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Summary of Significant Accounting Policies - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) plant unit segment | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Property, Plant and Equipment [Line Items] |  |  |
Number of operating segments | segment | 3 |  |
Number of jointly owned electric generation plants | plant | 2 |  |
Number of jointly owned major transmission lines | plant | 5 |  |
Number of reporting units | unit | 3 |  |
Electric Plant in Service |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Interest costs capitalized | $ | $ 0.6 | $ 2.1 | $ 1.7
OTP | Lignite Sales Agreement | Coyote Creek Mining Company, L.L.C. (CCMC) |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | $ | $ 45.0 |  |
Variable interest entity, reporting entity involvement, maximum loss exposure (as a percent) | 35.00% |  |
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 |  |

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