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Investments (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Investments [Abstract]          
ALLETE Properties $ 28.8   $ 28.8   $ 31.7
Available-for-sale Securities 21.0 [1]   21.0 [1]   18.8
Cash Equivalents 2.2   2.2   1.3
Other 3.8   3.8   3.8
Total Other Investments 55.8   55.8   $ 55.6
Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract]          
One Year or Less 1.1   1.1    
One Year to Less Than Three Years 3.2   3.2    
Three Years to Less Than Five Years 4.2   4.2    
Five or More Years 1.6   1.6    
Impairment of Land Inventory $ 0.0 $ 0.0 $ 0.0 $ 0.0  
[1] As of September 30, 2017, 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 $3.2 million, in three years to less than five years was $4.2 million and in five or more years was $1.6 million.