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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands | 12 Months Ended
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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

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