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Income Taxes (Details 1) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Taxes [Line Items]      
Due to parent $ 10,600,000 $ 0  
Due from parent $ 0 11,600,000  
Statutory federal income tax rate (as a percent) 35.00%    
Valuation allowance $ 0 0  
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 28,700,000 31,800,000 $ 25,600,000
Reductions related to expirations of statute of limitations 4,900,000 16,300,000 0
Unrecognized tax benefits reasonably possible to be recognized in next fiscal year 15,400,000    
State tax credits 25,700,000 25,900,000  
Interest on income recognized on uncertain tax positions 500,000 1,000,000 3,800,000
Penalties recognized on uncertain tax positions 0 0 0
Accrued interest 8,000,000 7,500,000  
AEPC      
Income Taxes [Line Items]      
Due to parent   10,600,000  
Due from parent   11,600,000  
Federal taxes paid to AEPC under the Tax Sharing Agreement 45,000,000 57,500,000 120,100,000
State      
Income Taxes [Line Items]      
Increase in tax benefit realized (2,800,000) (3,100,000) (2,000,000)
Kansas      
Income Taxes [Line Items]      
Net income tax benefit recognized from Kansas High Performance Incentive Program 5,700,000 4,300,000 2,800,000
Kansas High Performance Incentive Program (HPIP) credits for qualified business facility investment earned 8,700,000 6,700,000 4,300,000
Kansas | State      
Income Taxes [Line Items]      
Tax credit carryforwards 1,400,000    
Oklahoma      
Income Taxes [Line Items]      
Net income tax benefit recognized from qualified manufacturing facility investment 3,100,000 2,900,000 2,500,000
Qualified manufacturing facility investment credit earned 4,800,000 $ 4,400,000 $ 3,900,000
Oklahoma | State      
Income Taxes [Line Items]      
Tax credit carryforwards $ 25,700,000