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Income Taxes (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Reduction in net deferred income tax liability $ 1,273.0      
Current Federal Tax Expense (Benefit) 4.9 $ 0.0 $ 0.0  
Current State and Local Tax Expense (Benefit) (4.2) (5.7) (5.2)  
Current Income Tax Expense (Benefit) 0.7 (5.7) (5.2)  
Deferred Federal Income Tax Expense (Benefit) (75.9) 126.0 98.8  
Deferred State and Local Income Tax Expense (Benefit) 26.0 28.0 4.5  
Deferred Income Tax Expense (Benefit) (49.9) 154.0 103.3  
Investment Tax Credit (0.1) (0.2) (0.7)  
Income Tax Expense (Benefit) $ (49.3) $ 148.1 $ 97.4  
Statutory federal tax rate 35.00% 35.00% 35.00%  
Federal deferred tax revaluation (41.20%) 0.00% 0.00%  
Federal renewable energy credit (A) [1] (4.80%) (6.80%) (8.90%)  
Remeasurement of state deferred tax liabilities 0.40% 0.90% (0.80%)  
401(k) dividends (0.50%) (0.60%) (0.70%)  
Federal investment tax credits, net (0.10%) (0.80%) (0.20%)  
State income taxes, net of federal income tax benefit 2.00% 1.90% 0.10%  
Uncertain tax positions 0.00% 0.10% 0.70%  
Amortization of net unfunded deferred taxes 0.70% 0.70% 0.90%  
Other (0.10%) 0.10% 0.30%  
Effective income tax rate (8.60%) 30.50% 26.40%  
Accrued liabilities $ (7.4) $ (16.1)    
Accrued vacation (2.1) (3.5)    
Uncollectible accounts (0.4) (0.6)    
Accelerated depreciation and other property related differences 1,449.6 2,103.2    
Investment in Enable Midstream Partners 441.7 657.3    
Regulatory asset 18.9 34.4    
Income taxes (recoverable from) refundable to customers, net (244.3) 24.1    
Company Pension Plan 11.5 16.5    
Bond redemption-unamortized costs 2.6 4.3    
Derivative instruments 1.6 2.2    
Federal tax credits (218.5) (220.6)    
State tax credits (141.7) (112.2)    
Postretirement medical and life insurance benefits (25.2) (48.9)    
Net operating losses (21.1) (31.7)    
Regulatory liabilities (16.8) (34.6)    
Asset retirement obligations (19.2) (24.5)    
Deferred federal investment tax credits (0.5) (0.8)    
Other (0.9) (14.0)    
Total deferred income tax liabilities, net 1,227.8 2,334.5    
Unrecognized Tax Benefits 20.7 20.7 $ 20.2 $ 16.1
Current year additions 0.0 0.5 4.1  
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued 0.0      
Tax Credit Carryforward, Deferred Tax Asset 218.5 220.6    
Reduction in income tax expense 234.7      
Increased Regulatory Liabilities due to Income Taxes 1,038.0      
State operating loss [Member]        
Operating Loss Carryforwards 472.1      
Deferred Tax Assets, Operating Loss Carryforwards, State and Local $ 21.1      
Tax Credit Carryforward, Expiration Date Dec. 31, 2030      
Federal tax credits [Member]        
Federal tax credits $ (218.5)      
Tax Credit Carryforward, Amount 218.5      
Tax Credit Carryforward, Deferred Tax Asset $ 218.5      
Tax Credit Carryforward, Expiration Date Dec. 31, 2029      
Oklahoma investment tax credits [Member]        
Federal tax credits $ (113.8)      
Tax Credit Carryforward, Amount 144.1      
Tax Credit Carryforward, Deferred Tax Asset 113.8      
Oklahoma capital investment board credits [Member]        
Federal tax credits (8.5)      
Tax Credit Carryforward, Amount 8.5      
Tax Credit Carryforward, Deferred Tax Asset 8.5      
Oklahoma zero emission tax credits [Member]        
Federal tax credits (19.4)      
Tax Credit Carryforward, Amount 24.1      
Tax Credit Carryforward, Deferred Tax Asset $ 19.4      
Tax Credit Carryforward, Expiration Date Dec. 31, 2020      
OGE Energy Corp. [Member]        
Unrecognized Tax Benefits $ 16.4      
Og and E [Member]        
Unrecognized Tax Benefits   $ 13.5 $ 13.2  
[1] Represents credits associated with the production from OG&E's wind farms.