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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Workers' Compensation Liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of changes in the workers' compensation liability    
Beginning balance $ 49,539 $ 54,439
Accruals increase 7,162 7,654
Payments (11,320) (10,837)
Interest accretion 1,606 1,454
Valuation gain 6,397 (3,171)
Ending balance $ 53,384 $ 49,539
Estimated present value of future obligations and other information    
Workers' compensation discount rate 2.81% 3.89%
Letters of credit outstanding to secure workers compensation obligations $ 90,200 $ 90,500
Other long-term assets    
Estimated present value of future obligations and other information    
Receivables for traumatic injury claims $ 7,700 $ 8,100