Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 776,140		$ 811,685
Less allowance for bad debts	(57,995)		(58,565)
Accounts receivable, net	718,145		753,120
Inventories [Abstract]
Inventories	474,359		618,232
Property, plant and equipment, net [Abstract]
Property, plant and equipment	34,009,185		33,714,367
Accumulated depreciation and amortization	(11,731,341)		(11,658,351)
Property, plant and equipment, net	22,672,686		22,353,367
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	27,393,092		27,254,541
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	3,700,424		3,676,754
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,484,878		1,546,643
Plant to be Retired [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	115,401	[1]	151,184	[1]
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,315,390		1,085,245
Nuclear Fuel [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,062,790		1,939,299
Accumulated depreciation and amortization	(1,667,948)		(1,641,948)
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	207,729		202,699
Fuel [Member]
Inventories [Abstract]
Inventories	176,874		236,023
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 89,756		$ 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 taken out of service. Amounts are presented net of accumulated depreciation.