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Investment in ATC (Tables) | 9 Months Ended
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Sep. 30, 2017
Equity Method Investments and Joint Ventures [Abstract] |
ALLETE's Investment in ATC [Table Text Block] |
ALLETE’s Investment in ATC |
Millions |
Equity Investment Balance as of December 31, 2016 |  | $135.6 |
Cash Investments | 6.6 |
Equity in ATC Earnings | 17.3 |
Distributed ATC Earnings | (13.5 | )
Equity Investment Balance as of September 30, 2017 |  | $146.0 |

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- Definition Tabular disclosure of equity method investments including, but not limited to, name of each investee or group of investments, percentage ownership, difference between recorded amount of an investment and the value of the underlying equity in the net assets, and summarized financial information. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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