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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2021			$ 318.0
Cash Investments			3.9	$ 17.1
Equity in ATC Earnings	$ 5.3	$ 5.1	10.8	$ 9.9
Equity Investment Balance as of June 30, 2022	$ 322.0		$ 322.0
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage	8.00%		8.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2021			$ 154.5
Cash Investments			3.9
Equity in ATC Earnings			10.7
Distributed Earnings			(8.6)
Amortization of the Remeasurement of Deferred Income Taxes			0.7
Equity Investment Balance as of June 30, 2022	$ 161.2		$ 161.2
Authorized Return on Equity			10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%		10.52%
Approximate Refund for Reversal of FERC ROE Order	$ 66.0		$ 66.0
Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportion	5.0		5.0
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share	$ 1.0		$ 1.0
Basis Point Incentive adder	50.00%		50.00%

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ALLETE's Investment in ATC [Roll Forward]

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