Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share - Basic [Abstract]
Net Income (Loss) Attributable to ALLETE	$ 40.7	$ 31.2	$ 127.1	$ 135.9
Average Common Shares	51.9	51.7	51.8	51.6
Earnings Per Share	$ 0.78	$ 0.60	$ 2.45	$ 2.63
Earnings Per Share - Diluted [Abstract]
Net Income	$ 40.7	$ 31.2	$ 127.1	$ 135.9
Average Common Shares	52.0	51.8	51.9	51.7
Earnings Per Share	$ 0.78	$ 0.60	$ 2.45	$ 2.63
Dilutive Securities (Shares)	0.1	0.1	0.1	0.1