Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share - Basic [Abstract]
Net Income (Loss) Attributable to ALLETE	$ 66.3	$ 70.5
Average Common Shares	51.7	51.6
Earnings Per Share	$ 1.28	$ 1.37
Earnings Per Share - Diluted [Abstract]
Net Income	$ 66.3	$ 70.5
Average Common Shares	51.8	51.7
Earnings Per Share	$ 1.28	$ 1.37
Dilutive Securities (Shares)	0.1	0.1