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Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 0.0	$ 0.0
Equity Investments	322.7	318.0
Goodwill	154.9
Taconite Harbor [Member] \| Resource Package [Member] \| MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	25.0
Boswell Unit 3 [Member] \| 2021 Integrated Resource Plan [Member] \| MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	235.0
Boswell Unit 4 [Member] \| 2021 Integrated Resource Plan [Member] \| MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	435.0
ATC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Investments	165.4	154.5	$ 149.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] \| Corporate and Other [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Impairment of Long-Lived Assets Held-for-use	0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] \| ATC [Member] \| Regulated Operations [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Method Investment, Indicators of Impairment	$ 0.0	$ 0.0