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ORGANIZATION AND PRESENTATION - Organization (Details) - shares
6 Months Ended
Jul. 28, 2017
Jun. 30, 2017
Ownership interests    
Ownership percentage by limited partners   100.00%
MGP II | ARLP | Subsequent event    
Ownership interests    
Units owned by parent 56,100,000  
Exchange Transaction | MGP | ARLP | Subsequent event    
Ownership interests    
Common units issued in exchange 56,100,000  
Exchange Transaction | SGP | ARLP | Subsequent event    
Ownership interests    
Common units issued in exchange 7,181  
MGP    
Ownership interests    
Subsidiaries holding percentage   100.00%
ARLP    
Ownership interests    
Units owned by parent   31,088,338
ARLP | Subsequent event    
Ownership interests    
Common units issued in exchange 56,100,000  
Units owned by parent 87,188,338  
ARLP | SGP    
Ownership interests    
Ownership percentage by noncontrolling owners   0.01%
ARLP | Exchange Transaction | SGP | Subsequent event    
Ownership interests    
Ownership percentage by noncontrolling owners 0.01%  
Intermediate Partnership | Subsequent event    
Ownership interests    
Ownership percentage by general partners 1.0001%  
Intermediate Partnership | SGP    
Ownership interests    
Ownership percentage by noncontrolling owners   0.01%
Intermediate Partnership | Exchange Transaction | SGP | Subsequent event    
Ownership interests    
Ownership percentage by noncontrolling owners 0.01%  
Alliance Coal | Subsequent event    
Ownership interests    
Ownership percentage by general partners 0.001%  
MGP II | Subsequent event    
Ownership interests    
Subsidiaries holding percentage 100.00%