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Equity Investments - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2022 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) MW	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Equity Investments [Line Items]
Cash Investments			$ 8.2	$ 5.9	$ 17.6
Wind Turbine Generators | Tenaska | Nobles 2 PPA | Minnesota Power
Equity Investments [Line Items]
Generating capacity counterparty owned (MW) | MW			250
Contract term			20 years
ATC
Equity Investments [Line Items]
Approximate refund for reversal of FERC ROE order	$ 31.0
Approximate refund for reversal of FERC ROE Order, ALLETE's proportionate share	$ 2.4
Basis point incentive adder			50.00%
Approximate reduction in pretax income for FERC proposal to limit incentive adder, ALLETE's proportionate share			$ 1.0
ATC
Equity Investments [Line Items]
Ownership percentage			8.00%
Cash Investments			$ 8.2	$ 5.9
Authorized return on equity			10.02%
Authorized return on equity, including incentive adder			10.52%
ATC | Forecast
Equity Investments [Line Items]
Expected additional investment		$ 5.8
Nobles 2
Equity Investments [Line Items]
Ownership percentage			49.00%

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