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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Kansas High Performance Incentive Program (HPIP) credits        
Unrecognized tax benefits $ 44.8   $ 44.8  
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 18.8   18.8  
Accrued interest 2.0   2.0  
Effective tax rate (as a percent) 27.40% 36.90% 28.40% 36.20%
Combined federal and state expected statutory tax rate (as a percent)     39.20% 39.40%
AEPC
       
Kansas High Performance Incentive Program (HPIP) credits        
Federal taxes payable to AEPC under the Tax Allocation Agreement 33.7   33.7  
Federal taxes paid to AEPC under the Tax Allocation Agreement $ 95.0 $ 65.1 $ 234.0 $ 65.1