Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 80,727	$ 70,410
Less allowance for bad debts	(5,376)	(5,380)
Accounts receivable, net	75,351	65,030
Inventories [Abstract]
Inventories	25,190	35,575
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,458,945	4,296,061
Less accumulated depreciation	(1,738,170)	(1,701,329)
Property, plant and equipment, net	2,720,775	2,594,732
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,304,461	4,142,389
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	154,484	153,672
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	18,547	17,472
Fuel [Member]
Inventories [Abstract]
Inventories	$ 6,643	$ 18,103