Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Operating Revenues [Abstract]
Contracts with Customers - Utility	$ 292.2	$ 308.7	$ 604.8	$ 637.7
Contracts with Customers - Non-utility	239.9	62.1	490.9	113.8
Other - Non-utility	1.3	2.3	2.6	5.1
Total Operating Revenue	533.4	373.1	1,098.3	756.6
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	107.3	143.2	225.9	280.6
Transmission Services - Utility	23.5	18.3	43.6	38.2
Cost of Sales - Non-utility	193.2	41.5	403.7	58.5
Operating and Maintenance	84.9	79.6	170.6	154.9
Depreciation and Amortization	62.8	61.0	125.1	122.7
Taxes Other than Income Taxes	8.2	15.8	27.6	34.6
Total Operating Expenses	479.9	359.4	996.5	689.5
Operating Income	53.5	13.7	101.8	67.1
Other Income (Expense) [Abstract]
Interest Expense	(21.1)	(18.6)	(40.4)	(36.9)
Equity Earnings	5.4	5.3	11.4	10.8
Other	2.5	12.1	6.6	14.1
Total Other Expense	(13.2)	(1.2)	(22.4)	(12.0)
Income Before Income Taxes	40.3	12.5	79.4	55.1
Income Tax Benefit	(0.4)	(8.3)	1.1	(12.2)
Net Income	40.7	20.8	78.3	67.3
Net Loss Attributable to Non-Controlling Interest	(10.8)	(16.8)	(31.4)	(36.6)
Net Income Attributable to ALLETE	$ 51.5	$ 37.6	$ 109.7	$ 103.9
Earnings Per Share [Abstract]
Basic (Shares)	57.3	56.1	57.3	54.9
Diluted (Shares)	57.4	56.1	57.4	54.9
Basic Earnings Per Share of Common Stock	$ 0.90	$ 0.67	$ 1.91	$ 1.89
Diluted Earnings Per Share of Common Stock	$ 0.90	$ 0.67	$ 1.91	$ 1.89