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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Accrued Workers Compensation And Pneumoconiosis Benefits        
Interest cost $ 1,054 $ 1,216 $ 2,108 $ 2,432
Net amortization 1,063 982 2,127 1,961
Net periodic benefit cost 673 1,010 1,345 2,020
Pneumoconiosis benefits        
Accrued Workers Compensation And Pneumoconiosis Benefits        
Service cost 883 646 1,761 1,291
Interest cost 749 762 1,499 1,522
Net amortization (171) (1,146) (343) (2,291)
Net periodic benefit cost $ 1,461 $ 262 $ 2,917 $ 522