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Regulated Operations (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 0.0 $ 39.5
Current Regulatory Assets 15.5 48.7
Non-Current Regulatory Assets 301.6 306.0
Current Regulatory Liabilities 51.6 29.0
Fuel clause over recoveries 39.6 4.8
Non-Current Regulatory Liabilities 1,199.4 1,223.5
Reserve for Tax Refund [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 1.3 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] 5.8 2.0
Other (B)    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 4.9 6.9
Non-Current Regulatory Liabilities 5.0 5.8
Income taxes recoverable from customers, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 875.5 899.2
Accrued removal obligations, net    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 318.9 318.5
Benefit obligations regulatory asset    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 150.2 167.2
Deferred storm expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 70.2 65.5
Smart Grid    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 13.1 18.4
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 19.9 20.6
Unamortized loss on reacquired debt    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 9.9 10.6
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 8.4 8.0
Other Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 9.7 5.5
Non-Current Regulatory Assets 16.9 13.4
Pension tracker    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 8.1 2.3
SPP Cost Tracker Rider Under Recovery    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] 4.6 0.0
COVID-19 Deferred Expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 4.9 0.0
Generation Capacity Replacement Rider    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [2] $ 1.2 $ 3.7
[1] Included in Other Current Liabilities in the Condensed Consolidated Balance Sheets.    
[2] Included in Other Current Assets in the Condensed Consolidated Balance Sheets.