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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.