Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2025	Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 97.9	$ 112.7
Regulatory Assets, Current		109.3	123.5
Regulatory Assets, Noncurrent		561.8	568.1
Regulatory Liability, Current		13.4	20.3
Fuel clause over recoveries		6.3	9.3
Regulatory Liability, Noncurrent		985.2	1,016.4
ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause over recoveries		6.3	9.3
OKLAHOMA [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		97.9	112.7
Energy Efficiency Rider Over Recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	3.8	0.0
Investment Tax Credit Over Recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.8	6.1
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	0.5	4.9
Regulatory Liability, Noncurrent		3.5	3.7
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		748.6	780.8
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		233.1	231.9
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		91.6	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]	4.6	0.0
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		87.6	90.8
Winter Storm Uri costs [Member] | ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		59.0	63.0
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		248.3	244.3
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		5.9	6.3
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		16.0	16.4
SAP S/4 HANA Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		16.4	14.3
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		10.1	11.0
Vegetation Management Program Deferred Costs [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		7.1	6.4
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	6.8	3.8
Regulatory Assets, Noncurrent		10.7	12.0
COVID-19 Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		4.8	5.4
Capacity Power Purchase Agreements [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 4.3	$ 3.5

[1] Included in Other Current Liabilities in the condensed balance sheets.

[2] Included in Other Current Assets in the condensed balance sheets.