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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Obligations (Details) - Pneumoconiosis benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Reconciliation of the changes in black lung benefit obligations      
Benefit obligations at beginning of year $ 104,287 $ 111,316  
Service cost 2,698 3,798 $ 4,021
Interest cost 4,951 2,991 2,545
Actuarial loss (gain) 25,615 (9,840)  
Benefits and expenses paid (5,107) (3,978)  
Benefit obligations at end of year $ 132,444 $ 104,287 $ 111,316