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Investments (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments [Abstract]      
ALLETE Properties $ 26.4 $ 31.7  
Available-for-sale Securities [1] 19.1 18.8  
Cash Equivalents 3.8 1.3  
Other 3.8 3.8  
Total Other Investments 53.1 55.6  
Available-for-sale Corporate Debt Securities, Maturities [Abstract]      
One Year or Less 0.7    
One Year to Less Than Three Years 3.2    
Three Years to Less Than Five Years 3.6    
Five or More Years 1.4    
Impairment of Land Inventory $ 0.0 $ 0.0 $ 36.3
[1] As of December 31, 2017, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.7 million, in one year to less than three years was $3.2 million, in three years to less than five years was $3.6 million, and in five or more years was $1.4 million.