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INVENTORIES (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
INVENTORIES    
Coal $ 49,804 $ 19,756
Supplies (net of reserve for obsolescence of $5,545 and $5,547, respectively) 35,177 36,651
Total inventories, net 84,981 56,407
Reserve for obsolescence $ 5,545 $ 5,547