Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 287,169		$ 341,737
Less allowance for bad debts	(22,025)		(24,698)
Accounts receivable, net	265,144		317,039
Inventories [Abstract]
Inventories	242,444		253,997
Property, plant and equipment, net [Abstract]
Property, plant and equipment	13,621,894		13,082,501
Less accumulated depreciation	(3,776,988)		(3,606,930)
Property, plant and equipment, net	9,844,906		9,475,571
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	9,741,933		9,469,434
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,535,274		2,456,275
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	748,553		763,513
Plant to be Retired [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	105,573	[1]	151,184	[1]
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	490,561		242,095
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	53,851		53,318
Fuel [Member]
Inventories [Abstract]
Inventories	96,647		94,874
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 91,946		$ 105,805

[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.