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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - Pneumoconiosis benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accrued Workers Compensation And Pneumoconiosis Benefits      
Service cost $ 3,526 $ 2,593 $ 2,525
Interest cost 2,998 3,044 2,542
Net amortization (686) (4,582) 2
Net periodic benefit cost 5,838 1,055 5,069
Workers' compensation expense 12,305 17,541 11,270
Total pneumoconiosis expense $ 18,143 $ 18,596 $ 16,339