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Quarterly Financial Data (Unaudited) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
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, 2021
Dec. 31, 2020
Dec. 31, 2019
Selected Quarterly Financial Information [Abstract]                              
Operating Revenue $ 399.0 $ 345.4 $ 335.6 $ 339.2 $ 320.4 $ 293.9 $ 243.2 $ 311.6 $ 304.6 $ 288.3 $ 290.4 $ 357.2 $ 1,419.2 $ 1,169.1 $ 1,240.5
Operating Income 50.0 31.1 28.2 42.0 36.4 41.6 12.7 60.2 49.8 37.0 36.2 56.8 151.3 150.9 179.8
Net Income Attributable to ALLETE $ 61.9 $ 27.6 $ 27.9 $ 51.8 $ 47.1 $ 40.7 $ 20.1 $ 66.3 $ 49.7 $ 31.2 $ 34.2 $ 70.5 $ 169.2 $ 174.2 $ 185.6
Basic Earnings Per Share of Common Stock $ 1.18 $ 0.53 $ 0.53 $ 0.99 $ 0.91 $ 0.78 $ 0.39 $ 1.28 $ 0.96 $ 0.60 $ 0.66 $ 1.37 $ 3.23 $ 3.36 $ 3.59
Diluted Earnings Per Share of Common Stock $ 1.18 $ 0.53 $ 0.53 $ 0.99 $ 0.90 $ 0.78 $ 0.39 $ 1.28 $ 0.96 $ 0.60 $ 0.66 $ 1.37 $ 3.23 $ 3.35 $ 3.59