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Acquisitions (Details) | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Business Combinations [Abstract] |  |
Reason for Business Acquisitions | The following acquisition is consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services businesses to complement its regulated businesses, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth. | The following acquisition is consistent with ALLETE?s stated strategy of investing in energy infrastructure and related services businesses to complement its regulated businesses, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.

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