CONSOLIDATED STATEMENT OF EQUITY Unaudited - USD ($) $ in Millions	Total	Common Stock	Accumulated Other Comprehensive Loss	Retained Earnings	Non-Controlling Interest in Subsidiaries
Balance, Beginning of Period at Dec. 31, 2022		$ 1,781.5	$ (24.4)	$ 934.8	$ 656.4
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		15.7
Unrealized Gain on Debt Securities	$ 0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(0.2)		(0.2)
Net Income Attributable to ALLETE	195.6			195.6
Common Stock Dividends				(116.5)
Proceeds from Non-Controlling Interest in Subsidiaries – Net					9.9
Net Loss Attributable to Non-Controlling Interest	47.7				(47.7)
Distributions to Non-Controlling Interest					(8.5)
Balance, End of Period at Sep. 30, 2023	3,396.7	1,797.2	(24.5)	1,013.9	610.1
Balance, Beginning of Period at Dec. 31, 2022	0.0
Redeemable Non-Controlling Interest
Proceeds from Non-Controlling Interest in Subsidiaries	0.0
Net Loss Attributable to Non-Controlling Interest	0.0
Total Redeemable Non-Controlling Interest at Sep. 30, 2023	$ 0.0
Redeemable Non-Controlling Interest
Dividends Per Share of Common Stock (in usd per share)	$ 2.0325
Balance, Beginning of Period at Jun. 30, 2023		1,791.6	(24.4)	966.9	634.4
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		5.6
Unrealized Gain on Debt Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	(0.1)		(0.1)
Net Income Attributable to ALLETE	85.9			85.9
Common Stock Dividends				(38.9)
Proceeds from Non-Controlling Interest in Subsidiaries – Net					0.0
Net Loss Attributable to Non-Controlling Interest	16.3				(16.3)
Distributions to Non-Controlling Interest					(8.0)
Balance, End of Period at Sep. 30, 2023	3,396.7	1,797.2	(24.5)	1,013.9	610.1
Balance, Beginning of Period at Jun. 30, 2023	0.0
Redeemable Non-Controlling Interest
Proceeds from Non-Controlling Interest in Subsidiaries	0.0
Net Loss Attributable to Non-Controlling Interest	0.0
Total Redeemable Non-Controlling Interest at Sep. 30, 2023	$ 0.0
Redeemable Non-Controlling Interest
Dividends Per Share of Common Stock (in usd per share)	$ 0.6775
Balance, Beginning of Period at Dec. 31, 2023	$ 3,406.6	1,803.7	(20.5)	1,026.4	597.0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		15.4
Unrealized Gain on Debt Securities	0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(0.6)		(0.6)
Net Income Attributable to ALLETE	128.7			128.7
Common Stock Dividends				(122.0)
Proceeds from Non-Controlling Interest in Subsidiaries – Net					1.4
Net Loss Attributable to Non-Controlling Interest	39.3				(36.8)
Distributions to Non-Controlling Interest					(0.9)
Balance, End of Period at Sep. 30, 2024	3,391.9	1,819.1	(21.0)	1,033.1	560.7
Balance, Beginning of Period at Dec. 31, 2023	0.5
Redeemable Non-Controlling Interest
Proceeds from Non-Controlling Interest in Subsidiaries	2.7
Net Loss Attributable to Non-Controlling Interest	(2.5)
Total Redeemable Non-Controlling Interest at Sep. 30, 2024	$ 0.7
Redeemable Non-Controlling Interest
Dividends Per Share of Common Stock (in usd per share)	$ 2.115
Balance, Beginning of Period at Jun. 30, 2024		1,813.8	(20.9)	1,028.8	573.0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		5.3
Unrealized Gain on Debt Securities	$ 0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(0.2)		(0.2)
Net Income Attributable to ALLETE	45.0			45.0
Common Stock Dividends				(40.7)
Proceeds from Non-Controlling Interest in Subsidiaries – Net					0.0
Net Loss Attributable to Non-Controlling Interest	12.3				(12.1)
Distributions to Non-Controlling Interest					(0.2)
Balance, End of Period at Sep. 30, 2024	3,391.9	$ 1,819.1	$ (21.0)	$ 1,033.1	$ 560.7
Balance, Beginning of Period at Jun. 30, 2024	0.9
Redeemable Non-Controlling Interest
Proceeds from Non-Controlling Interest in Subsidiaries	0.0
Net Loss Attributable to Non-Controlling Interest	(0.2)
Total Redeemable Non-Controlling Interest at Sep. 30, 2024	$ 0.7
Redeemable Non-Controlling Interest
Dividends Per Share of Common Stock (in usd per share)	$ 0.705