ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accrued Workers Compensation And Pneumoconiosis Benefits
Total recognized in accumulated other comprehensive income/(loss)		$ 1,290	$ (26,128)	$ 32,545
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Net actuarial (loss) gain		(750)	(2,029)	16,750
Reversal of amortization item: net actuarial (gain) loss	[1]	(451)	(1,051)	670
Total recognized in accumulated other comprehensive income/(loss)		$ (1,201)	$ (3,080)	$ 17,420

[1] Amortization of actuarial gain or loss is included in the computation of net periodic benefit cost (see Notes 14 and 18 for additional details).