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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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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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