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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Dec. 31, 2013
Income Taxes          
Due to parent $ 28.0   $ 28.0   $ 0.1
Unrecognized tax benefits 52.6   52.6    
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 23.5   23.5    
Unrecognized tax benefits related to state tax credits 13.9   13.9    
Accrued interest 4.6   4.6    
Effective tax rate (as a percent) 23.90% 26.80% 24.30% 28.50%  
Combined federal and state expected statutory tax rate (as a percent)     39.60% 39.20%  
AEPC
          
Income Taxes          
Due to parent 28.0   28.0    
Federal taxes paid to AEPC under the Tax Allocation Agreement $ 98.1 $ 139.0 $ 98.1 $ 139.0