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Goodwill and Intangible Assets - Goodwill (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Roll Forward]      
Beginning Balance $ 131.2 $ 130.6  
Acquired Goodwill [1] 16.9 3.9  
Impairment Charge 0.0 (3.3) [2] $ 0.0
Other Adjustments [3] 0.2    
Ending Balance 148.3 131.2 130.6
ALLETE Clean Energy [Member]      
Goodwill [Roll Forward]      
Beginning Balance 0.0 3.3  
Acquired Goodwill [1] 0.0 0.0  
Impairment Charge   (3.3) [2] 0.0
Other Adjustments [3] 0.0    
Ending Balance 0.0 0.0 3.3
U.S. Water Services [Member]      
Goodwill [Roll Forward]      
Beginning Balance 131.2 127.3  
Acquired Goodwill [1] 16.9 3.9  
Impairment Charge 0.0 0.0 [2] 0.0
Other Adjustments [3] 0.2    
Ending Balance $ 148.3 $ 131.2 $ 127.3
[1] See Note 6. Acquisitions.
[2] The facts and circumstances that led to an impairment of ALLETE Clean Energy’s goodwill primarily relate to lower estimated energy prices in periods not under PSAs. Impairment Charge is included in Operating Expenses – Other on the Consolidated Statement of Income. (See Note 1. Operations and Significant Accounting Policies.) ALLETE Clean Energy’s goodwill was primarily related to the acquisition of Storm Lake II in 2014.
[3] Finalization of purchase price accounting for U.S. Water Services’ acquisition of WEST was completed in 2017 resulting in an adjustment to the goodwill recorded at the time of the initial acquisition.