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COMMITMENTS AND CONTINGENCIES - General Litigation (Details) - USD ($)
$ in Thousands
12 Months Ended
Mar. 09, 2018
Dec. 31, 2018
Loss Contingency, Information about Litigation Matters [Abstract]    
Settlement per agreement $ 93,000  
Purchase of coal reserves, agreed amount 2,000  
Legal and other costs of settlement 13,000  
Net gain from settlement $ 80,000 $ 80,000