Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Mar. 31, 2023	Dec. 31, 2022
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 370.1	$ 514.9
Regulatory Assets, Current		378.6	527.3
Regulatory Assets, Noncurrent		531.6	524.3
Regulatory Liability, Current		9.3	5.5
Regulatory Liability, Noncurrent		1,123.2	1,147.1
OKLAHOMA [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		334.7	474.3
ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		35.4	40.6
SPP Cost Tracker Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	6.9	3.0
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.4	2.5
Regulatory Liability, Noncurrent		1.8	1.9
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		880.2	894.7
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		241.2	250.5
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		77.7	57.2
Oklahoma Energy Efficiency Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	3.3	7.7
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		212.4	206.3
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		100.6	119.7
Winter Storm Uri costs [Member] | ARKANSAS [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		76.4	78.2
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		17.9	18.1
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		13.5	12.3
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		7.8	8.0
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	5.2	4.7
Regulatory Assets, Noncurrent		12.9	11.6
COVID-19 Deferred Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		7.6	7.7
Deferred Plant Expense [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 4.8	$ 5.2

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

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