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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accrued Workers Compensation And Pneumoconiosis Benefits      
Workers' compensation expense $ 17,541 $ 11,270 $ 12,215
Total expense 18,596 16,339 14,933
Pneumoconiosis benefits      
Accrued Workers Compensation And Pneumoconiosis Benefits      
Service cost 2,593 2,525 2,255
Interest cost 3,044 2,542 2,555
Net amortization (4,582) 2 (2,092)
Net periodic benefit cost $ 1,055 $ 5,069 $ 2,718