ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accrued Workers Compensation And Pneumoconiosis Benefits
Workers’ compensation expense	$ 9,063	$ 9,759	$ 7,776
Total expense	11,504	14,483	12,411
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost	2,578	3,081	3,424
Interest cost	2,506	2,094	2,262
Net amortization	(2,643)	(451)	(1,051)
Net periodic benefit cost	$ 2,441	$ 4,724	$ 4,635