HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 2,041	$ 2,404	$ 6,138	$ 7,054
Combined benefit fund payments	462	342	1,387	1,366
Workers' compensation benefits	102	120	324	380
Black lung benefits	196	449	173	1,446
Total	$ 2,801	$ 3,315	$ 8,022	$ 10,246