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Retirement Plans and Postretirement Benefit Plans (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment   $ 42.9        
Defined Benefit Plan, Plan Assets, Payment for Settlement   26.4        
Payment for Pension Benefits     $ 20.0 $ 20.0    
Defined Contribution Plan, Employer Matching Contribution, Percent of Match     200.00%      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay     5.00%      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]            
Plan settlements   (27.9)        
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Plan settlements   (27.9)        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Net periodic benefit cost     $ 32.9 22.5 $ 40.2  
Plan settlements   (27.9)        
Effect of One Percentage Point Increase on Service and Interest Cost Components     0.0 0.0 0.0  
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation     0.1 0.2 0.2  
Effect of One Percentage Point Decrease on Service and Interest Cost Components     0.0 0.0 0.1  
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation     0.3 0.7 0.7  
Expected Future Benefit Payments, Next Twelve Months $ 65.6   65.6      
Expected Future Benefit Payments, Year Two 62.0   62.0      
Expected Future Benefit Payments, Year Three 63.4   63.4      
Expected Future Benefit Payments, Year Four 62.3   62.3      
Expected Future Benefit Payments, Year Five 61.1   61.1      
Expected Future Benefit Payments, Five Fiscal Years Thereafter 285.0   285.0      
Defined Contribution Plan, Cost     13.2 11.9 11.6  
Net Effect of Settlement on Other Comprehensive Income and Regulatory Assets   2.1        
Pension Plans [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment     0.0 0.0    
Pension Plans, Defined Benefit [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Plan Assets, Benefits Paid     (14.8) (53.8)    
Accumulated Benefit Obligation 626.9   626.9 608.0    
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]            
Benefit Obligation, Beginning   672.2 672.2 680.0    
Service cost     15.5 15.8 16.1  
Interest cost     26.2 25.5 26.1  
Plan settlements     (50.2) 0.0    
Participants' contributions     0.0 0.0    
Actuarial gains (losses)     38.6 4.7    
Benefit Obligation, Ending 687.5   687.5 672.2 680.0  
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   595.9 595.9 581.7    
Actual return on plans' assets     84.4 48.0    
Employer contributions     20.0 20.0    
Plan settlements     (50.2) 0.0    
Fair Value of Plan Assets, Ending 635.3   635.3 595.9 581.7  
Funded Status of Plan (52.2)   (52.2) (76.3)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Service cost     15.5 15.8 16.1  
Interest cost     26.2 25.5 26.1  
Expected return on plan assets     (42.6) (41.5) (46.0)  
Defined Benefit Plan, Amortization of Gain (Loss)     (17.4) (16.5) (18.0)  
Amortization of unrecognized prior service cost [1]     (0.1) (0.1) 0.4  
Settlement $ 15.3   15.3 0.0 21.7  
Net periodic benefit cost     31.7 16.2 36.3  
Amount paid by unconsolidated affiliates     4.3 5.1 4.2  
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2]     27.4 11.1 32.1  
Plan settlements     (50.2) 0.0    
Capitalized Portion of Net Periodic Benefit Cost     $ 4.4 $ 4.0 $ 5.0  
Assumptions Used Calculating Benefit Obligation, Discount Rate 3.60%   3.60% 4.00% 4.00%  
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets     7.50% 7.50% 7.50%  
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase     4.20% 4.20% 4.20%  
Fair Value of Plan Assets, Beginning   595.9 $ 595.9 $ 581.7    
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant     0.0 0.0    
Defined Benefit Plan, Benefit Obligation, Benefits Paid     (14.8) (53.8)    
Pension Plans, Defined Benefit [Member] | OKLAHOMA            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Additional Pension Expense to Meet State Requirements     (2.3) 9.9 $ (3.1)  
Pension Plans, Defined Benefit [Member] | ARKANSAS            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Settlement     (1.1) (0.1) (1.9)  
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning       0.0    
Fair Value of Plan Assets, Ending         0.0  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning       0.0    
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] $ 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   237.1 237.1      
Fair Value of Plan Assets, Ending 225.9   225.9 237.1    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   237.1 237.1      
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   237.1 237.1      
Fair Value of Plan Assets, Ending 225.9   225.9 237.1    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   237.1 237.1      
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3],[4] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [4]   122.3 122.3      
Fair Value of Plan Assets, Ending [4] 169.7   169.7 122.3    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [4]   122.3 122.3      
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [4]   122.3 122.3      
Fair Value of Plan Assets, Ending [4] 169.7   169.7 122.3    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [4]   122.3 122.3      
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [4]   0.0 0.0      
Fair Value of Plan Assets, Ending [4] 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [4]   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   59.2 59.2      
Fair Value of Plan Assets, Ending 43.4   43.4 59.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   59.2 59.2      
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   59.2 59.2      
Fair Value of Plan Assets, Ending 43.4   43.4 59.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   59.2 59.2      
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   137.6 137.6      
Fair Value of Plan Assets, Ending 153.8   153.8 137.6    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   137.6 137.6      
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   137.6 137.6      
Fair Value of Plan Assets, Ending 153.8   153.8 137.6    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   137.6 137.6      
Pension Plans, Defined Benefit [Member] | Commingled fund [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3],[5] 29.9   29.9 23.8    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [5]   23.8 23.8      
Fair Value of Plan Assets, Ending [5] 29.9   29.9 23.8    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [5]   23.8 23.8      
Pension Plans, Defined Benefit [Member] | Commingled fund [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [5]   0.0 0.0      
Fair Value of Plan Assets, Ending [5] 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [5]   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Commingled fund [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning [5]   0.0 0.0      
Fair Value of Plan Assets, Ending [5] 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning [5]   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   5.2 5.2      
Fair Value of Plan Assets, Ending 4.0   4.0 5.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   5.2 5.2      
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   5.2 5.2      
Fair Value of Plan Assets, Ending 4.0   4.0 5.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   5.2 5.2      
Pension Plans, Defined Benefit [Member] | Municipal bonds [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   1.9 1.9      
Fair Value of Plan Assets, Ending 1.2   1.2 1.9    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   1.9 1.9      
Pension Plans, Defined Benefit [Member] | Municipal bonds [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Municipal bonds [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   1.9 1.9      
Fair Value of Plan Assets, Ending 1.2   1.2 1.9    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   1.9 1.9      
Pension Plans, Defined Benefit [Member] | Money market funds [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 4.3   4.3 2.2    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   2.2 2.2      
Fair Value of Plan Assets, Ending 4.3   4.3 2.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   2.2 2.2      
Pension Plans, Defined Benefit [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Money market funds [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Mutual fund [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   9.0 9.0      
Fair Value of Plan Assets, Ending 7.8   7.8 9.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   9.0 9.0      
Pension Plans, Defined Benefit [Member] | Mutual fund [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   9.0 9.0      
Fair Value of Plan Assets, Ending 7.8   7.8 9.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   9.0 9.0      
Pension Plans, Defined Benefit [Member] | Mutual fund [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Treasury futures, receivable [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   10.7 10.7      
Fair Value of Plan Assets, Ending 13.4   13.4 10.7    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   10.7 10.7      
Pension Plans, Defined Benefit [Member] | Treasury futures, receivable [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Treasury futures, receivable [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   10.7 10.7      
Fair Value of Plan Assets, Ending 13.4   13.4 10.7    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   10.7 10.7      
Pension Plans, Defined Benefit [Member] | Treasury futures, payable [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   (2.3) (2.3)      
Fair Value of Plan Assets, Ending (11.4)   (11.4) (2.3)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   (2.3) (2.3)      
Pension Plans, Defined Benefit [Member] | Treasury futures, payable [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Treasury futures, payable [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   (2.3) (2.3)      
Fair Value of Plan Assets, Ending (11.4)   (11.4) (2.3)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   (2.3) (2.3)      
Pension Plans, Defined Benefit [Member] | Cash collateral [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.3 0.3      
Fair Value of Plan Assets, Ending 0.3   0.3 0.3    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.3 0.3      
Pension Plans, Defined Benefit [Member] | Cash collateral [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.3 0.3      
Fair Value of Plan Assets, Ending 0.3   0.3 0.3    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.3 0.3      
Pension Plans, Defined Benefit [Member] | Cash collateral [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 0.0   0.0 0.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.2 0.2      
Fair Value of Plan Assets, Ending 0.1   0.1 0.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.2 0.2      
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.2 0.2      
Fair Value of Plan Assets, Ending 0.1   0.1 0.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.2 0.2      
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Alternative Investment, Fair Value Disclosure [3] 34.2   34.2 26.0    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   607.2 607.2      
Fair Value of Plan Assets, Ending 642.4   642.4 607.2    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   607.2 607.2      
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   368.7 368.7      
Fair Value of Plan Assets, Ending 403.7   403.7 368.7    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   368.7 368.7      
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   212.5 212.5      
Fair Value of Plan Assets, Ending 204.5   204.5 212.5    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   212.5 212.5      
Pension Plans, Defined Benefit [Member] | Receivable from broker for securities sold [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Pension Plans, Defined Benefit [Member] | Interest and dividends receivable [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   3.0 3.0      
Fair Value of Plan Assets, Ending 3.2   3.2 3.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   3.0 3.0      
Pension Plans, Defined Benefit [Member] | Payable to broker for securities purchased [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   (14.3) (14.3)      
Fair Value of Plan Assets, Ending (10.3)   (10.3) (14.3)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   (14.3) (14.3)      
Pension Plans, Defined Benefit [Member] | Total Plan assets [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   595.9 595.9      
Fair Value of Plan Assets, Ending 635.3   635.3 595.9    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   595.9 595.9      
Restoration of Retirement Income Plan [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Plan Assets, Benefits Paid     (0.2) 0.0    
Accumulated Benefit Obligation 7.5   7.5 6.1    
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]            
Benefit Obligation, Beginning   7.0 7.0 25.1    
Service cost     0.3 0.3 1.3  
Interest cost     0.3 0.4 0.7  
Plan settlements     0.0 (20.6)    
Participants' contributions     0.0 0.0    
Actuarial gains (losses)     0.7 1.8    
Benefit Obligation, Ending 8.1   8.1 7.0 25.1  
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0 0.0    
Actual return on plans' assets     0.0 0.0    
Employer contributions     0.2 20.6    
Plan settlements     0.0 (20.6)    
Fair Value of Plan Assets, Ending 0.0   0.0 0.0 0.0  
Funded Status of Plan (8.1)   (8.1) (7.0)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Service cost     0.3 0.3 1.3  
Interest cost     0.3 0.4 0.7  
Expected return on plan assets     0.0 0.0 0.0  
Defined Benefit Plan, Amortization of Gain (Loss)     (0.4) (0.7) (0.6)  
Amortization of unrecognized prior service cost [1]     0.1 0.1 0.1  
Settlement     0.0 8.6 0.0  
Net periodic benefit cost     1.1 10.1 2.7  
Amount paid by unconsolidated affiliates     0.0 0.3 0.1  
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2]     1.1 9.8 2.6  
Plan settlements     0.0 (20.6)    
Fair Value of Plan Assets, Beginning   0.0 0.0 0.0    
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant     0.0 0.0    
Defined Benefit Plan, Benefit Obligation, Benefits Paid     (0.2) 0.0    
Other Postretirement Benefits Plan [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Plan Assets, Benefits Paid     (15.7) (15.7)    
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]            
Benefit Obligation, Beginning   215.9 215.9 225.3    
Service cost     0.6 0.8 1.5  
Interest cost     7.2 9.5 10.3  
Plan settlements     (28.1) 0.0    
Participants' contributions     3.5 3.6    
Actuarial gains (losses)     5.6 (7.6)    
Benefit Obligation, Ending 149.4   149.4 215.9 225.3  
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   53.1 53.1 55.3    
Actual return on plans' assets     2.8 2.0    
Employer contributions     34.6 7.9    
Plan settlements     (28.1) 0.0    
Fair Value of Plan Assets, Ending 50.2   50.2 53.1 55.3  
Funded Status of Plan $ (99.2)   (99.2) (162.8)    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Service cost     0.6 0.8 1.5  
Interest cost     7.2 9.5 10.3  
Expected return on plan assets     (2.2) (2.3) (2.4)  
Defined Benefit Plan, Amortization of Gain (Loss)     (2.0) (2.6) (13.9)  
Amortization of unrecognized prior service cost [1]     (3.5) (8.8) (16.5)  
Settlement     0.6 0.0 0.0  
Net periodic benefit cost     4.7 1.8 6.8  
Amount paid by unconsolidated affiliates     0.3 0.2 1.3  
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2]     4.4 1.6 5.5  
Plan settlements     (28.1) 0.0    
Capitalized Portion of Net Periodic Benefit Cost     $ 1.2 $ 0.8 $ 1.9  
Assumptions Used Calculating Benefit Obligation, Discount Rate 3.70%   3.70% 4.20% 4.25%  
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets     4.00% 4.00% 4.00%  
Health Care Cost Trend Rate Assumed for Next Fiscal Year 7.50%   7.50% 6.75% 6.10% 6.50%
Ultimate Health Care Cost Trend Rate 4.50%   4.50% 4.50% 4.50%  
Year that Rate Reaches Ultimate Trend Rate     2030 2026 2026  
Fair Value of Plan Assets, Beginning   53.1 $ 53.1 $ 55.3    
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant     3.5 3.6    
Defined Benefit Plan, Benefit Obligation, Benefits Paid     (15.7) (15.7)    
Other Postretirement Benefits Plan [Member] | OKLAHOMA            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Additional Postretirement Medical Expense to Meet State Requirements     6.2 7.9 $ 5.8  
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   8.4 8.4      
Fair Value of Plan Assets, Ending $ 10.0   10.0 8.4    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   8.4 8.4      
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 2 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Fair Value of Plan Assets, Ending 0.0   0.0 0.0    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   0.0 0.0      
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member]            
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]            
Fair Value of Plan Assets, Beginning   44.7 44.7      
Fair Value of Plan Assets, Ending $ 40.2   40.2 44.7    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Fair Value of Plan Assets, Beginning   $ 44.7 44.7      
Restoration of Retirement Income Plan [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment     0.0 0.0    
Postretirement Benefit Plan [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment     $ (39.6) $ 0.0    
Less Than 90% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     50.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     50.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
95% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     58.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     42.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
100% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     65.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     35.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
105% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     73.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     27.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
110% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     80.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     20.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
115% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     85.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     15.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
120% [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Projected Benefit Obligation Funded Status Thresholds Fixed Income     90.00%      
Projected Benefit Obligation Funded Status Thresholds Equity     10.00%      
Projected Benefit Obligation Funded Status Thresholds     100.00%      
Domestic All-Cap/Large Cap Equity [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Target Plan Asset Allocations 40.00%   40.00%      
Domestic Mid-Cap Equity [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Target Plan Asset Allocations 15.00%   15.00%      
Domestic Small-Cap Equity [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Target Plan Asset Allocations 25.00%   25.00%      
International Equity [Member]            
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]            
Target Plan Asset Allocations 20.00%   20.00%      
Minimum [Member] | Domestic All-Cap/Large Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.35      
Minimum [Member] | Domestic Mid-Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.05      
Minimum [Member] | Domestic Small-Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.05      
Minimum [Member] | International Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.1      
Maximum [Member] | Domestic All-Cap/Large Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.6      
Maximum [Member] | Domestic Mid-Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.25      
Maximum [Member] | Domestic Small-Cap Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.3      
Maximum [Member] | International Equity [Member]            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Defined Benefit Plan, Target Allocation Percentage     0.3      
[1] Unamortized prior service cost is amortized on a straight-line basis over the average remaining service period to the first eligibility age of participants who are expected to receive a benefit and are active at the date of the plan amendment.
[2] In addition to the $32.9 million, $22.5 million and $40.2 million of net periodic benefit cost recognized in 2017, 2016 and 2015, respectively, OG&E recognized the following: •a change in pension expense in 2017, 2016 and 2015 of $(2.3) million, $9.9 million and $(3.1) million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory asset or liability (see Note 1); •an increase in postretirement medical expense in 2017, 2016 and 2015 of $6.2 million, $7.9 million and $5.8 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory asset or liability (see Note 1); and •a deferral of pension expense in 2017, 2016 and 2015 of $1.1 million, $0.1 million and $1.9 million related to the Arkansas jurisdictional portion of the pension settlement charge of $15.3 million, $8.6 million and $21.7 million, respectively.
[3] GAAP allows the measurement of certain investments that do not have a readily determinable fair value at the net asset value. These investments do not consider the observability of inputs; therefore, they are not included within the fair value hierarchy.
[4] This category represents U.S. Treasury notes and bonds with a Moody's Investors Services rating of Aaa and Government Agency Bonds with a Moody's Investors Services rating of A1 or higher.
[5] This category represents units of participation in a commingled fund that primarily invested in stocks of international companies and emerging markets.