ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligations:
Benefit obligations at beginning of year	$ 64,988	$ 61,693
Service cost	2,255	2,578	$ 3,081
Interest cost	2,555	2,506	2,094
Actuarial loss	7,938	205
Benefits and expenses paid	(2,877)	(1,994)
Benefit obligations at end of year	$ 74,859	$ 64,988	$ 61,693