Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic
Net Income Attributable to ALLETE	$ 85.9	$ 33.7	$ 195.6	$ 137.6
Average common shares (in shares)	57.4	57.1	57.3	55.5
Earnings Per Share (in usd per share)	$ 1.50	$ 0.59	$ 3.41	$ 2.48
Diluted
Net Income Attributable to ALLETE	$ 85.9	$ 33.7	$ 195.6	$ 137.6
Average common shares adjustment (in shares)	0.1	0.1	0.1	0.1
Average common shares (in shares)	57.5	57.2	57.4	55.6
Earnings Per Share (in usd per share)	$ 1.49	$ 0.59	$ 3.41	$ 2.48