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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, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Earnings Per Share - Basic [Abstract]                      
Net Income Attributable to ALLETE $ 33.0 $ 25.2 $ 14.0 $ 32.5 $ 28.9 $ 29.4 $ 14.4 $ 24.4 $ 104.7 $ 97.1 $ 93.8
Average Common Shares                 39.7 37.6 35.3
Earnings Per Share $ 0.82 $ 0.63 $ 0.36 $ 0.83 $ 0.76 $ 0.78 $ 0.39 $ 0.66 $ 2.64 $ 2.59 $ 2.66
Earnings Per Share - Diluted [Abstract]                      
Net Income Attributable to ALLETE                 $ 104.7 $ 97.1 $ 93.8
Average Common Shares                 39.8 37.6 35.4
Earnings Per Share $ 0.82 $ 0.63 $ 0.35 $ 0.83 $ 0.75 $ 0.78 $ 0.39 $ 0.66 $ 2.63 $ 2.58 $ 2.65
Dilutive Securities (Shares)                 0.1 0 0.1