Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Accounts receivable, net [Abstract]
Accounts receivable	$ 336,830		$ 299,379
Less allowance for bad debts	(21,811)		(21,918)
Accounts receivable, net	315,019		277,461
Inventories [Abstract]
Inventories	181,360		223,794
Property, plant and equipment, net [Abstract]
Property, plant and equipment	13,987,066		13,786,224
Less accumulated depreciation	(3,834,295)		(3,755,233)
Property, plant and equipment, net	10,152,771		10,030,991
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	9,853,170		9,782,163
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,612,393		2,583,394
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	763,607		761,712
Plant to be Retired [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	141,038	[1]	152,730	[1]
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	616,858		506,225
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	55,748		54,486
Fuel [Member]
Inventories [Abstract]
Inventories	82,715		89,246
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 42,897		$ 80,062

[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.