Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 290.4	$ 200.8	$ 583.4	$ 466.1
Contracts with Customers - Non-utility	42.3	39.6	85.7	83.1
Other - Non-utility	2.9	2.8	5.7	5.6
Total Operating Revenue	335.6	243.2	674.8	554.8
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	128.9	69.3	249.3	158.3
Transmission Services - Utility	19.2	16.4	36.9	34.9
Cost of Sales - Non-utility	15.8	16.3	32.6	33.2
Operating and Maintenance	67.1	59.0	133.4	120.0
Depreciation and Amortization	57.9	54.5	115.9	107.9
Taxes Other than Income Taxes	18.5	15.0	36.5	27.6
Total Operating Expenses	307.4	230.5	604.6	481.9
Operating Income	28.2	12.7	70.2	72.9
Other Income (Expense) [Abstract]
Interest Expense	(17.4)	(15.9)	(34.5)	(31.6)
Equity Earnings	5.1	6.4	9.9	11.6
Other	1.8	5.2	5.1	6.2
Total Other Expense	(10.5)	(4.3)	(19.5)	(13.8)
Income Before Income Taxes	17.7	8.4	50.7	59.1
Income Tax Benefit	(4.0)	(8.5)	(14.4)	(22.3)
Net Income	21.7	16.9	65.1	81.4
Net Loss Attributable to Non-Controlling Interest	(6.2)	(3.2)	(14.6)	(5.0)
Net Income Attributable to ALLETE	$ 27.9	$ 20.1	$ 79.7	$ 86.4
Earnings Per Share [Abstract]
Basic (Shares)	52.2	51.8	52.2	51.8
Diluted (Shares)	52.3	51.9	52.2	51.8
Basic Earnings Per Share of Common Stock	$ 0.53	$ 0.39	$ 1.53	$ 1.67
Diluted Earnings Per Share of Common Stock	$ 0.53	$ 0.39	$ 1.53	$ 1.67