Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2024	Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current		$ 10.9	$ 6.5
Regulatory Assets, Noncurrent		586.2	577.6
Regulatory Liability, Current		37.6	33.2
Regulatory Liability, Noncurrent		1,030.5	1,061.6
ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		18.7	4.9
OKLAHOMA [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		11.1	15.6
SPP Cost Tracker Over Recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	7.2	10.4
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	0.6	2.3
Regulatory Liability, Noncurrent		1.8	1.6
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		811.3	838.0
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		217.4	222.0
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		98.6	91.9
Oklahoma Energy Efficiency Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	5.7	3.8
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		262.3	261.1
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		91.4	89.4
Winter Storm Uri costs [Member] | ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		67.5	71.0
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		16.8	17.2
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		11.9	12.7
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.8	7.2
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	5.2	2.7
Regulatory Assets, Noncurrent		11.1	10.7
COVID-19 Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.0	6.8
SAP S/4 HANA Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		10.9	6.0
Deferred Plant Expense [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 2.9	$ 3.6

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

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