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, 2014	Mar. 31, 2013
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 2,276	$ 3,176
Combined benefit fund payments	512	576
Workers' compensation benefits	136	116
Black lung benefits (credit)	620	83
Total	$ 3,544	$ 3,951