INVENTORIES (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Coal stockpiles	$ 31,170	$ 44,692
Coal fuel inventories	5,384	6,816
Materials and supplies	87,386	77,628
Reserve for obsolete inventory	(3,642)	(3,621)
Total	$ 120,298	$ 125,515