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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 38.5 $ 112.7
Regulatory Assets, Current 47.6 123.5
Regulatory Assets, Noncurrent 546.7 568.1
Regulatory Liability, Current 24.5 20.3
Fuel clause over recoveries 10.0 9.3
Regulatory Liability, Noncurrent 971.2 1,016.4
ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause over recoveries 10.0 9.3
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 38.5 112.7
Energy Efficiency Rider Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 11.9 0.0
Investment Tax Credit Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 2.6 6.1
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 0.0 4.9
Regulatory Liability, Noncurrent 3.6 3.7
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 733.8 780.8
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 233.8 231.9
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 89.8 94.7
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 0.0 7.0
SPP cost tracker under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 3.9 0.0
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 86.0 90.8
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 56.2 63.0
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 236.1 244.3
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.7 6.3
SB 998 Deferred Costs, Net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.0 0.0
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 15.8 16.4
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 16.7 14.3
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 9.6 11.0
Vegetation Management Program Deferred Costs [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 4.5 6.4
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 5.2 3.8
Regulatory Assets, Noncurrent 10.3 12.0
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 4.4 5.4
Capacity Power Purchase Agreements [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 6.6 $ 3.5
[1] Included in Other Current Liabilities in the condensed balance sheets.
[2] Included in Other Current Assets in the condensed balance sheets.