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Acquisitions - Acquisition of Non-Controlling Interest (Details) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended
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Apr. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Acquisition of Non-Controlling Interest [Line Items] |  |  |
Payment to Acquire Non-Controlling Interest |  | $ 0.0 | $ 8.0
ALLETE Clean Energy [Member] |  |  |
Acquisition of Non-Controlling Interest [Line Items] |  |  |
Payment to Acquire Non-Controlling Interest | $ 8.0 |  |
Gain or Loss in Net Income or Other Comprehensive Income | $ 0.0 |  |

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- Definition Amount of gain (loss) related to nonoperating activities, classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7,9) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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- Definition The cash outflow associated with the purchase of noncontrolling interest during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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