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Fair Value Measurements Carrying and Fair Value Amounts (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
OG&E Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-Term Debt, Carrying Amount $ 2,855.2 $ 2,854.3
Long-Term Debt, Fair Value 3,101.4 3,242.8
OG&E Industrial Authority Bonds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-Term Debt, Carrying Amount 135.4 135.4
Long-Term Debt, Fair Value 135.4 135.4
OG&E Tinker Debt [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-Term Debt, Carrying Amount 9.7 9.7
Long-Term Debt, Fair Value 9.1 9.8
Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative Assets (Liabilities), at Fair Value, Net $ 0.0 $ 0.0