Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 36,810,345		$ 35,511,843
Less accumulated depreciation	(12,344,995)		(12,048,697)
Property, plant and equipment, net	24,813,411		23,809,348
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,017,319		28,285,031
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,903,127		3,836,335
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,475,534		1,480,558
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	115,466	[1]	152,730	[1]
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,298,899		1,757,189
Nuclear fuel
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,142,145		2,090,801
Less accumulated depreciation	$ (1,794,084)		$ (1,744,599)

[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 2012, Cherokee Unit 1 was retired. Amounts are presented net of accumulated depreciation.