Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2023	Dec. 31, 2022
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 0.0	$ 514.9
Regulatory Assets, Current		6.5	527.3
Regulatory Assets, Noncurrent		577.6	524.3
Regulatory Liability, Current		33.2	5.5
Regulatory Liability, Noncurrent		1,061.6	1,147.1
OKLAHOMA [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		0.0	474.3
ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		0.0	40.6
SPP Cost Tracker Over Recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	10.4	3.0
Oklahoma fuel clause over recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current		15.6	0.0
Arkansas fuel clause over recoveries [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current		4.9	0.0
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		838.0	894.7
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		222.0	250.5
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		91.9	57.2
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.3	2.5
Regulatory Liability, Noncurrent		1.6	1.9
Oklahoma Energy Efficiency Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	3.8	7.7
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		89.4	119.7
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		261.1	206.3
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		7.2	8.0
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		12.7	12.3
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		17.2	18.1
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	2.7	4.7
Regulatory Assets, Noncurrent		10.7	10.9
Deferred Plant Expense [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		3.6	5.2
COVID-19 Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.8	7.7
SAP S/4 HANA Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.0	0.7
Winter Storm Uri costs [Member] | ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 71.0	$ 78.2

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

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