WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Workers' Compensation Liability (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the changes in workers' compensation liability
Beginning balance		$ 59,981	$ 56,403	$ 54,558	$ 48,131	$ 54,558	$ 54,558
Accruals increase		1,999	2,480		16,673	7,904
Payments		(2,357)	(2,238)		(7,799)	(8,226)
Interest accretion		421	492		1,261	1,476
Valuation loss					1,778	1,425
Ending balance	$ 59,981	$ 60,044	$ 57,137	$ 56,403	$ 60,044	$ 57,137	$ 48,131	$ 54,558
Workers' compensation discount rate	3.38%			2.89%			3.52%	3.63%