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Regulated Operations (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 35.5 $ 51.3
Current Regulatory Assets 84.2 121.8
Non-Current Regulatory Assets 373.4 404.8
Current Regulatory Liabilities 3.3 12.3
Non-Current Regulatory Liabilities 331.7 299.7
Other (B)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 3.3 12.3
Non-Current Regulatory Liabilities 9.3 1.4
Accrued removal obligations, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 282.0 262.8
Pension tracker    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 40.4 35.5
Oklahoma demand program rider under recovery (A)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 38.3 51.0
SPP cost tracker under recovery (A)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 7.8 10.0
Benefit obligations regulatory asset    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 189.0 232.6
Income taxes recoverable from customers, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 76.3 62.3
Deferred storm expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 43.3 35.7
Smart Grid    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 34.7 43.2
Unamortized loss on reacquired debt    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 12.5 13.4
Other Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 2.6 9.5
Non-Current Regulatory Assets $ 17.6 $ 17.6
[1] Included in Other Current Liabilities on the Condensed Consolidated Balance Sheets.
[2] Included in Other Current Assets on the Condensed Consolidated Balance Sheets.