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Fair Value - Narrative (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Recurring Fair Value Measures [Line Items] |  |  |
Equity Investments | $ 331,200,000 | $ 322,700,000 |
Goodwill | 154,900,000 |  |
Impairment of long-lived assets held-for-use | 0 | 0 | $ 0
Boswell Unit 3 | 2021 Integrated Resource Plan | MPUC |  |  |
Recurring Fair Value Measures [Line Items] |  |  |
Net book value | 220,000,000 |  |
Boswell Unit 4 | 2021 Integrated Resource Plan | MPUC |  |  |
Recurring Fair Value Measures [Line Items] |  |  |
Net book value | 420,000,000 |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis |  |  |
Recurring Fair Value Measures [Line Items] |  |  |
Impairment of long-lived assets held-for-use | 0 | 0 |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis | Regulated Operations |  |  |
Recurring Fair Value Measures [Line Items] |  |  |
Indicators of impairment | 0 | 0 |
ATC |  |  |
Recurring Fair Value Measures [Line Items] |  |  |
Equity Investments | $ 179,700,000 | $ 165,400,000 | $ 154,500,000

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