UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 127,479	$ 124,945	$ 372,982	$ 362,542
Costs and expenses:
Cost of natural gas and NGLs	$ 26,879	$ 18,177	$ 71,549	$ 36,328
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Operation and maintenance	$ 24,382	$ 22,303	$ 73,452	$ 70,011
General and administrative	11,740	13,289	39,666	40,370
Depreciation and amortization	26,743	28,927	80,204	86,184
Transaction costs	0	0	0	119
Loss (gain) on asset sales, net	6	460	(6)	530
Long-lived asset impairment	1,540	1,290	2,127	1,577
Total costs and expenses	91,290	84,446	266,992	235,119
Other income	58	79	78	214
Interest expense	(14,862)	(17,614)	(44,821)	(51,883)
Early extinguishment of debt	0	0	0	(22,020)
Deferred Purchase Price Obligation	37,204	70,499	(53,759)	54,674
Income before income taxes and (loss) income from equity method investees	58,589	93,463	7,488	108,408
Income tax benefit (expense)	35	(176)	(88)	(417)
(Loss) income from equity method investees	(1,169)	350	(3,703)	(3,691)
Net income	57,455	93,637	3,697	104,300
Net income attributable to noncontrolling interest	25	91	168	282
Net income attributable to SMLP	57,430	93,546	3,529	104,018
Less net income attributable to General Partner, including IDRs	3,279	3,999	6,477	8,442
Net income (loss) attributable to limited partners	54,151	89,547	(2,948)	95,576
Series A Preferred Units
Costs and expenses:
Net income (loss) attributable to limited partners	7,125	0	21,375	0
Common units
Costs and expenses:
Net income (loss) attributable to limited partners	$ 47,026	$ 89,547	$ (24,323)	$ 95,576
Earnings (loss) per limited partner unit:
Basic (in dollars per share)	$ 0.64	$ 1.23	$ (0.33)	$ 1.32
Diluted (in dollars per share)	$ 0.64	$ 1.22	$ (0.33)	$ 1.31
Weighted-average limited partner units outstanding:
Basic (shares)	73,356	73,059	73,283	72,583
Diluted (shares)	73,756	73,433	73,283	72,901
Gathering Services and Related Fees
Revenues:
Total revenues	$ 86,427	$ 96,070	$ 260,373	$ 298,884
Natural Gas, NGLs and Condensate Sales
Revenues:
Total revenues	34,017	22,940	92,025	44,655
Other Revenues
Revenues:
Total revenues	$ 7,035	$ 5,935	$ 20,584	$ 19,003