Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earnings Per Share [Abstract]
Net Income Attributable to ALLETE	$ 51.5	$ 37.6	$ 109.7	$ 103.9
Net Income Attributable to ALLETE	$ 51.5	$ 37.6	$ 109.7	$ 103.9
Basic (Shares)	57.3	56.1	57.3	54.9
Dilutive Securities (Shares)	0.1	0.0	0.1	0.0
Diluted (Shares)	57.4	56.1	57.4	54.9
Basic Earnings Per Share of Common Stock	$ 0.90	$ 0.67	$ 1.91	$ 1.89
Diluted Earnings Per Share of Common Stock	$ 0.90	$ 0.67	$ 1.91	$ 1.89