Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 329.0	$ 293.0
Contracts with Customers - Non-utility	51.7	43.4
Other - Non-utility	2.8	2.8
Total Operating Revenue	383.5	339.2
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	137.4	120.4
Transmission Services - Utility	19.9	17.7
Cost of Sales - Non-utility	17.0	16.8
Operating and Maintenance	75.3	66.3
Depreciation and Amortization	61.7	58.0
Taxes Other than Income Taxes	18.8	18.0
Total Operating Expenses	330.1	297.2
Operating Income	53.4	42.0
Other Income (Expense) [Abstract]
Interest Expense	(18.3)	(17.1)
Equity Earnings	5.5	4.8
Other	2.0	3.3
Total Other Expense	(10.8)	(9.0)
Income Before Income Taxes	42.6	33.0
Income Tax Benefit	(3.9)	(10.4)
Net Income	46.5	43.4
Net Loss Attributable to Non-Controlling Interest	(19.8)	(8.4)
Net Income Attributable to ALLETE	$ 66.3	$ 51.8
Earnings Per Share [Abstract]
Basic (Shares)	53.3	52.1
Diluted (Shares)	53.3	52.2
Basic Earnings Per Share of Common Stock	$ 1.24	$ 0.99
Diluted Earnings Per Share of Common Stock	$ 1.24	$ 0.99