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Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Earnings Per Share [Abstract]                      
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share                 0.3 0.5 0.6
Earnings Per Share, Basic [Abstract]                      
Net Income Attributable to ALLETE $ 19.1 $ 20.5 $ 17.0 $ 37.2 $ 13.3 $ 19.6 $ 19.4 $ 23.0 $ 93.8 $ 75.3 $ 61.0
Common Shares                 35.3 34.2 32.2
Per Share of Common Stock $ 0.53 $ 0.57 $ 0.49 $ 1.07 $ 0.38 $ 0.57 $ 0.57 $ 0.68 $ 2.66 $ 2.20 $ 1.89
Dilutive Securities (Shares)                 0.1 0.1 0
Earnings Per Share, Diluted [Abstract]                      
Net Income Attributable to ALLETE                 $ 93.8 $ 75.3 $ 61.0
Common Shares                 35.4 34.3 32.2
Per Share of Common Stock $ 0.53 $ 0.57 $ 0.48 $ 1.07 $ 0.38 $ 0.56 $ 0.57 $ 0.68 $ 2.65 $ 2.19 $ 1.89