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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, 2021	Dec. 31, 2020	Dec. 31, 2019
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	$ 0.0
Equity Investments	318.0	301.2
Taconite Harbor [Member] | Resource Package [Member] | MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	45.0
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	245.0
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	154.5	149.0	$ 141.6
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

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- Definition

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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