INVENTORIES (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Inventory Disclosure [Abstract]
Coal stockpiles	$ 51,960	$ 38,636
Coal fuel inventories	7,619	7,194
Materials and supplies	86,741	78,784
Reserve for obsolete inventory	(2,921)	(2,756)
Total	$ 143,399	$ 121,858