Asset Acquisition and Sale (Details) (USD $)	Oct. 31, 2010 Lubbock Electric Distribution Assets [Member]	Nov. 30, 2009 Lubbock Electric Distribution Assets [Member]	Dec. 31, 2011 Blue Spruce Energy Center and Rocky Mountain Energy Center [Member]	Dec. 31, 2010 Blue Spruce Energy Center and Rocky Mountain Energy Center [Member]	Dec. 31, 2011 Blue Spruce Energy Center [Member] MW	Dec. 31, 2011 Rocky Mountain Energy Center [Member] MW
Acquisition of Generation Assets [Abstract]
Acquisition cost of Blue Spruce Energy Center and Rocky Mountain Energy Center				$ 739,000,000
Working capital adjustment to the acquisition cost of Blue Spruce Energy Center and Rocky Mountain Energy Center			200,000	3,000,000
Accrued operating liabilities of the acquired plants				6,500,000
Generating capacity (in MW)					310	652
Assets acquired [Abstract]
Inventory			3,791,000
Property, plant and equipment			735,959,000
Total assets acquired			739,750,000
Liabilities assumed [Abstract]
Accrued expenses			7,437,000
Total liabilities assumed			7,437,000
Net assets acquired			732,313,000
Sale of Lubbock Electric Distribution Assets [Abstract]
Proceeds from sale of electric distribution system assets		87,000,000
Number of customers served within Lubbock, Texas		24,000
Number of customers as a percentage of total customers in the dually certified service area (in hundredths)		25.00%
Additional number of years under new contract (in years)		25Y
Initial ROE (in hundredths)		10.50%
Deferred regulatory liability associated with sale of electric distribution system assets	$ 20,000,000