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Fair Value Measurements Carrying and Fair Value Amounts (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
OG&E Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-Term Debt, Carrying Amount $ 3,350.2 $ 3,349.6
Long-Term Debt, Fair Value 3,831.0 4,182.1
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 9.4 9.4
Long-Term Debt, Fair Value 9.9 10.7
Fair Value, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative Assets (Liabilities), at Fair Value, Net $ 0.0 $ 0.0