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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries $ 53.0 $ 514.9
Regulatory Assets, Current 61.0 527.3
Regulatory Assets, Noncurrent 581.8 524.3
Regulatory Liability, Current 12.5 5.5
Regulatory Liability, Noncurrent 1,079.3 1,147.1
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 45.2 474.3
ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 7.8 40.6
SPP Cost Tracker Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 10.6 3.0
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 1.9 2.5
Regulatory Liability, Noncurrent 1.6 1.9
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 853.1 894.7
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 224.6 250.5
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 87.2 57.2
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 3.0 7.7
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 262.7 206.3
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 94.7 119.7
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 72.5 78.2
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 17.4 18.1
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 13.0 12.3
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 7.4 8.0
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 5.0 4.7
Regulatory Assets, Noncurrent 10.3 10.9
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 7.1 7.7
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 5.5 0.7
Deferred Plant Expense [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 4.0 $ 5.2
[1] Included in Other Current Liabilities in the condensed balance sheets.
[2] Included in Other Current Assets in the condensed balance sheets.