EARNINGS PER SHARE AND COMMON STOCK (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Basic
Net Income Attributable to ALLETE	$ 33.0	$ 51.5	$ 83.7	$ 109.7
Average common shares (in shares)	57.7	57.3	57.7	57.3
Earnings Per Share (in usd per share)	$ 0.57	$ 0.90	$ 1.45	$ 1.91
Diluted
Net Income Attributable to ALLETE	$ 33.0	$ 51.5	$ 83.7	$ 109.7
Average common shares adjustment (in shares)	0.1	0.1	0.0	0.1
Average common shares (in shares)	57.8	57.4	57.7	57.4
Earnings Per Share (in usd per share)	$ 0.57	$ 0.90	$ 1.45	$ 1.91