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Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Income Taxes [Line Items]          
Due from related party $ 2.7   $ 2.7   $ 11.6
Unrecognized tax benefits 44.1   44.1    
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 28.7   28.7    
Unrecognized tax benefits related to state tax credits 25.9   25.9    
Accrued interest $ 7.4   $ 7.4    
Effective tax rate (as a percent) 55.60% 18.90% 13.30% 21.90%  
Effective tax rate at a combined federal and state income tax rate (as a percent) 39.40% 39.60% 39.40% 39.60%  
AEPC          
Income Taxes [Line Items]          
Due from related party $ 2.7   $ 2.7    
Income tax payments to AEPC $ 15.0 $ 20.0 $ 15.0 $ 47.5