment Reporting Information [Line Items]
Assets	425,873	452,152
Revenues	92,695	105,651
Depreciation and amortization	25,911	19,829
Capital expenditures	8,767	30,861
Reportable Segments | DJ Basin
Segment Reporting Information [Line Items]
Assets	199,920	205,308
Revenues	28,070	26,050
Depreciation and amortization	6,146	3,732
Capital expenditures	12,829	80,487
Reportable Segments | Permian Basin
Segment Reporting Information [Line Items]
Assets	165,765	185,708
Revenues	29,533	20,303
Depreciation and amortization	5,455	4,868
Capital expenditures	7,014	44,955
Reportable Segments | Piceance Basin
Segment Reporting Information [Line Items]
Assets	579,800	631,140
Revenues	113,890	133,638
Depreciation and amortization	45,203	47,018
Capital expenditures	1,370	1,946
Reportable Segments | Barnett Shale
Segment Reporting Information [Line Items]
Assets	336,629	350,638
Revenues	54,459	71,802
Depreciation and amortization	16,112	16,575
Capital expenditures	1,878	184
Reportable Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Assets	176,441	184,631
Revenues	25,741	24,471
Depreciation and amortization	9,195	9,141
Capital expenditures	700	693
Corporate and Other
Segment Reporting Information [Line Items]
Assets	146,076	83,153
Revenues	2,576	30,552
Depreciation and amortization	3,352	2,752
Capital expenditures	$ 3,613	19,263
Eliminations
Segment Reporting Information [Line Items]
Revenues		$ (2,930)

SEGMENT INFORMATION - Narrative

SEGMENT INFORMATION - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Equity method investments	$ 392,740	$ 309,728
Capital expenditures	43,128	182,291
Corporate
Segment Reporting Information [Line Items]
Capital expenditures		1,600
Corporate and Other
Segment Reporting Information [Line Items]
Capital expenditures	3,613	19,263
Corporate and Other | Double E
Segment Reporting Information [Line Items]
Equity method investments	132,900	34,700
Reportable Segments
Segment Reporting Information [Line Items]
Capital expenditures	39,515	163,028
Reportable Segments | Barnett Shale
Segment Reporting Information [Line Items]
Sales tax reimbursement		1,100
Capital expenditures	$ 1,878	$ 184

SEGMENT INFORMATION - Concentra

SEGMENT INFORMATION - Concentration Risk (Details) - Sales Revenue, Net - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Counterparty A - Piceance Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	13.00%	11.00%
Counterparty B - Williston Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage		10.00%
Counterparty C - Utica Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	5.00%
Counterparty C - Permian Basin
Segment Reporting Information [Line Items]
Concentration risk, percentage	5.00%
Counterparty C - Barnett Shale
Segment Reporting Information [Line Items]
Concentration risk, percentage	1.00%

SEGMENT INFORMATION - Adjusted

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 282,702	$ 317,503
Utica Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	32,783	29,292
Ohio Gathering
Segment Reporting Information [Line Items]
Adjusted EBITDA	31,056	39,126
Williston Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	52,060	69,437
DJ Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	19,449	18,668
Permian Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	4,426	(879)
Piceance Basin
Segment Reporting Information [Line Items]
Adjusted EBITDA	88,820	98,765
Barnett Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	32,093	43,043
Marcellus Shale
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 22,015	$ 20,051

SEGMENT INFORMATION - Reconcili

SEGMENT INFORMATION - Reconciliation of Net Income (Loss) to Adjusted EBITDA (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting [Abstract]
Income (loss) before income taxes and equity method investment income (loss)	$ 177,661	$ (54,644)
Add:
Corporate and Other expense	59,585	44,808
Interest expense	78,894	91,966
Gain on early extinguishment of debt	(203,062)	0
Depreciation and amortization	119,070	111,574
Proportional adjusted EBITDA for equity method investees	31,056	39,126
Adjustments related to MVC shortfall payments		3,476
Adjustments related to capital reimbursement activity	(1,395)	(2,156)
Unit-based and noncash compensation	8,111	8,171
Gain on asset sales, net	(307)	(1,536)
Long-lived asset impairment	13,089	60,507
Goodwill impairment	0	16,211
Total of reportable segments' measures of profit	$ 282,702	$ 317,503

SUBSEQUENT EVENTS - Narrative (

SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ in Millions	May 28, 2020	Feb. 28, 2021
Subsequent Event [Line Items]
Line of credit facility description	(i) a Term Loan Credit Agreement (the “ECP NewCo Term Loan Credit Agreement”), with SMP TopCo, LLC, a Delaware limited liability company and affiliate of ECP (“ECP NewCo”), as lender and administrative agent, and Mizuho Bank (USA) (“Mizuho”), as collateral agent, in a principal amount of $28.2 million (the “ECP NewCo Loan”), and (ii) a Term Loan Credit Agreement (the “ECP Holdings Term Loan Credit Agreement” and together with the ECP NewCo Term Loan Credit Agreement, the “ECP Term Loan Credit Agreements”), with ECP Holdings, as lender, and ECP NewCo, as administrative agent and Mizuho, as collateral agent, in a principal amount of $6.8 million (the “ECP Holdings Loan” and together with the ECP NewCo Loan, the “ECP Loans”). The ECP Loans were set to mature on March 31, 2021 and bore interest at a fixed rate of 8.00% per annum, with the interest payment due at maturity of the ECP Loans. With borrowings under the Partnership’s Revolving Credit Facility, the Partnership fully repaid all amounts outstanding under the ECP Loans ($35 million of principal and $0.6 million of accrued interest) on August 7, 2020.
Subsequent Event
Subsequent Event [Line Items]
Proceeds from secured lines of credit		$ 175
Line of credit facility description		The Credit Facilities are non-recourse to SMLP and mature seven years after the date of initial borrowing.
Line of credit facility, expiration period		7 years
Subsequent Event | Term Loan Credit Facility
Subsequent Event [Line Items]
Proceeds from secured lines of credit		$ 160
Subsequent Event | Working Capital Credit Facility
Subsequent Event [Line Items]
Proceeds from secured lines of credit		15
Subsequent Event | Revolving credit facility
Subsequent Event [Line Items]
Letter of credit		$ 15