HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 3,179	$ 2,850	$ 9,446	$ 8,526
Combined benefit fund payments	576	561	1,727	1,682
Workers' compensation benefits	99	121	331	382
Black lung benefits (credit)	203	349	(387)	1,153
Total	$ 4,057	$ 3,881	$ 11,117	$ 11,743