Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to ALLETE	$ 66.3	$ 51.8
Net Income Attributable to ALLETE	$ 66.3	$ 51.8
Basic (Shares)	53.3	52.1
Dilutive Securities (Shares)	0.0	0.1
Diluted (Shares)	53.3	52.2
Basic Earnings Per Share of Common Stock	$ 1.24	$ 0.99
Diluted Earnings Per Share of Common Stock	$ 1.24	$ 0.99