Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 66,789	$ 70,410
Less allowance for bad debts	(4,722)	(5,380)
Accounts receivable, net	62,067	65,030
Inventories [Abstract]
Inventories	31,337	35,575
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,616,344	4,296,061
Less accumulated depreciation	(1,754,588)	(1,701,329)
Property, plant and equipment, net	2,861,756	2,594,732
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,379,208	4,142,389
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	237,136	153,672
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	18,129	17,472
Fuel [Member]
Inventories [Abstract]
Inventories	$ 13,208	$ 18,103