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Income Taxes (Details 1) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Taxes [Line Items]      
Due from parent $ 11,600,000 $ 44,500,000  
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 31,800,000 25,600,000 $ 19,100,000
Reductions related to expirations of statute of limitations 16,300,000 0 0
Unrecognized tax benefits reasonably possible to be recognized in next fiscal year 11,600,000    
State tax credits 25,900,000 13,500,000  
Interest on income recognized on uncertain tax positions 1,000,000 3,800,000 2,200,000
Penalties recognized on uncertain tax positions 0 0 0
Accrued interest 7,500,000 6,500,000  
Accrued penalties 0 0  
AEPC      
Income Taxes [Line Items]      
Due from parent 11,600,000 44,500,000  
Federal taxes paid to AEPC under the Tax Sharing Agreement 57,500,000 120,100,000 260,000,000
Kansas      
Income Taxes [Line Items]      
Net income tax benefit recognized from Kansas High Performance Incentive Program 4,300,000 2,800,000 7,800,000
Kansas High Performance Incentive Program (HPIP) credits for qualified business facility investment earned 6,700,000 4,300,000 12,000,000
Oklahoma      
Income Taxes [Line Items]      
Net income tax benefit recognized from qualified manufacturing facility investment 2,900,000 2,500,000 1,200,000
Qualified manufacturing facility investment credit earned 4,400,000 $ 3,900,000 $ 1,800,000
Oklahoma | State      
Income Taxes [Line Items]      
Tax credit carryforwards $ 25,900,000