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UNIT-BASED AND NONCASH COMPENSATION - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2013
Phantom Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period 3 years  
Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period   4 years
SMLP LTIP    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Reserved for issuance (in shares) 5,000,000  
Units remaining available 3,600,000  
Unrecognized unit-based compensation $ 5.7  
Remaining vesting period 2 years 2 months 12 days  
Forfeitures 0