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Regulated Operations (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause over recoveries $ 38.7 $ 1.7
Current Regulatory Assets 19.7 40.8
Non-Current Regulatory Assets 270.2 283.0
Current Regulatory Liabilities 41.2 3.9
Non-Current Regulatory Liabilities 1,296.1 1,283.4
Other (B)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 2.5 2.2
Non-Current Regulatory Liabilities 7.3 7.2
Income taxes recoverable from customers, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 944.2 955.5
Accrued removal obligations, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 300.2 288.4
Pension tracker    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 44.4 32.3
Oklahoma demand program rider under recovery (A)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 16.7 31.6
SPP cost tracker under recovery (A)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 0.8 7.7
Benefit obligations regulatory asset    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 171.9 177.2
Deferred storm expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 39.0 42.2
Smart Grid    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 29.3 32.8
Unamortized loss on reacquired debt    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 11.8 12.3
Other Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 2.2 1.5
Non-Current Regulatory Assets $ 18.2 $ 18.5
[1] Included in Other Current Liabilities on the Condensed Consolidated Balance Sheets.
[2] Included in Other Current Assets on the Condensed Consolidated Balance Sheets.