INVENTORIES (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal stockpiles	$ 38,636	$ 41,795
Coal fuel inventories	7,194	6,531
Materials and supplies	78,784	88,584
Reserve for obsolete inventory	(2,756)	(3,055)
Total	$ 121,858	$ 133,855