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Short-Term Debt and Credit Facilities (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Short-term Debt [Line Items]    
Advances to parent $ 299.7 $ 112.5
Letters of Credit Outstanding $ 0.3  
Weighted Average Interest Rate 1.05%  
OG&E [Member]    
Short-term Debt [Line Items]    
Line of Credit Facility, Current Borrowing Capacity $ 450.0  
Intercomany Borrowings, Maximum Borrowing Capacity 350.0  
Outstanding Intercompany Borrowings 0.0  
Commercial Paper 0.0  
Short Term Borrowing Capacity That Has Regulatory Approval $ 800.0  
Period For Which Regulatory Approval Has Been Given to Acquire Short Term Debt 2 years