Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 14,423,728		$ 13,786,224
Less accumulated depreciation	(3,921,853)		(3,755,233)
Property, plant and equipment, net	10,501,875		10,030,991
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	10,045,683		9,782,163
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,662,749		2,583,394
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	749,437		761,712
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	115,753	[1]	152,730	[1]
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 850,106		$ 506,225

[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.