ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accrued Workers Compensation And Pneumoconiosis Benefits
Workers’ compensation expense	$ 12,215	$ 9,063	$ 9,759
Total expense	14,933	11,504	14,483
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost	2,255	2,578	3,081
Interest cost	2,555	2,506	2,094
Net amortization	(2,092)	(2,643)	(451)
Net periodic benefit cost	$ 2,718	$ 2,441	$ 4,724