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RELATED-PARTY TRANSACTIONS - Affliliate Royalty Agreements - SGP Land and SGP (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
SGP Land, LLC | MC Mining LLC      
Related Party Transaction      
Payments for royalties $ 0.6 $ 1.9 $ 0.9
SGP Land, LLC | MC Mining LLC | Mineral lease      
Related Party Transaction      
Annual minimum royalties 0.3    
Cumulative annual minimum and/or earned royalty payments 6.0    
SGP | Tunnel Ridge | Coal lease      
Related Party Transaction      
Annual minimum royalties 3.0    
Accrued earned royalties payable $ 0.9