ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Black lung benefits:
Service cost	$ 3,798	$ 4,021	$ 3,526
Interest cost	$ 2,991	$ 2,545	$ 2,998
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)	Other Nonoperating Income (Expense)
Net amortization	$ 1,038	$ 4,172	$ (686)
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)	Other Nonoperating Income (Expense)
Total pneumoconiosis expense	$ 7,827	$ 10,738	$ 5,838
Workers' compensation expense	11,675	8,339	12,305
Net periodic benefit cost	$ 19,502	$ 19,077	$ 18,143