ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total recognized in accumulated other comprehensive loss	$ 23,445	$ (9,681)	$ (31,122)
Pneumoconiosis benefits
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial loss	(161)	(7,787)	(23,298)
Reversal of amortization item: Net actuarial loss (gain)	4,172	(686)	(4,582)
Total recognized in accumulated other comprehensive loss	$ 4,011	$ (8,473)	$ (27,880)
Estimated present value of future obligations and other information
Pneumoconiosis discount rate	2.73%	2.38%	3.12%
Amount recognized in accumulated other comprehensive income consists of:
Net actuarial loss	$ 36,388	$ 40,399	$ 31,927

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Amount, after tax and reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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