Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 987.3	$ 1,042.4	$ 1,059.5
Contracts with Customers – Non-utility	170.5	186.5	415.5
Other – Non-utility	11.3	11.6	23.6
Operating Revenue	1,169.1	1,240.5	1,498.6
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	358.6	390.7	407.5
Transmission Services - Utility	67.0	69.8	69.9
Cost of Sales - Non-utility	66.7	80.6	218.0
Operating and Maintenance	252.0	264.3	340.5
Depreciation and Amortization	217.8	202.0	205.6
Taxes Other than Income Taxes	56.1	53.3	57.9
Other	0.0	0.0	(2.0)
Total Operating Expenses	1,018.2	1,060.7	1,297.4
Operating Income	150.9	179.8	201.2
Other Income (Expense) [Abstract]
Interest Expense	(65.6)	(64.9)	(67.9)
Equity Earnings	22.1	21.7	17.5
Gain on Sale of U.S. Water Services	0.0	23.6	0.0
Other	14.7	18.7	7.8
Total Other Expense	(28.8)	(0.9)	(42.6)
Income Before Non-Controlling Interest and Income Taxes	122.1	178.9	158.6
Income Tax Benefit	(39.5)	(6.6)	(15.5)
Net Income	161.6	185.5	174.1
Net Loss Attributable to Non-Controlling Interest	(12.6)	(0.1)	0.0
Net Income Attributable to ALLETE	$ 174.2	$ 185.6	$ 174.1
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	51.9	51.6	51.3
Diluted (Shares)	51.9	51.7	51.5
Basic Earnings Per Share of Common Stock	$ 3.36	$ 3.59	$ 3.39
Diluted Earnings Per Share of Common Stock	$ 3.35	$ 3.59	$ 3.38