Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share - Basic [Abstract]
Net Income Attributable ALLETE	$ 27.6	$ 40.7	$ 107.3	$ 127.1
Average Common Shares	52.4	51.9	52.3	51.8
Earnings Per Share	$ 0.53	$ 0.78	$ 2.05	$ 2.45
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 27.6	$ 40.7	$ 107.3	$ 127.1
Average Common Shares	52.5	52.0	52.3	51.9
Earnings Per Share	$ 0.53	$ 0.78	$ 2.05	$ 2.45
Dilutive Securities (Shares)	0.1	0.1	0.0	0.1