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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current $ 12.7 $ 11.4
Regulatory Assets, Noncurrent 562.4 557.2
Regulatory Liability, Current 163.9 34.9
Fuel clause over recoveries 152.0 27.1
Regulatory Liability, Noncurrent 935.6 957.1
Arkansas Fuel Clause Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 20.2 11.4
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 131.8 15.7
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 1.7 2.0
Regulatory Liability, Noncurrent 4.4 3.5
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 689.9 718.9
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 241.3 234.7
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 84.2 88.7
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 10.2 5.8
SPP cost tracker under recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 6.0 7.1
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 82.7 85.6
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 207.1 224.7
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.1 5.5
SB 998 Deferred Costs, Net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 64.5 22.9
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 50.0 54.2
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 15.1 15.5
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 16.3 16.5
Vegetation Management Program Deferred Costs [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 9.0 13.9
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 8.4 9.3
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 6.7 4.3
Regulatory Assets, Noncurrent 8.8 9.6
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 3.5 4.1
Capacity Power Purchase Agreements [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 7.7 $ 6.7
[1] Included in Other Current Liabilities in the condensed balance sheets.
[2] Included in Other Current Assets in the condensed balance sheets.