Business Combinations and Asset Acquisitions (Details) - New Energy - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2022	Jun. 30, 2022	Apr. 15, 2022
Business Acquisition [Line Items]
Percentage of Voting Interests Acquired				100.00%
Transaction Purchase Price				$ 165.5
Business Combination, Consideration Transferred			$ 158.8
Business Acquisition, Pro Forma Information, Description			The Company has not presented separate results of operations since closing or combined pro forma financial information of the Company and New Energy since the beginning of 2021, as the results of operations for New Energy are not material to the Company's consolidated financials.
Business Combination, Reason for Business Combination			The acquisition of New Energy is consistent with ALLETE’s stated strategy of additional investment in renewable energy and related infrastructure across North America to support the Company’s sustainability-in-action strategy while providing potential long-term earnings growth.
Business Combination, Goodwill Recognized, Description			The goodwill recorded is primarily attributable to the highly skilled workforce of New Energy and synergies expected to arise as a result of the acquisition.
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual		$ 16.8
Cash and Equivalents				3.9
Account Receivable				1.4
Inventory				23.2
Other Current Assets				13.2
Property, Plant, and Equipment				16.4
Goodwill	[1]			150.3
Other Noncurrent Assets				2.2
Total Assets Acquired				210.6
Current Liabilities				17.4
Long-Term Debt Due Within One Year				28.3
Long-Term Debt				5.9
Other Non-Current Liabilities				0.2
Total Liabilities Assumed				51.8
Net Identifiable Assets Acquired				158.8
Business Acquisition, Goodwill, Expected Tax Deductible Amount				$ 150.3
Business Combination, Acquisition Related Costs			$ 3.0

[1] For tax purpose, the purchase price allocation resulted in $150.3 million of deductible goodwill.