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Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Income Taxes [Line Items]        
Unrecognized tax benefits $ 62.6   $ 62.6  
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 30.1   30.1  
Unrecognized tax benefits related to state tax credits 21.6   21.6  
Accrued interest $ 8.5   $ 8.5  
Effective tax rate (as a percent) 23.20% 23.90% 22.90% 24.30%
Effective tax rate at a combined federal and state income tax rate (as a percent) 39.60% 39.60% 39.60% 39.60%
AEPC        
Income Taxes [Line Items]        
Due to parent $ 20.8   $ 20.8  
Federal taxes paid to AEPC under the Tax Allocation Agreement $ 27.5 $ 98.1 $ 27.5 $ 98.1