ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of the changes in workers' compensation liability
Balance at the beginning of the period	$ 73,201	$ 67,687
Accruals	24,812	22,254
Payments	(10,477)	(11,235)
Interest accretion	2,739	3,174
Valuation gain	(13,229)	(8,679)
Balance at the end of the period	$ 77,046	$ 73,201