HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Heritage Health Benefit Expenses [Abstract]
Health care benefits	$ 8,179	$ 9,473	$ 12,579
Combined benefit fund payments	1,794	1,966	2,240
Workers' compensation benefits (credit)	(583)	230	(1,212)
Black lung benefits (credit)	5,183	1,719	(189)
Total	$ 14,573	$ 13,388	$ 13,418