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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2022	$ 322.7
Cash Investments	0.8	$ 2.7
Equity in ATC Earnings	6.0	$ 5.5
Equity Investment Balance as of March 31, 2023	$ 323.9
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, 2022	$ 165.4
Cash Investments	0.8
Equity in ATC Earnings	6.0
Distributed Earnings	(4.8)
Amortization of the Remeasurement of Deferred Income Taxes	0.4
Equity Investment Balance as of March 31, 2023	$ 167.8
Authorized Return on Equity	10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%
Approximate Annual Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share	$ 1.0
Basis Point Incentive adder	50.00%

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ALLETE's Investment in ATC [Roll Forward]

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Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments

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Basis Point Incentive adder

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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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Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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