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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - Pneumoconiosis benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Black lung benefits:      
Service cost $ 4,021 $ 3,526 $ 2,593
Interest cost 2,545 2,998 3,044
Net amortization 4,172 (686) (4,582)
Total pneumoconiosis expense 19,077 18,143 18,596
Workers' compensation expense 8,339 12,305 17,541
Net periodic benefit cost $ 10,738 $ 5,838 $ 1,055