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Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 0.0 $ 51.3
Fuel clause over recoveries 61.3 0.0
Regulatory Assets, Current 46.5 121.8
Regulatory Assets, Noncurrent 402.2 404.8
Regulatory Liability, Current 68.8 12.3
Regulatory Liability, Noncurrent 273.6 299.7
Prior Period Reclassification Adjustment 1.0  
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 7.5 12.3
Regulatory Liability, Noncurrent [2] 1.0 1.4
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 254.9 262.8
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 17.7 35.5
Oklahoma demand program rider under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [3] 36.6 51.0
SPP cost tracker under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [3] 4.5 10.0
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [3] 5.4 9.5
Regulatory Assets, Noncurrent 17.3 17.6
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 242.2 232.6
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 56.7 62.3
Smart Grid [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 43.6 43.2
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 27.6 35.7
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 14.8 $ 13.4
[1] Included in Other Current Liabilities on the Consolidated Balance Sheets.
[2] Prior year amount of $1.0 million reclassified from Deferred Other Liabilities to Non-Current Regulatory Liabilities.
[3] Included in Other Current Assets on the Consolidated Balance Sheets.