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Goodwill and Intangible Assets (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014
Goodwill [Roll Forward] |  |  |
Balance as of December 31, 2014 |  | $ 2.9 |
Acquired Goodwill |  | 128.1 |
Balance as of September 30, 2015 |  | 131.0 |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets |  | 65.3 | $ 1.9
Definite-Lived Intangible Assets, Additions | [1] | 66.5 |
Definite-Lived Intangible Assets, Amortization |  | (2.8) |
Definite-Lived Intangible Assets, Other | [2] | (0.3) |
Total Intangible Assets |  | 81.9 | 1.9
Total Intangible Assets, Additions | [1] | 83.1 |
Total Intangible Assets, Other | [2] | (0.3) |
Definite-Lived Intangible Assets, Accumulated Amortization |  | 2.9 | 0.1
Definite-Lived Intangible Assets, Estimated Annual Amortization Expense [Abstract] |  |  |
Definite-Lived Intangible Assets, Estimated Amortization Expense, Remainder of 2015 |  | 1.1 |
Definite-Lived Intangible Assets, Estimated Amortization Expense, 2016 |  | 4.3 |
Definite-Lived Intangible Assets, Estimated Amortization Expense, 2017 |  | 4.2 |
Definite-Lived Intangible Assets, Estimated Amortization Expense, 2018 |  | 4.1 |
Definite-Lived Intangible Assets, Estimated Amortization Expense, 2019 |  | 4.0 |
Definite-Lived Intangible Assets, Estimated Amortization Expense, Thereafter |  | $ 47.6 |
Minimum [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 3 years |
Maximum [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 22 years |
Weighted Average [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 21 years |
Trademarks and Trade Names [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Indefinite-Lived Intangible Assets |  | $ 16.6 | 0.0
Indefinite-Lived Intangible Assets, Additions | [1] | 16.6 |
Indefinite-Lived Intangible Assets, Other | [2] | 0.0 |
Customer Relationships [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets |  | 57.9 | 0.0
Definite-Lived Intangible Assets, Additions | [1] | 60.1 |
Definite-Lived Intangible Assets, Amortization |  | (2.2) |
Definite-Lived Intangible Assets, Other | [2] | $ 0.0 |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 22 years |
Developed Technology and Other [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets | [3] | $ 7.4 | $ 1.9
Definite-Lived Intangible Assets, Additions | [1],[3] | 6.4 |
Definite-Lived Intangible Assets, Amortization | [3] | (0.6) |
Definite-Lived Intangible Assets, Other | [2],[3] | $ (0.3) |
Developed Technology and Other [Member] | Minimum [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 3 years |
Developed Technology and Other [Member] | Maximum [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 13 years |
Developed Technology and Other [Member] | Weighted Average [Member] |  |  |
Intangible Assets [Abstract] |  |  |
Definite-Lived Intangible Assets, Useful Life (Years) |  | 9 years |
ALLETE Clean Energy [Member] |  |  |
Goodwill [Roll Forward] |  |  |
Balance as of December 31, 2014 |  | $ 2.9 |
Acquired Goodwill |  | 0.4 |
Balance as of September 30, 2015 |  | 3.3 |
U.S. Water Services [Member] |  |  |
Goodwill [Roll Forward] |  |  |
Balance as of December 31, 2014 |  | 0.0 |
Acquired Goodwill |  | 127.7 |
Balance as of September 30, 2015 |  | $ 127.7 |
[1] | Additions are primarily the result of the U.S. Water Services acquisition. (See Note 4. Acquisitions.)
[2] | 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.
[3] | Developed Technology and Other includes patents, non-compete agreements, and land easements.

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