PARTNERS' CAPITAL - Partners' Capital and Schedule of Units (Details)	3 Months Ended
	Mar. 31, 2019 shares
Series A Preferred Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	300,000
Units, ending balance (in shares)	300,000
Conversion of General Partner economic interests (in shares)	0
Net units issued under the SMLP LTIP (in shares)	0
Common
Rollforwards of the number of partner units
Units, beginning balance (in shares)	73,390,853
Units, ending balance (in shares)	82,695,311
Conversion of General Partner economic interests (in shares)	8,750,000
Net units issued under the SMLP LTIP (in shares)	554,458
General Partner Units
Rollforwards of the number of partner units
Units, beginning balance (in shares)	1,490,999
Units, ending balance (in shares)	0
Conversion of General Partner economic interests (in shares)	(1,490,999)
Net units issued under the SMLP LTIP (in shares)	0