Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 1,227.9	$ 987.3	$ 1,042.4
Contracts with Customers – Non-utility	179.9	170.5	186.5
Other – Non-utility	11.4	11.3	11.6
Total Operating Revenue	1,419.2	1,169.1	1,240.5
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	562.4	358.6	390.7
Transmission Services - Utility	75.3	67.0	69.8
Cost of Sales - Non-utility	68.8	66.7	80.6
Operating and Maintenance	259.2	252.0	264.3
Depreciation and Amortization	231.7	217.8	202.0
Taxes Other than Income Taxes	70.5	56.1	53.3
Total Operating Expenses	1,267.9	1,018.2	1,060.7
Operating Income	151.3	150.9	179.8
Other Income (Expense) [Abstract]
Interest Expense	(69.1)	(65.6)	(64.9)
Equity Earnings	20.0	22.1	21.7
Gain on Sale of U.S. Water Services	0.0	0.0	23.6
Other	8.7	14.7	18.7
Total Other Expense	(40.4)	(28.8)	(0.9)
Income Before Non-Controlling Interest and Income Taxes	110.9	122.1	178.9
Income Tax Benefit	(26.9)	(39.5)	(6.6)
Net Income	137.8	161.6	185.5
Net Loss Attributable to Non-Controlling Interest	(31.4)	(12.6)	(0.1)
Net Income Attributable to ALLETE	$ 169.2	$ 174.2	$ 185.6
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	52.4	51.9	51.6
Diluted (Shares)	52.5	51.9	51.7
Basic Earnings Per Share of Common Stock	$ 3.23	$ 3.36	$ 3.59
Diluted Earnings Per Share of Common Stock	$ 3.23	$ 3.35	$ 3.59