PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	0	0	0
Subordinated units conversion		0
Units issued in connection with offering	300,000	0	0
Net units issued under SMLP LTIP (in shares)	0	0	0
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	300,000	0	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	72,111,121	42,062,644	34,426,513
Subordinated units conversion		24,409,850
Units issued in connection with offering	0	5,500,000	7,475,000
Net units issued under SMLP LTIP (in shares)	211,327	138,627	161,131
Units issued under ATM program (in shares)	763,548		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	73,085,996	72,111,121	42,062,644
Limited partners, Subordinated
Rollforwards of the number of partner units
Units, beginning balance (in shares)	0	24,409,850	24,409,850
Subordinated units conversion		(24,409,850)
Units issued in connection with offering	0	0	0
Net units issued under SMLP LTIP (in shares)	0	0	0
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	0	0	0
Units, ending balance (in shares)	0	0	24,409,850
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,471,187	1,354,700	1,200,651
Subordinated units conversion		0
Units issued in connection with offering	0	0	0
Net units issued under SMLP LTIP (in shares)	0	4,242	1,498
Units issued under ATM program (in shares)	0		0
General Partner 2% capital contribution (in shares)	19,812	112,245	152,551
Units, ending balance (in shares)	1,490,999	1,471,187	1,354,700