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Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Investments [Abstract]    
ALLETE Properties $ 25.9 $ 26.4
Available-for-sale Securities 21.4 [1] 19.1
Cash Equivalents 1.5 3.8
Other 3.7 3.8
Total Other Investments 52.5 $ 53.1
Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract]    
One Year or Less 1.1  
One Year to Less Than Three Years 3.9  
Three Years to Less Than Five Years 2.5  
Five or More Years $ 1.1  
[1] As of June 30, 2018, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $1.1 million, in one year to less than three years was $3.9 million, in three years to less than five years was $2.5 million and in five or more years was $1.1 million.