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Investments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Investments [Abstract]    
ALLETE Properties $ 49.1 $ 50.1
Available-for-sale Securities 17.4 [1] 18.5
Cash Equivalents 2.5 2.0
Other 3.9 4.0
Total Other Investments 72.9 $ 74.6
Impairment of Land Inventory 0.0  
Available-for-sale Securities Corporate Debt Securities, Maturities [Abstract]    
One Year or Less 0.0  
One Year to Less Than Three Years 1.6  
Three Years to Less Than Five Years 4.1  
Five or More Years $ 4.7  
[1] As of March 31, 2016, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was zero, in one year to less than three years was $1.6 million, in three years to less than five years was $4.1 million, and in five or more years was $4.7 million.