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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 2,404	$ 3,179	$ 7,054	$ 9,446
Combined benefit fund payments	342	576	1,366	1,727
Workers' compensation benefits	120	99	380	331
Black lung benefits	449	203	1,446	(387)
Total	$ 3,315	$ 4,057	$ 10,246	$ 11,117