ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total adjustments	$ (31,122)	$ 5,069	$ (13,400)
Pneumoconiosis benefits
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial loss	(23,298)	4,599	(7,938)
Reversal of amortization item: net actuarial gain	(4,582)	2	(2,092)
Total adjustments	$ (27,880)	$ 4,601	$ (10,030)
Estimated present value of future obligations and other information
Pneumoconiosis discount rate	3.12%	4.13%	3.49%
Amount recognized in accumulated other comprehensive income consists of:
Net actuarial loss	$ 31,927	$ 4,047	$ 8,648

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