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LONG-LIVED ASSET IMPAIRMENTS (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2020
Jun. 30, 2020
Asset impairment charges    
Asset impairments   $ 24,977
Illinois Basin    
Asset impairment charges    
Asset impairments $ 23,500  
Coal Royalties    
Asset impairment charges    
Asset impairments $ 1,500