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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2020	$ 301.2
Cash Investments	1.0	$ 27.8
Equity in ATC Earnings	4.8	$ 5.2
Equity Investment Balance as of March 31, 2021	$ 301.5
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage	8.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2020	$ 149.0
Equity in ATC Earnings	5.3
Distributed ATC Earnings	(4.3)
Amortization of the Remeasurement of Deferred Income Taxes	0.3
Equity Investment Balance as of March 31, 2021	$ 150.3
Authorized Return on Equity	10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%
Equity Method Investment, Expected Additional Investment Next Fiscal Year	$ 2.0

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ALLETE's Investment in ATC [Roll Forward]

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The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting.

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Additional investments expected to be made in the next fiscal year in an equity method investment.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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