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Goodwill and Intangible Assets (Tables) | 9 Months Ended
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Sep. 30, 2015
Goodwill and Intangible Assets [Abstract] |
Schedule of Goodwill [Table Text Block] | The following table summarizes changes to goodwill by business segment for the nine months ended September 30, 2015:
| ALLETE Clean Energy |  |  | U.S. Water Services |  |  | Total |
Millions |  |  |  |  |
Balance as of December 31, 2014 |  | $2.9 |  |  | — |  |  |  | $2.9 |
Acquired Goodwill | 0.4 |  |  |  | $127.7 |  |  | 128.1 |
Balance as of September 30, 2015 |  | $3.3 |  |  |  | $127.7 |  |  |  | $131.0 |
Schedule of Intangible Assets [Table Text Block] | Balances of intangible assets, net, excluding goodwill as of September 30, 2015, are as follows:
| December 31, 2014 |  |  | Additions (a) |  | Amortization |  | Other (b) |  | September 30, 2015 |
Millions |  |  |  |  |  |  |  |  |
Intangible Assets |  |  |  |  |  |  |  |  |
Definite-Lived Intangible Assets |  |  |  |  |  |  |  |  |
Customer Relationships | — |  |  |  | $60.1 |  |  | $(2.2) |  | — |  |  |  | $57.9 |
Developed Technology and Other (c) |  | $1.9 |  |  | 6.4 |  |  | (0.6) |  | $(0.3) |  | 7.4 |
Total Definite-Lived Intangible Assets | 1.9 |  |  | 66.5 |  |  | (2.8) |  | (0.3) |  | 65.3 |
Indefinite-Lived Intangible Assets |  |  |  |  |  |  |  |  |
Trademarks and Trade Names | — |  |  | 16.6 |  |  | n/a |  | — |  |  | 16.6 |
Total Intangible Assets |  | $1.9 |  |  |  | $83.1 |  |  | $(2.8) |  | $(0.3) |  |  | $81.9 |
(a) | Additions are primarily the result of the U.S. Water Services acquisition. (See Note 4. 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 Goodwill and Intangible Assets [Abstract] + References No definition available. + Details
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- Definition Tabular disclosure or assets, excluding financial assets and goodwill, lacking physical substance, by either major class or business segment. + References No definition available. + Details
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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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