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VARIABLE INTEREST ENTITIES - Other VIEs (Details) - company
12 Months Ended
Jul. 30, 2015
Dec. 31, 2017
Nov. 17, 2014
Variable Interest Entities      
Percentage of available cash distributed   100.00%  
MGP | ARLP      
Variable Interest Entities      
Percentage of available cash distributed   100.00%  
ARLP      
Variable Interest Entities      
Ownership interest (as a percent)   66.70%  
Ownership percentage by general partners   1.0001%  
MGP      
Variable Interest Entities      
Ownership interest (as a percent)   100.00%  
Variable Interest Entity, Not Primary Beneficiary | White Oak | ARLP      
Variable Interest Entities      
Ownership interest in VIE (as a percent) 40.00%    
Variable Interest Entity, Primary Beneficiary | ARLP | Limited Partners' Capital      
Variable Interest Entities      
Ownership interest (as a percent)   66.70%  
Variable Interest Entity, Primary Beneficiary | ARLP | General Partner Interest      
Variable Interest Entities      
Ownership percentage by general partners   100.00%  
WKY CoalPlay | Variable Interest Entity, Not Primary Beneficiary      
Variable Interest Entities      
Number of limited liability companies related to MGP, that formed the related party together with SGP Land     2