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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Workers' Compensation Liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Reconciliation of changes in the workers' compensation liability    
Beginning balance $ 54,739 $ 53,384
Accruals increase 5,168 5,146
Payments (10,725) (8,482)
Interest accretion 926 1,278
Valuation loss 3,340 3,413
Ending balance $ 53,448 $ 54,739
Estimated present value of future obligations and other information    
Workers' compensation discount rate 2.41% 1.95%
Letters of credit outstanding to secure workers compensation obligations $ 100,400 $ 95,200
Other long-term assets    
Estimated present value of future obligations and other information    
Receivables for traumatic injury claims $ 5,700 $ 7,100