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Summary of Significant Accounting Policies - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2024 USD ($) segment plan foundation plant reportingUnit | Dec. 31, 2023 USD ($) foundation | Dec. 31, 2022 USD ($) foundation
Property, Plant and Equipment [Line Items] |  |  |
Number of reportable segments | segment | 3 |  |
Number of jointly owned electric generation plants | plant | 2 |  |
Number of reporting units | reportingUnit | 3 |  |
Amortization period | 10 years |  |
Amortization expense | $ 3.0 | $ 1.3 | $ 1.4
Number of plans offered | plan | 2 |  |
Deferred compensation liability, company contributions | $ 29.1 | 24.6 |
Deferred compensation arrangement, contributions by employer | 1.3 | 1.2 | 0.9
Gains or (losses) recognized due to changes in payment obligations | (3.3) | $ (3.3) | $ 3.1
Coyote Creek Mining Company, L.L.C. (CCMC) | Lignite Sales Agreement | OTP |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | $ 40.0 |  |
Variable interest entity, reporting entity involvement, maximum loss exposure (as a percent) | 35.00% |  |
Contribution Obligation | Subsidiaries |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Number of foundations transacting with | foundation | 2 | 2 |
Contribution obligations | $ 5.5 | $ 5.5 |
Contribution Obligation Paid | Subsidiaries |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Number of foundations transacting with | foundation | 2 | 2 | 2
Amounts of transaction | $ 5.5 | $ 4.3 | $ 4.5
Minimum |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Intangible asset, useful life | 15 years |  |
Operating lease, term | 1 year |  |
Maximum |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Intangible asset, useful life | 20 years |  |
Operating lease, term | 10 years |  |
Electric Plant in Service |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Interest costs capitalized | $ 1.9 | $ 1.9 | $ 0.9

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