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LONG-LIVED ASSET IMPAIRMENTS (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Aug. 16, 2019
Aug. 15, 2019
Sep. 30, 2018
Asset impairment charges            
Asset impairments $ 24,977 $ 15,190 $ 40,483      
Total assets 2,166,016 2,586,694 2,394,748      
Dotiki Mine            
Asset impairment charges            
Asset impairments   15,200 34,300      
Total assets     51,000 $ 25,800 $ 35,900 $ 85,300
Accrued payments       $ 5,100    
Coal reserves not to exercise lease option            
Asset impairment charges            
Asset impairments     6,200      
Illinois Basin            
Asset impairment charges            
Asset impairments 25,000          
Total assets $ 1,018,916 $ 1,373,516 $ 1,380,912