Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 265.3	$ 282.2
Contracts with Customers - Non-utility	43.5	72.1
Other - Non-utility	2.8	2.9
Total Operating Revenue	311.6	357.2
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	89.0	109.8
Transmission Services - Utility	18.5	18.3
Cost of Sales - Non-utility	16.9	30.6
Operating and Maintenance	61.0	76.2
Depreciation and Amortization	53.4	51.9
Taxes Other than Income Taxes	12.6	13.6
Total Operating Expenses	251.4	300.4
Operating Income	60.2	56.8
Other Income (Expense) [Abstract]
Interest Expense	(15.7)	(16.5)
Equity Earnings	5.2	5.6
Gain on Sale of U.S. Water Services	0.0	20.1
Other	1.0	7.4
Total Other Income (Expense)	(9.5)	16.6
Income Before Income Taxes	50.7	73.4
Income Tax Expense (Benefit)	(13.8)	2.9
Net Income	64.5	70.5
Net Loss Attributable to Non-Controlling Interest	(1.8)	0.0
Net Income Attributable to ALLETE	$ 66.3	$ 70.5
Earnings Per Share [Abstract]
Basic (Shares)	51.7	51.6
Diluted (Shares)	51.8	51.7
Basic Earnings Per Share of Common Stock	$ 1.28	$ 1.37
Diluted Earnings Per Share of Common Stock	$ 1.28	$ 1.37