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