ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS (Details 5) (Black Lung, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Black Lung
Change in benefit obligations:
Benefit obligations at beginning of year	$ 60,991	$ 55,605
Service cost	3,810	3,758	3,345
Interest cost	2,253	2,372	2,382
Actuarial gain	(16,750)	(2,156)
Benefits and expenses paid	(744)	(715)
Acquisition of Onton (Note 3)		2,127
Benefit obligations at end of year	49,560	60,991	55,605
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial (gain) loss	$ (8,511)	$ 8,908