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Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Definite-Lived Intangible Assets [Roll Forward]    
Beginning Balance $ 58.2  
Amortization (1.5) $ (5.6)
Finite-Lived Intangible Assets, Other Changes [1] 55.7  
Ending Balance 1.0 58.2
Intangible Assets [Abstract]    
Total Intangible Assets 1.0 74.8
Total Intangible Assets, Amortization (1.5) (5.6)
Intangible Assets, Other Changes [1] 72.3  
Trademarks and Trade Names [Member]    
Indefinite-Lived Intangible Assets [Roll Forward]    
Beginning Balance 16.6  
Indefinite-Lived Intangible Assets, Other Changes [1] 16.6  
Ending Balance 0.0 16.6
Customer Relationships [Member]    
Definite-Lived Intangible Assets [Roll Forward]    
Beginning Balance 50.7  
Amortization (1.1)  
Finite-Lived Intangible Assets, Other Changes [1] 49.6  
Ending Balance 0.0 50.7
Intangible Assets [Abstract]    
Total Intangible Assets, Amortization (1.1)  
Developed Technology and Other [Member]    
Definite-Lived Intangible Assets [Roll Forward]    
Beginning Balance [2] 7.5  
Amortization [2] (0.4)  
Finite-Lived Intangible Assets, Other Changes [1],[2] 6.1  
Ending Balance [2] 1.0 $ 7.5
Intangible Assets [Abstract]    
Total Intangible Assets, Amortization [2] $ (0.4)  
[1] On March 26, 2019, ALLETE completed the sale of U.S. Water Services which resulted in the removal of the related intangible assets from the Consolidated Balance Sheet.
[2]
Developed Technology and Other includes land easements and trade names with finite lives.