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Quarterly Financial Data (Unaudited) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                      
Operating Revenue $ 380.6 $ 462.5 $ 323.3 $ 320.0 $ 290.7 $ 288.9 $ 260.7 $ 296.5 $ 1,486.4 $ 1,136.8 $ 1,018.4
Operating Income 29.6 85.2 39.5 56.4 51.5 60.8 28.2 48.3 210.7 188.8 154.1
Net Income Attributable to ALLETE $ 18.3 $ 60.4 $ 22.5 $ 39.9 $ 32.9 $ 41.6 $ 16.8 $ 33.5 $ 141.1 $ 124.8 $ 104.7
Basic Earnings Per Share of Common Stock $ 0.37 $ 1.24 $ 0.46 $ 0.85 $ 0.73 $ 0.97 $ 0.40 $ 0.81 $ 2.92 $ 2.91 $ 2.64
Diluted Earnings Per Share of Common Stock $ 0.37 $ 1.23 $ 0.46 $ 0.85 $ 0.73 $ 0.97 $ 0.40 $ 0.80 $ 2.92 $ 2.90 $ 2.63