Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 89,009	$ 70,410
Less allowance for bad debts	(4,938)	(5,380)
Accounts receivable, net	84,071	65,030
Inventories [Abstract]
Inventories	31,974	35,575
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,544,308	4,296,061
Less accumulated depreciation	(1,758,904)	(1,701,329)
Property, plant and equipment, net	2,785,404	2,594,732
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,344,849	4,142,389
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	199,459	153,672
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	19,022	17,472
Fuel [Member]
Inventories [Abstract]
Inventories	$ 12,952	$ 18,103