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