Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 698,853		$ 811,685
Less allowance for bad debts	(53,589)		(58,565)
Accounts receivable, net	645,264		753,120
Inventories [Abstract]
Inventories	473,137		618,232
Property, plant and equipment, net [Abstract]
Property, plant and equipment	34,504,465		33,714,367
Accumulated depreciation and amortization	(11,852,561)		(11,658,351)
Property, plant and equipment, net	23,047,854		22,353,367
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	27,717,392		27,254,541
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	3,735,411		3,676,754
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,454,246		1,546,643
Plant to be Retired [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	112,823	[1]	151,184	[1]
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,484,593		1,085,245
Nuclear Fuel [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,087,663		1,939,299
Accumulated depreciation and amortization	(1,691,713)		(1,641,948)
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	210,197		202,699
Fuel [Member]
Inventories [Abstract]
Inventories	182,281		236,023
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 80,659		$ 179,510

[1] In 2010, in response to the Clean Air Clean Jobs Act (CACJA), the Colorado Public Utilities Commission (CPUC) approved the early retirement of Cherokee Units 1, 2 and 3, Arapahoe Unit 3 and Valmont Unit 5 between 2011 and 2017. In 2011, Cherokee Unit 2 was retired and in May 2012, Cherokee Unit 1 was retired. Amounts are presented net of accumulated depreciation.