Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 255.1	$ 254.1	$ 721.2	$ 786.1
Contracts with Customers - Non-utility	35.9	31.3	119.0	141.1
Other - Non-utility	2.9	2.9	8.5	8.7
Total Operating Revenue	293.9	288.3	848.7	935.9
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	93.4	98.2	251.7	295.9
Transmission Services - Utility	14.9	18.3	49.8	55.8
Cost of Sales - Non-utility	15.4	14.7	48.6	61.8
Operating and Maintenance	61.9	58.1	181.9	201.0
Depreciation and Amortization	53.4	49.5	161.3	151.6
Taxes Other than Income Taxes	13.3	12.5	40.9	39.8
Total Operating Expenses	252.3	251.3	734.2	805.9
Operating Income	41.6	37.0	114.5	130.0
Other Income (Expense) [Abstract]
Interest Expense	(16.3)	(16.1)	(47.9)	(48.9)
Equity Earnings	5.1	4.9	16.7	15.3
Gain on Sale of U.S. Water Services	0.0	0.0	0.0	20.6
Other	2.9	3.0	9.1	14.6
Total Other Income (Expense)	(8.3)	(8.2)	(22.1)	1.6
Income Before Income Taxes	33.3	28.8	92.4	131.6
Income Tax Expense (Benefit)	(5.5)	(2.4)	(27.8)	(4.3)
Net Income	38.8	31.2	120.2	135.9
Net Loss Attributable to Non-Controlling Interest	(1.9)	0.0	(6.9)	0.0
Net Income Attributable to ALLETE	$ 40.7	$ 31.2	$ 127.1	$ 135.9
Earnings Per Share [Abstract]
Basic (Shares)	51.9	51.7	51.8	51.6
Diluted (Shares)	52.0	51.8	51.9	51.7
Basic Earnings Per Share of Common Stock	$ 0.78	$ 0.60	$ 2.45	$ 2.63
Diluted Earnings Per Share of Common Stock	$ 0.78	$ 0.60	$ 2.45	$ 2.63