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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, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Earnings Per Share - Basic [Abstract]                              
Net Income Attributable to ALLETE $ 47.1 $ 40.7 $ 20.1 $ 66.3 $ 49.7 $ 31.2 $ 34.2 $ 70.5 $ 61.1 $ 30.7 $ 31.3 $ 51.0 $ 174.2 $ 185.6 $ 174.1
Average Common Shares                         51.9 51.6 51.3
Earnings Per Share $ 0.91 $ 0.78 $ 0.39 $ 1.28 $ 0.96 $ 0.60 $ 0.66 $ 1.37 $ 1.19 $ 0.59 $ 0.61 $ 1.00 $ 3.36 $ 3.59 $ 3.39
Earnings Per Share - Diluted [Abstract]                              
Net Income (Loss) Available to Common Stockholders, Diluted                         $ 174.2 $ 185.6 $ 174.1
Average Common Shares                         51.9 51.7 51.5
Earnings Per Share $ 0.90 $ 0.78 $ 0.39 $ 1.28 $ 0.96 $ 0.60 $ 0.66 $ 1.37 $ 1.18 $ 0.59 $ 0.61 $ 0.99 $ 3.35 $ 3.59 $ 3.38
Dilutive Securities (Shares)                         0.0 0.1 0.2