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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current $ 4.0 $ 6.5
Regulatory Assets, Noncurrent 576.6 577.6
Regulatory Liability, Current 48.3 33.2
Regulatory Liability, Noncurrent 1,047.4 1,061.6
OKLAHOMA [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 19.9 15.6
ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Fuel clause under recoveries 16.6 4.9
SPP Cost Tracker Over Recoveries [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 10.8 10.4
Other Regulatory Liabilities [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current [1] 1.0 2.3
Regulatory Liability, Noncurrent 1.5 1.6
Income taxes recoverable from customers, net [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 823.9 838.0
Accrued removal obligations [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Noncurrent 222.0 222.0
Pension tracker [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 95.6 91.9
Oklahoma Energy Efficiency Rider Under Recovery [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 1.7 3.8
Deferred storm expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 254.9 261.1
Benefit obligations regulatory asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 92.8 89.4
Winter Storm Uri costs [Member] | ARKANSAS [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 69.2 71.0
Dry Scrubber Regulatory Asset [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 17.0 17.2
Deferred Pension Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 12.4 12.7
Unamortized loss on reacquired debt [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 7.0 7.2
Other Regulatory Assets [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current [2] 2.3 2.7
Regulatory Assets, Noncurrent 10.5 10.7
COVID-19 Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 6.4 6.8
SAP S/4 HANA Deferred Expenses [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent 7.6 6.0
Deferred Plant Expense [Member]    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Noncurrent $ 3.2 $ 3.6
[1] Included in Other Current Liabilities in the condensed balance sheets.
[2] Included in Other Current Assets in the condensed balance sheets.