Acquisitions (Details) - USD ($) $ in Millions			12 Months Ended
		Apr. 15, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows			$ 40.2	$ 47.7	$ 65.2	$ 92.5
Goodwill			154.9
New Energy
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired		100.00%
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		$ 3.9
Accounts Receivable		1.4
Inventory	[1]	25.3
Other Current Assets		12.8
Property, Plant, and Equipment		16.4
Goodwill	[2]	154.9
Other Noncurrent Assets		2.1
Total Assets Acquired		216.8
Total Liabilities Assumed		58.0
Net Identifiable Assets Acquired		158.8
Current Liabilities		23.6
Long-Term Debt due Within One Year		28.3
Long-Term Debt		5.9
Other No-Current Liabilities		0.2
Business Combinations, Fair Value Adjustments		11.6
Business Acquisition, Goodwill, Expected Tax Deductible Amount		154.9
Business Combination, Acquisition Related Costs			$ 2.7
Transaction Purchase Price		165.5
Business Combination, Consideration Transferred		$ 158.8
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			$ 80.2
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.

[1] Includes $11.6 million of purchase price accounting for certain projects under development at the time of acquisition.

[2] For tax purpose, the purchase price allocation resulted in $154.9 million of deductible goodwill.