ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in benefit obligations:
Benefit obligations at beginning of year	$ 61,693	$ 56,386
Service cost	2,578	3,081	$ 3,424
Interest cost	2,506	2,094	2,262
Actuarial loss	205	750
Acquisition		790
Benefits and expenses paid	(1,994)	(1,408)
Benefit obligations at end of year	64,988	61,693	56,386
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial (gain) loss	$ (1,382)	$ (4,230)	$ (5,431)