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Quarterly Financial Data (Unaudited) (Details) - USD ($)
$ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]                              
Operating Revenue $ 364.8 $ 407.2 $ 354.5 $ 403.3 $ 402.7 $ 378.8 $ 533.4 $ 564.9 $ 425.8 $ 388.3 $ 373.1 $ 383.5 $ 1,529.8 $ 1,879.8 $ 1,570.7
Operating Income 38.1 45.3 29.4 47.3 43.1 36.0 53.5 48.3 33.7 33.4 13.7 53.4 160.1 180.9 134.2
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
Basic Earnings Per Share of Common Stock (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 Earnings Per Share of Common Stock (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