Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2021	$ 318.0
Cash Investments	2.7	$ 1.0
Equity in ATC Earnings	5.5	$ 4.8
Equity Investment Balance as of March 31, 2022	$ 320.6
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, 2021	$ 154.5
Cash Investments	2.7
Equity in ATC Earnings	5.4
Distributed Earnings	(4.4)
Amortization of the Remeasurement of Deferred Income Taxes	0.3
Equity Investment Balance as of March 31, 2022	$ 158.5
Authorized Return on Equity	10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%
Approximate Refund for Reversal of FERC ROE Order	$ 66.0
Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportion	5.0
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share	$ 1.0
Basis Point Incentive adder	50.00%

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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Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share

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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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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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Percentage of public utility's approved return on equity.

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