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Acquisitions (Details) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014
Acquisitions [Abstract] |  |
Business Combination, Reason for Business Combinations | The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services businesses to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth. |
Business Acquisition, Pro Forma Information | not significant | not significant

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- Definition Description of the pro forma information, nature of adjustments made to historical information to arrive at pro forma amounts, amounts excluded from pro forma information, any material-nonrecurring items included in the pro forma information and limitations of pro forma disclosures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition This element represents a description of the primary reason for the business combination which may consist of general categories such as top-line growth, synergistic benefits, market share, and diversification and the more detailed factors that might apply. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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