Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]
Operating Revenue	$ 425.8	$ 388.3	$ 373.1	$ 383.5	$ 399.0	$ 345.4	$ 335.6	$ 339.2	$ 320.4		$ 293.9		$ 243.2		$ 311.6		$ 1,570.7	$ 1,419.2	$ 1,169.1
Operating Income	33.7	33.4	13.7	53.4	50.0	31.1	28.2	42.0	23.7		41.6		12.7		60.2		134.2	151.3	138.2
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
Basic Earnings Per Share of Common Stock	$ 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
Diluted Earnings Per Share of Common Stock	$ 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

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