Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Accounts receivable, net [Abstract]
Accounts receivable	$ 811,685		$ 773,037
Less allowance for bad debts	(58,565)		(54,563)
Accounts receivable, net	753,120		718,474
Inventories [Abstract]
Inventories	618,232		560,800
Property, plant and equipment, net [Abstract]
Property, plant and equipment	33,714,367		31,435,251
Accumulated depreciation and amortization	(11,658,351)		(11,068,820)
Property, plant and equipment, net	22,353,367		20,663,082
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	27,254,541		24,993,582
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	3,676,754		3,463,343
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,546,643		1,555,287
Plant to be Retired [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	151,184	[1]	236,606	[1]
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,085,245		1,186,433
Nuclear Fuel [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,939,299		1,837,697
Accumulated depreciation and amortization	(1,641,948)		(1,541,046)
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	202,699		196,081
Fuel [Member]
Inventories [Abstract]
Inventories	236,023		188,566
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 179,510		$ 176,153

[1] In 2010, in response to the CACJA, the CPUC approved the early retirement of Cherokee Units 1, 2 and 3, Arapahoe Unit 3 and Valmont Unit 5 between 2011 and 2017. Amounts are presented net of accumulated depreciation. See Item 1 - Public Utility Regulation for further discussion.