WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost		$ 644	$ 732	$ 1,292	$ 1,464
Interest cost		626	523	1,253	1,047
Amortization of net actuarial gain	[1]	(661)	(112)	(1,322)	(225)
Net periodic benefit cost		$ 609	$ 1,143	$ 1,223	$ 2,286

[1] Amortization of net actuarial gain is included in the Operating expenses (excluding depreciation, depletion and amortization) line item within our condensed consolidated statements of income.