ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Workers' Compensation Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the changes in workers' compensation liability
Beginning balance	$ 57,557	$ 62,909
Accruals increase	13,220	14,978
Payments	(8,906)	(10,563)
Interest accretion	1,954	2,585
Valuation gain	(9,267)	(12,352)
Ending balance	$ 54,558	$ 57,557