Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 322.6	$ 304.8	$ 960.3	$ 888.2
Contracts with Customers - Non-utility	64.5	37.7	178.3	123.4
Other - Non-utility	1.2	2.9	6.3	8.6
Total Operating Revenue	388.3	345.4	1,144.9	1,020.2
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	136.8	140.1	417.4	389.4
Transmission Services - Utility	19.3	19.2	57.5	56.1
Cost of Sales - Non-utility	38.4	15.2	96.9	47.8
Operating and Maintenance	83.2	66.7	238.1	200.1
Depreciation and Amortization	58.7	57.5	181.4	173.4
Taxes Other than Income Taxes	18.5	15.6	53.1	52.1
Total Operating Expenses	354.9	314.3	1,044.4	918.9
Operating Income	33.4	31.1	100.5	101.3
Other Income (Expense) [Abstract]
Interest Expense	(18.4)	(17.3)	(55.3)	(51.8)
Equity Earnings	2.3	4.4	13.1	14.3
Other	2.3	1.0	16.4	6.1
Total Other Expense	(13.8)	(11.9)	(25.8)	(31.4)
Income Before Income Taxes	19.6	19.2	74.7	69.9
Income Tax Benefit	(7.2)	(4.9)	(19.4)	(19.3)
Net Income	26.8	24.1	94.1	89.2
Net Loss Attributable to Non-Controlling Interest	(6.9)	(3.5)	(43.5)	(18.1)
Net Income Attributable to ALLETE	$ 33.7	$ 27.6	$ 137.6	$ 107.3
Earnings Per Share [Abstract]
Basic (Shares)	57.1	52.4	55.5	52.3
Diluted (Shares)	57.2	52.5	55.6	52.3
Basic Earnings Per Share of Common Stock	$ 0.59	$ 0.53	$ 2.48	$ 2.05
Diluted Earnings Per Share of Common Stock	$ 0.59	$ 0.53	$ 2.48	$ 2.05