Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock	Accumulated Other Comprehensive Loss	Retained Earnings	Non-Controlling Interest in Subsidiaries
Balance, Beginning of Period at Dec. 31, 2021		$ 1,536.7	$ (23.8)	$ 900.2	$ 533.2
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		240.5
Unrealized Gain (Loss) on Debt Securities	$ (0.4)		(0.4)
Defined Benefit Pension and Other Postretirement Plans	0.4		0.4
Net Income Attributable to ALLETE	137.6			137.6
Common Stock Dividends				(108.6)
Proceeds from Non-Controlling Interest in Subsidiaries – Net					182.9
Net Loss Attributable to Non-Controlling Interest	43.5				(43.5)
Distributions to Non-Controlling Interest					(1.3)
Balance, End of Period at Sep. 30, 2022	$ 3,353.9	1,777.2	(23.8)	929.2	671.3
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock (in usd per share	$ 1.95
Balance, Beginning of Period at Dec. 31, 2021		1,536.7	(23.8)	900.2	533.2
Balance, End of Period at Dec. 31, 2022	$ 3,348.3	1,781.5	(24.4)	934.8	656.4
Balance, Beginning of Period at Jun. 30, 2022		1,771.7	(23.9)	932.6	678.5
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		5.5
Unrealized Gain (Loss) on Debt Securities	0.0
Defined Benefit Pension and Other Postretirement Plans	0.1		0.1
Net Income Attributable to ALLETE	33.7			33.7
Common Stock Dividends				(37.1)
Net Loss Attributable to Non-Controlling Interest	6.9				(6.9)
Distributions to Non-Controlling Interest					(0.3)
Balance, End of Period at Sep. 30, 2022	$ 3,353.9	1,777.2	(23.8)	929.2	671.3
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock (in usd per share	$ 0.65
Balance, Beginning of Period at Dec. 31, 2022	$ 3,348.3	1,781.5	(24.4)	934.8	656.4
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		15.7
Unrealized Gain (Loss) 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
Consolidated Statement of Equity [Roll Forward]
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 (Loss) on Debt Securities	$ 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)
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
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock (in usd per share	$ 0.6775