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Investments (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Investments [Abstract]      
ALLETE Properties $ 47.8 $ 47.8 $ 50.1
Available-for-sale Securities 18.4 [1] 18.4 [1] 18.5
Cash Equivalents 2.3 2.3 2.0
Other 3.8 3.8 4.0
Total Other Investments 72.3 72.3 $ 74.6
Available-for-sale Securities Corporate Debt Securities, Maturities [Abstract]      
One Year or Less 0.2 0.2  
One Year to Less Than Three Years 2.5 2.5  
Three Years to Less Than Five Years 5.0 5.0  
Five or More Years 3.3 3.3  
Impairment of Land Inventory $ 0.0 $ 0.0  
[1] As of June 30, 2016, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $2.5 million, in three years to less than five years was $5.0 million, and in five or more years was $3.3 million.