Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 304.8	$ 255.1	$ 888.2	$ 721.2
Contracts with Customers - Non-utility	37.7	35.9	123.4	119.0
Other - Non-utility	2.9	2.9	8.6	8.5
Total Operating Revenue	345.4	293.9	1,020.2	848.7
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	140.1	93.4	389.4	251.7
Transmission Services - Utility	19.2	14.9	56.1	49.8
Cost of Sales - Non-utility	15.2	15.4	47.8	48.6
Operating and Maintenance	66.7	61.9	200.1	181.9
Depreciation and Amortization	57.5	53.4	173.4	161.3
Taxes Other than Income Taxes	15.6	13.3	52.1	40.9
Total Operating Expenses	314.3	252.3	918.9	734.2
Operating Income	31.1	41.6	101.3	114.5
Other Income (Expense) [Abstract]
Interest Expense	(17.3)	(16.3)	(51.8)	(47.9)
Equity Earnings	4.4	5.1	14.3	16.7
Other	1.0	2.9	6.1	9.1
Total Other Expense	(11.9)	(8.3)	(31.4)	(22.1)
Income Before Income Taxes	19.2	33.3	69.9	92.4
Income Tax Benefit	(4.9)	(5.5)	(19.3)	(27.8)
Net Income	24.1	38.8	89.2	120.2
Net Loss Attributable to Non-Controlling Interest	(3.5)	(1.9)	(18.1)	(6.9)
Net Income Attributable to ALLETE	$ 27.6	$ 40.7	$ 107.3	$ 127.1
Earnings Per Share [Abstract]
Basic (Shares)	52.4	51.9	52.3	51.8
Diluted (Shares)	52.5	52.0	52.3	51.9
Basic Earnings Per Share of Common Stock	$ 0.53	$ 0.78	$ 2.05	$ 2.45
Diluted Earnings Per Share of Common Stock	$ 0.53	$ 0.78	$ 2.05	$ 2.45