HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 3,176	$ 2,801
Combined benefit fund payments	576	561
Workers' compensation benefits (credit)	116	121
Black lung benefits	83	327
Total	$ 3,951	$ 3,810