ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Workers' Compensation Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the changes in workers' compensation liability
Beginning balance	$ 54,558	$ 57,557
Accruals	10,450	13,220
Payments	(10,415)	(8,906)
Interest accretion	1,967	1,954
Valuation gain	(8,429)	(9,267)
Ending balance	$ 48,131	$ 54,558	$ 57,557
Estimated present value of future obligations and other informaton
Workers' compensation discount rate	3.52%	3.63%	3.41%