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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Accrued Workers Compensation And Pneumoconiosis Benefits        
Interest cost $ 860 $ 1,054 $ 1,720 $ 2,108
Net amortization 1,142 1,063 2,283 2,127
Net periodic benefit cost 377 673 754 1,345
Pneumoconiosis benefits        
Accrued Workers Compensation And Pneumoconiosis Benefits        
Service cost 989 883 2,020 1,761
Interest cost 637 749 1,273 1,499
Net amortization 1,043 (171) 2,086 (343)
Net periodic benefit cost $ 2,669 $ 1,461 $ 5,379 $ 2,917