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, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	[1]	Sep. 30, 2020	[1]	Jun. 30, 2020	[1]	Mar. 31, 2020	[1]	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 51.7	$ 33.7	$ 37.6	$ 66.3	$ 61.9	$ 27.6	$ 27.9	$ 51.8	$ 38.3		$ 40.7		$ 20.1		$ 66.3		$ 189.3	$ 169.2	$ 165.4
Average Common Shares																	55.9	52.4	51.9
Earnings Per Share	$ 0.90	$ 0.59	$ 0.67	$ 1.24	$ 1.18	$ 0.53	$ 0.53	$ 0.99	$ 0.74		$ 0.78		$ 0.39		$ 1.28		$ 3.38	$ 3.23	$ 3.18
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE																	$ 189.3	$ 169.2	$ 165.4
Average Common Shares																	56.0	52.5	51.9
Earnings Per Share	$ 0.90	$ 0.59	$ 0.67	$ 1.24	$ 1.18	$ 0.53	$ 0.53	$ 0.99	$ 0.73		$ 0.78		$ 0.39		$ 1.28		$ 3.38	$ 3.23	$ 3.18
Dilutive Securities (Shares)																	0.1	0.1	0.0

[1] See Immaterial Revision to Prior Period in Note 1. Operations and Significant Accounting Policies.