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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of the changes in black lung benefit obligations      
Benefit obligations at beginning of year $ 97,683 $ 72,095  
Service cost 3,526 2,593 $ 2,525
Interest cost 2,998 3,044 2,542
Actuarial (gain) loss 7,787 23,298  
Benefits and expenses paid (3,498) (3,347)  
Benefit obligations at end of year $ 108,496 $ 97,683 $ 72,095