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Goodwill and Intangible Assets Goodwill and Intangible Assets (Tables) | 12 Months Ended
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Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract] |
Schedule of Goodwill [Table Text Block] | The following table summarizes changes to goodwill by reportable segment for the year ended December 31, 2015:
| ALLETE Clean Energy |  |  | U.S. Water Services |  |  | Total |
Millions |  |  |  |  |
Balance as of December 31, 2014 |  | $2.9 |  |  | — |  |  |  | $2.9 |
Acquired Goodwill | 0.4 |  |  |  | $127.3 |  |  | 127.7 |
Balance as of December 31, 2015 |  | $3.3 |  |  |  | $127.3 |  |  |  | $130.6 |
Schedule of Intangible Assets [Table Text Block] | Balances of intangible assets, net, excluding goodwill as of December 31, 2015, are as follows:
| December 31, 2014 |  |  | Additions (a) |  | Amortization |  | Other (b) |  | December 31, 2015 |
Millions |  |  |  |  |  |  |  |  |
Intangible Assets |  |  |  |  |  |  |  |  |
Definite-Lived Intangible Assets |  |  |  |  |  |  |  |  |
Customer Relationships | — |  |  |  | $64.0 |  |  | $(3.2) |  | — |  |  |  | $60.8 |
Developed Technology and Other (c) |  | $1.9 |  |  | 6.4 |  |  | (0.8) |  | $(0.3) |  | 7.2 |
Total Definite-Lived Intangible Assets | 1.9 |  |  | 70.4 |  |  | (4.0) |  | (0.3) |  | 68.0 |
Indefinite-Lived Intangible Assets |  |  |  |  |  |  |  |  |
Trademarks and Trade Names | — |  |  | 16.6 |  |  | n/a |  | — |  |  | 16.6 |
Total Intangible Assets |  | $1.9 |  |  |  | $87.0 |  |  | $(4.0) |  | $(0.3) |  |  | $84.6 |
(a) | Additions are primarily the result of the U.S. Water Services acquisition. (See Note 7. Acquisitions.)
(b) | Armenia Mountain was acquired on July 1, 2015, at which time the purchase option intangible asset was reclassified as a component of the acquisition consideration.
(c) | Developed Technology and Other includes patents, non-compete agreements, and land easements.

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- Definition Tabular disclosure of goodwill by reportable segment and in total which includes a rollforward schedule. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267 + Details
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