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Income Taxes Reconciliation of Effective Income Tax Rate (Details)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]      
Federal statutory income tax rate 35.00% 35.00% 35.00%
State income taxes, net of federal income tax benefit 3.00% 2.00% 2.00%
Federal income tax credits (9.00%) [1] (10.00%) [1] (8.00%) [1]
Effects of ratemaking (1.00%) 0.00% (2.00%)
Other (1.00%) 1.00% 0.00%
Effective income tax rate 27.00% 28.00% 27.00%
Years eligible for renewable electricity production tax credits 10 years    
[1] Primarily attributable to the impact of federal renewable electricity production tax credits related to qualifying wind-powered generating facilities that extend 10 years from the date the facilities were placed in service.