Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 1,259.3	$ 1,227.9	$ 987.3
Contracts with Customers – Non-utility	303.8	179.9	170.5
Other – Non-utility	7.6	11.4	11.3
Total Operating Revenue	1,570.7	1,419.2	1,169.1
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	545.5	562.4	358.6
Transmission Services - Utility	76.7	75.3	67.0
Cost of Sales - Non-utility	182.8	68.8	66.7
Operating and Maintenance	318.9	259.2	252.0
Depreciation and Amortization	242.2	231.7	217.8
Taxes Other than Income Taxes	70.4	70.5	56.1
Impairment of Property, Plant and Equipment	0.0	0.0	12.7
Total Operating Expenses	1,436.5	1,267.9	1,030.9
Operating Income	134.2	151.3	138.2
Other Income (Expense) [Abstract]
Interest Expense	(75.2)	(69.1)	(65.6)
Equity Earnings	18.7	20.0	22.1
Other	22.4	8.7	14.7
Total Other Expense	(34.1)	(40.4)	(28.8)
Income Before Non-Controlling Interest and Income Taxes	100.1	110.9	109.4
Income Tax Benefit	(31.2)	(26.9)	(43.4)
Net Income	131.3	137.8	152.8
Net Loss Attributable to Non-Controlling Interest	(58.0)	(31.4)	(12.6)
Net Income Attributable to ALLETE	$ 189.3	$ 169.2	$ 165.4
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	55.9	52.4	51.9
Diluted (Shares)	56.0	52.5	51.9
Basic Earnings Per Share of Common Stock	$ 3.38	$ 3.23	$ 3.18
Diluted Earnings Per Share of Common Stock	$ 3.38	$ 3.23	$ 3.18