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Earnings Per Share and Common Stock (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Basic    
Net Income Attributable to ALLETE $ 50.7 $ 58.2
Average common shares (in shares) 57.6 57.3
Earnings Per Share (in usd per share) $ 0.88 $ 1.02
Diluted    
Net Income Attributable to ALLETE $ 50.7 $ 58.2
Average common shares adjustment (in shares) 0.1 0.0
Average common shares (in shares) 57.7 57.3
Earnings Per Share (in usd per share) $ 0.88 $ 1.02