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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Income Taxes          
Due to parent $ 15.8   $ 15.8   $ 0.1
Unrecognized tax benefits 55.0   55.0    
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 25.1   25.1    
Unrecognized tax benefits related to state tax credits 14.2   14.2    
Accrued interest 5.5   5.5    
Effective tax rate (as a percent) 16.50% 27.40% 23.90% 28.40%  
Effective tax rate at a combined federal and state income tax rate (as a percent)     39.60% 39.20%  
AEPC
          
Income Taxes          
Due to parent 15.8   15.8    
Federal taxes paid to AEPC under the Tax Allocation Agreement $ 22.0 $ 95.0 $ 120.1 $ 234.0