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ACQUISITIONS - MAC (Details) - USD ($)
$ in Thousands
1 Months Ended
Jan. 01, 2015
Mar. 31, 2006
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Acquisitions          
Goodwill     $ 136,399 $ 136,399  
White County Coal | MAC          
Acquisitions          
Purchase of equity investment   $ 1,000      
MAC          
Acquisitions          
Equity interest (as a percent)   50.00%      
Balance of equity investment         $ 1,600
Remaining equity interest acquired (as a percent) 50.00%        
Cash on hand $ 5,500        
Liabilities assumed 200        
Assets acquired 7,300        
Goodwill $ 4,200