ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accrued Workers Compensation And Pneumoconiosis Benefits
Total recognized adjustments		$ (13,400)	$ (3,983)	$ 1,290
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Net actuarial loss		(7,938)	(205)	(750)
Reversal of amortization item: net actuarial (gain) loss	[1]	(2,092)	(2,643)	(451)
Total recognized adjustments		$ (10,030)	$ (2,848)	$ (1,201)
Other information
Pneumoconiosis discount rate		3.49%	3.97%	4.16%
Net actuarial loss (gain)		$ 8,648	$ (1,382)	$ (4,230)

[1] Amortization of prior service cost and actuarial gain or loss is included in the computation of net periodic benefit cost (see Notes 13 and 17 for additional details).