ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in benefit obligations:
Benefit obligations at beginning of year	$ 56,386	$ 49,560
Service cost	3,081	3,424	$ 3,810
Interest cost	2,094	2,262	2,253
Actuarial loss	750	2,029
Acquisition	790
Benefits and expenses paid	(1,408)	(889)
Benefit obligations at end of year	61,693	56,386	49,560
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial (gain) loss	$ (4,230)	$ (5,431)	$ (8,511)