WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Workers' Compensation Liability (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the changes in workers' compensation liability
Beginning balance	$ 56,403	$ 55,671	$ 54,558	$ 57,557	$ 57,557
Accruals increase	2,480	2,959	7,904	9,126
Payments	(2,238)	(2,250)	(8,226)	(6,864)
Interest accretion	492	489	1,476	1,466
Valuation loss (gain)			1,425	(4,416)
Ending balance	$ 57,137	$ 56,869	$ 57,137	$ 56,869	$ 54,558	$ 57,557
Workers' compensation discount rate			2.89%	3.71%	3.63%	3.41%