INVENTORIES (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Inventory Disclosure [Abstract]
Coal stockpiles	$ 44,692				$ 38,934
Coal fuel inventories	6,816				7,194
Materials and supplies	77,628				78,784
Reserve for obsolete inventory	(3,621)				(2,756)
Total	$ 125,515	$ 129,907	$ 130,215	$ 143,765	$ 122,156	$ 124,451	$ 138,822	$ 134,036