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Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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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
Quarterly Financial Data (Unaudited) [Abstract] |  |  |  |  |  |  |  |  |  |  |
Operating Revenue | $ 239.2 | $ 226.9 | $ 219.9 | $ 242.2 | $ 238.1 | $ 224.1 | $ 211.2 | $ 233.6 | $ 928.2 | $ 907.0 | $ 759.1
Operating Income | 34.2 | 38.9 | 26.1 | 50.8 | 22.7 | 35.3 | 31.7 | 46.1 | 150.0 | 135.8 | 106.0
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
Basic Earnings 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
Diluted Earnings 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

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