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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Income Taxes        
Federal taxes payable to AEPC under the Tax Sharing Agreement $ 28.3   $ 28.3  
Unrecognized tax benefits 17.8   17.8  
Accrued interest 0.1   0.1  
Unrecognized tax benefits which, if recognized, would impact the Company's effective tax rate $ 0.2   $ 0.2  
Effective tax rate (as a percent) 35.50% 36.40% 35.30% 36.60%
Combined federal and state expected statutory tax rate (as a percent)     39.40% 39.70%