Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share - Basic [Abstract]
Net Income (Loss) Attributable to ALLETE	$ 20.1	$ 34.2	$ 86.4	$ 104.7
Average Common Shares	51.8	51.6	51.8	51.6
Earnings Per Share	$ 0.39	$ 0.66	$ 1.67	$ 2.03
Earnings Per Share - Diluted [Abstract]
Net Income	$ 20.1	$ 34.2	$ 86.4	$ 104.7
Average Common Shares	51.9	51.7	51.8	51.7
Earnings Per Share	$ 0.39	$ 0.66	$ 1.67	$ 2.02
Dilutive Securities (Shares)	0.1	0.1	0.0	0.1