INVENTORIES (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Inventory Disclosure [Abstract]
Coal stockpiles	$ 929	$ 989
Coal fuel inventories	5,227	3,048
Materials and supplies	35,402	34,954
Reserve for obsolete inventory	(1,137)	(1,257)
Total	$ 40,421	$ 37,734