INVENTORIES (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal stockpiles	$ 37,313	$ 41,795
Coal fuel inventories	8,769	6,531
Materials and supplies	81,032	88,584
Reserve for obsolete inventory	(2,676)	(3,055)
Total	$ 124,438	$ 133,855