HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 2,376	$ 3,089	$ 4,650	$ 6,266
Combined benefit fund payments	512	576	1,025	1,152
Workers' compensation benefits	124	117	260	232
Black lung benefits (credit)	376	(673)	997	(590)
Total	$ 3,388	$ 3,109	$ 6,932	$ 7,060