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Acquisitions (Details) | 9 Months Ended | 12 Months Ended
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Sep. 30, 2017 | Dec. 31, 2016
Business Combinations [Abstract] |  |
Reason for Business Acquisitions | The following acquisitions are 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 acquisitions are 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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