Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Basic
Net Income Attributable to ALLETE	$ 51.5	$ 85.9	$ 51.5	$ 58.2	$ 51.7	$ 33.7	$ 37.6	$ 66.3	$ 61.9	$ 27.6	$ 27.9	$ 51.8	$ 247.1	$ 189.3	$ 169.2
Average Common Shares (in shares)													57.3	55.9	52.4
Earnings Per Share (in dollars per share)	$ 0.89	$ 1.50	$ 0.90	$ 1.02	$ 0.90	$ 0.59	$ 0.67	$ 1.24	$ 1.18	$ 0.53	$ 0.53	$ 0.99	$ 4.31	$ 3.38	$ 3.23
Diluted
Net Income Attributable to ALLETE													$ 247.1	$ 189.3	$ 169.2
Average Common Shares (in shares)													57.4	56.0	52.5
Earnings Per Share (in dollars per share)	$ 0.89	$ 1.49	$ 0.90	$ 1.02	$ 0.90	$ 0.59	$ 0.67	$ 1.24	$ 1.18	$ 0.53	$ 0.53	$ 0.99	$ 4.30	$ 3.38	$ 3.23
Dilutive Securities (Shares)													0.1	0.1	0.1