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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 112.7 $ 0.0
Regulatory Assets, Current 123.5 6.5
Regulatory Assets, Noncurrent 568.1 577.6
Regulatory Liability, Current 20.3 33.2
Regulatory Liability, Noncurrent 1,016.4 1,061.6
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 112.7 0.0
SPP Cost Tracker Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 4.1 10.4
Oklahoma fuel clause over recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 0.0 15.6
Arkansas fuel clause over recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 9.3 4.9
Investment Tax Credit Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 6.1 0.0
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 780.8 838.0
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 231.9 222.0
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 94.7 91.9
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 0.8 2.3
Regulatory Liability, Noncurrent 3.7 1.6
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 7.0 3.8
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 90.8 89.4
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 63.0 71.0
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 244.3 261.1
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 6.3 7.2
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 16.4 17.2
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 14.3 6.0
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 11.0 12.7
Vegetation Management Program Deferred Costs [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 6.4 0.0
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 3.8 2.7
Regulatory Assets, Noncurrent 12.0 13.7
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.4 6.8
Capacity Power Purchase Agreements [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 3.5 $ 0.6
[1] Included in Other Current Liabilities in the balance sheets.
[2] Included in Other Current Assets in the balance sheets.