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Income Taxes - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Due from parent $ 5,100,000 $ 0  
Due to parent $ 0 $ 10,600,000  
Statutory federal income tax rate (as a percent) 35.00% 35.00% 35.00%
Operating loss carryforwards $ 27,500,000    
Tax cuts and jobs act, income tax benefit 200,500,000    
Valuation allowance 0 $ 0  
Unrecognized tax benefits which, if recognized, would impact the company's effective tax rate 22,700,000 28,700,000 $ 31,800,000
Reductions related to expirations of statute of limitations 15,400,000 4,900,000 16,300,000
Unrecognized tax benefits reasonably possible to be recognized in next fiscal year 5,800,000    
State tax credits 25,800,000 25,700,000  
Interest on income recognized on uncertain tax positions 7,000,000 500,000 1,000,000
Penalties recognized on uncertain tax positions 0 0 0
Accrued interest 1,000,000 8,000,000 7,500,000
Kansas      
Income Taxes [Line Items]      
Net income tax benefit recognized from Kansas High Performance Incentive Program 4,300,000 5,700,000 4,300,000
Kansas High Performance Incentive Program (HPIP) credits for qualified business facility investment earned 6,600,000 8,700,000 6,700,000
Oklahoma      
Income Taxes [Line Items]      
Net income tax benefit recognized from qualified manufacturing facility investment 2,600,000 3,100,000 2,900,000
Qualified manufacturing facility investment credit earned 4,000,000 4,800,000 4,400,000
State      
Income Taxes [Line Items]      
Increase in tax benefit realized (14,300,000) (1,700,000)  
State | Kansas      
Income Taxes [Line Items]      
Tax credit carryforwards 9,300,000    
State | Oklahoma      
Income Taxes [Line Items]      
Tax credit carryforwards 29,800,000    
AEPC      
Income Taxes [Line Items]      
Due from parent 5,100,000    
Due to parent   10,600,000  
Federal taxes paid to AEPC under the Tax Sharing Agreement $ 15,000,000 $ 45,000,000 $ 57,500,000