Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Non-controlling Interest [Member]
Balance, Beginning Balance at Dec. 31, 2020		$ 1,460.9	$ (31.1)	$ 864.8	$ 505.6
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		6.7
Unrealized Loss on Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.4		0.4
Net Income Attributable ALLETE	51.8			51.8
Common Stock Dividends	32.8			(32.8)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs	28.9				28.9
Net Loss Attributable to Non-Controlling Interest	(8.4)				(8.4)
Distributions to Non-controlling Interest					(0.4)
Balance, End of Period at Mar. 31, 2021	$ 2,846.4	1,467.6	(30.7)	883.8	525.7
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.63
Balance, Beginning Balance at Dec. 31, 2021	$ 2,946.3	1,536.7	(23.8)	900.2	533.2
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		4.6
Unrealized Loss on Securities	(0.3)		(0.3)
Defined Benefit Pension and Other Postretirement Plans	0.1		0.1
Net Income Attributable ALLETE	66.3			66.3
Common Stock Dividends	34.5			(34.5)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs	154.1				181.2
Net Loss Attributable to Non-Controlling Interest	(19.8)				(19.8)
Distributions to Non-controlling Interest					(0.4)
Balance, End of Period at Mar. 31, 2022	$ 3,143.5	$ 1,541.3	$ (24.0)	$ 932.0	$ 694.2
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.65