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Regulated Operations (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 0.0 $ 39.5
Current Regulatory Assets 16.7 48.7
Non-Current Regulatory Assets 299.4 306.0
Current Regulatory Liabilities 52.4 29.0
Fuel clause over recoveries 37.2 4.8
Non-Current Regulatory Liabilities 1,206.2 1,223.5
Reserve for Tax Refund [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 5.9 12.7
SPP Cost Tracker Rider Over Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 0.0 2.6
Oklahoma Demand Program Over Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 3.6 2.0
Other (B)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 5.7 6.9
Non-Current Regulatory Liabilities 5.3 5.8
Income taxes recoverable from customers, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 882.6 899.2
Accrued removal obligations, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 317.9 318.5
Pension tracker    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 0.4 0.0
Production Tax Credit Rider [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 4.1 3.7
Benefit obligations regulatory asset    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 163.0 167.2
Deferred storm expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 63.8 65.5
Smart Grid    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 14.8 18.4
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 20.2 20.6
Unamortized loss on reacquired debt    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 10.2 10.6
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 7.6 8.0
Other Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 6.8 5.5
Non-Current Regulatory Assets 16.0 13.4
Pension tracker    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 0.0 2.3
SPP Cost Tracker Rider Under Recovery    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 5.8 0.0
COVID-19 Deferred Expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets $ 3.8 $ 0.0
[1] Included in Other Current Liabilities in the Condensed Consolidated Balance Sheets. 
[2] Included in Other Current Assets in the Condensed Consolidated Balance Sheets.