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COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)
1 Months Ended 9 Months Ended 12 Months Ended
Jan. 31, 2017
Sep. 30, 2018
Dec. 31, 2015
Maximum | Meadowlark Midstream Gathering System | Principal Owner      
Loss Contingencies [Line Items]      
Coverage from pollution liability insurance policy   $ 25,000,000.0 $ 25,000,000.0
Coverage from property and business interruption insurance policy   $ 200,000,000.0  
Business Interruption      
Loss Contingencies [Line Items]      
Recoveries recognized $ 2,600,000    
Property Recoveries      
Loss Contingencies [Line Items]      
Recoveries recognized $ 400,000