Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 200.8	$ 249.8	$ 466.1	$ 532.0
Contracts with Customers - Non-utility	39.6	37.7	83.1	109.8
Other - Non-utility	2.8	2.9	5.6	5.8
Total Operating Revenue	243.2	290.4	554.8	647.6
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	69.3	87.9	158.3	197.7
Transmission Services - Utility	16.4	19.2	34.9	37.5
Cost of Sales - Non-utility	16.3	16.5	33.2	47.1
Operating and Maintenance	59.0	66.7	120.0	142.9
Depreciation and Amortization	54.5	50.2	107.9	102.1
Taxes Other than Income Taxes	15.0	13.7	27.6	27.3
Total Operating Expenses	230.5	254.2	481.9	554.6
Operating Income	12.7	36.2	72.9	93.0
Other Income (Expense) [Abstract]
Interest Expense	(15.9)	(16.3)	(31.6)	(32.8)
Equity Earnings	6.4	4.8	11.6	10.4
Gain on Sale of U.S. Water Services	0.0	0.5	0.0	20.6
Other	5.2	4.2	6.2	11.6
Total Other Income (Expense)	(4.3)	(6.8)	(13.8)	9.8
Income Before Income Taxes	8.4	29.4	59.1	102.8
Income Tax Expense (Benefit)	(8.5)	(4.8)	(22.3)	(1.9)
Net Income	16.9	34.2	81.4	104.7
Net Loss Attributable to Non-Controlling Interest	(3.2)	0.0	(5.0)	0.0
Net Income Attributable to ALLETE	$ 20.1	$ 34.2	$ 86.4	$ 104.7
Earnings Per Share [Abstract]
Basic (Shares)	51.8	51.6	51.8	51.6
Diluted (Shares)	51.9	51.7	51.8	51.7
Basic Earnings Per Share of Common Stock	$ 0.39	$ 0.66	$ 1.67	$ 2.03
Diluted Earnings Per Share of Common Stock	$ 0.39	$ 0.66	$ 1.67	$ 2.02