ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accrued Workers Compensation And Pneumoconiosis Benefits
Total recognized adjustments		$ (3,983)	$ 1,290	$ (26,128)
Estimated present value of future obligations and other informaton
Letters of credit to secure workers compensation obligation		89,100	90,000
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Net actuarial loss		(205)	(750)	(2,029)
Reversal of amortization item: net actuarial (gain) loss	[1]	(2,643)	(451)	(1,051)
Total recognized adjustments		$ (2,848)	$ (1,201)	$ (3,080)
Estimated present value of future obligations and other informaton
Pneumoconiosis discount rate		3.97%	4.16%	3.82%

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