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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 43.3 $ 0.0
Regulatory Assets, Current 53.0 6.5
Regulatory Assets, Noncurrent 572.7 577.6
Regulatory Liability, Current 28.2 33.2
Fuel clause over recoveries 16.4 20.5
Regulatory Liability, Noncurrent 1,014.6 1,061.6
ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause over recoveries 16.4 4.9
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 43.3 0.0
Fuel clause over recoveries 0.0 15.6
SPP Cost Tracker Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 7.9 10.4
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 13.1 10.7
Regulatory Liability, Current [1] 3.9 2.3
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 796.1 838.0
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 216.6 222.0
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 102.0 91.9
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 5.0 3.8
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 247.1 261.1
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 89.8 89.4
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 64.8 71.0
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 16.6 17.2
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 11.4 12.7
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 6.6 7.2
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 4.7 2.7
Regulatory Assets, Noncurrent 1.9 1.6
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.7 6.8
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 13.1 6.0
Deferred Plant Expense [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 2.5 $ 3.6
[1] Included in Other Current Liabilities in the condensed balance sheets.
[2] Included in Other Current Assets in the condensed balance sheets.