ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total defined benefit pension plan adjustments	$ (9,681)	$ (31,122)	$ 5,069
Pneumoconiosis benefits
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial gain (loss)	(7,787)	(23,298)	4,599
Reversal of amortization item: Net actuarial (gain) loss	(686)	(4,582)	2
Total defined benefit pension plan adjustments	$ (8,473)	$ (27,880)	$ 4,601
Estimated present value of future obligations and other information
Pneumoconiosis discount rate	2.38%	3.12%	4.13%
Amount recognized in accumulated other comprehensive income consists of:
Net actuarial loss	$ 40,399	$ 31,927	$ 4,047

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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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