ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of the changes in black lung benefit obligations
Benefit obligations at beginning of year	$ 111,316	$ 108,496
Service cost	3,798	4,021	$ 3,526
Interest cost	2,991	2,545	2,998
Actuarial loss (gain)	(9,840)	161
Benefits and expenses paid	(3,978)	(3,907)
Benefit obligations at end of year	$ 104,287	$ 111,316	$ 108,496