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Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 0.0 $ 51.3
Fuel clause over recoveries 1.7 0.0
Regulatory Assets, Current 40.8 121.8
Regulatory Assets, Noncurrent 283.0 404.8
Regulatory Liability, Current 3.9 12.3
Regulatory Liability, Noncurrent 1,283.4 299.7
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 2.2 12.3
Regulatory Liability, Noncurrent 7.2 1.4
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 955.5 0.0
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 288.4 262.8
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 32.3 35.5
Oklahoma demand program rider under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 31.6 51.0
SPP cost tracker under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 7.7 10.0
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 1.5 9.5
Regulatory Assets, Noncurrent 18.5 17.6
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 177.2 232.6
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 0.0 62.3
Smart Grid [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 32.8 43.2
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 42.2 35.7
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 12.3 $ 13.4