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Fair Value Measurements Carrying and Fair Value Amounts (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
OG&E Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-Term Debt, Carrying Amount $ 3,348.5 $ 3,050.3
Long-Term Debt, Fair Value 4,024.7 3,500.4
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 9.5 9.5
Long-Term Debt, Fair Value 10.7 10.0
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