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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Benefit cost (Details) - Pneumoconiosis benefits - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Black lung benefits:    
Service cost $ 859 $ 861
Interest cost $ 1,672 $ 1,558
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense) Other Nonoperating Income (Expense)
Net amortization $ 226 $ 839
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Amortization of Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense) Other Nonoperating Income (Expense)
Net periodic benefit cost $ 2,757 $ 3,258