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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, 2024
Sep. 30, 2024
Jun. 30, 2024
Mar. 31, 2024
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, 2024
Dec. 31, 2023
Dec. 31, 2022
Basic                              
Net Income Attributable to ALLETE $ 50.6 $ 45.0 $ 33.0 $ 50.7 $ 51.5 $ 85.9 $ 51.5 $ 58.2 $ 51.7 $ 33.7 $ 37.6 $ 66.3 $ 179.3 $ 247.1 $ 189.3
Average Common Shares (in shares)                         57.7 57.3 55.9
Earnings Per Share (in usd per share) $ 0.88 $ 0.78 $ 0.57 $ 0.88 $ 0.89 $ 1.50 $ 0.90 $ 1.02 $ 0.90 $ 0.59 $ 0.67 $ 1.24 $ 3.11 $ 4.31 $ 3.38
Diluted                              
Net Income Attributable to ALLETE                         $ 179.3 $ 247.1 $ 189.3
Average Common Shares (in shares)                         57.8 57.4 56.0
Earnings Per Share (in usd per share) $ 0.87 $ 0.78 $ 0.57 $ 0.88 $ 0.89 $ 1.49 $ 0.90 $ 1.02 $ 0.90 $ 0.59 $ 0.67 $ 1.24 $ 3.10 $ 4.30 $ 3.38
Dilutive Securities (Shares)                         0.1 0.1 0.1