ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - AOCL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total recognized in accumulated other comprehensive loss	$ 23,175	$ 23,445	$ (9,681)
Pneumoconiosis benefits
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial gain (loss)	9,840	(161)	(7,787)
Reversal of amortization item: Net actuarial loss (gain)	1,038	4,172	(686)
Total recognized in accumulated other comprehensive loss	$ 10,878	$ 4,011	$ (8,473)
Estimated present value of future obligations and other information
Pneumoconiosis discount rate	5.00%	2.73%	2.38%
Amount recognized in accumulated other comprehensive income consists of:
Net actuarial loss	$ 25,510	$ 36,388	$ 40,399