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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Jun. 30, 2026 | Dec. 31, 2025
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Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Current |  | $ 12.7 | $ 11.4
Regulatory Assets, Noncurrent |  | 562.4 | 557.2
Regulatory Liability, Current |  | 163.9 | 34.9
Fuel clause over recoveries |  | 152.0 | 27.1
Regulatory Liability, Noncurrent |  | 935.6 | 957.1
Arkansas Fuel Clause Over Recoveries [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Current |  | 20.2 | 11.4
OKLAHOMA [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Current |  | 131.8 | 15.7
Other Regulatory Liabilities [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Current | [1] | 1.7 | 2.0
Regulatory Liability, Noncurrent |  | 4.4 | 3.5
Income taxes recoverable from customers, net [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Noncurrent |  | 689.9 | 718.9
Accrued removal obligations [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Noncurrent |  | 241.3 | 234.7
Pension tracker [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 84.2 | 88.7
Oklahoma Energy Efficiency Rider Under Recovery [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Liability, Current | [1] | 10.2 | 5.8
SPP cost tracker under recovery [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Current | [2] | 6.0 | 7.1
Benefit obligations regulatory asset [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 82.7 | 85.6
Deferred storm expenses [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 207.1 | 224.7
Unamortized loss on reacquired debt [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 5.1 | 5.5
SB 998 Deferred Costs, Net [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 64.5 | 22.9
Winter Storm Uri costs [Member] | ARKANSAS [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 50.0 | 54.2
Dry Scrubber Regulatory Asset [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 15.1 | 15.5
SAP S/4 HANA Deferred Expenses [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 16.3 | 16.5
Vegetation Management Program Deferred Costs [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 9.0 | 13.9
Deferred Pension Expenses [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 8.4 | 9.3
Other Regulatory Assets [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Current | [2] | 6.7 | 4.3
Regulatory Assets, Noncurrent |  | 8.8 | 9.6
COVID-19 Deferred Expenses [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | 3.5 | 4.1
Capacity Power Purchase Agreements [Member] |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets, Noncurrent |  | $ 7.7 | $ 6.7
[1] | Included in Other Current Liabilities in the condensed balance sheets.
[2] | Included in Other Current Assets in the condensed balance sheets.

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478742/980-340-50-1 + Details
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