Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
	Apr. 05, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to ALLETE		$ 33.7	$ 27.6	$ 137.6	$ 107.3
Net Income Attributable to ALLETE		$ 33.7	$ 27.6	$ 137.6	$ 107.3
Basic (Shares)		57.1	52.4	55.5	52.3
Dilutive Securities (Shares)		0.1	0.1	0.1	0.0
Diluted (Shares)		57.2	52.5	55.6	52.3
Basic Earnings Per Share of Common Stock		$ 0.59	$ 0.53	$ 2.48	$ 2.05
Diluted Earnings Per Share of Common Stock		$ 0.59	$ 0.53	$ 2.48	$ 2.05
Stock Issued During Period, Shares, New Issues	3.7
Proceeds from Issuance or Sale of Equity	$ 224.0