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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Income Taxes [Line Items]          
Due to related party $ 2.6   $ 2.6   $ 10.6
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 netted with deferred tax asset carryforwards 25.7   25.7    
Accrued interest $ 10.1   $ 10.1    
Effective tax rate 26.70% 55.60% 28.20% 13.30%  
Federal and state combined statutory tax rate 39.30% 39.40% 39.30% 39.40%  
AEPC          
Income Taxes [Line Items]          
Due to related party $ 2.6   $ 2.6    
Income tax payments to AEPC $ 0.0 $ 15.0 $ 10.0 $ 15.0