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Investments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Investments [Abstract]      
ALLETE Properties $ 31.6   $ 31.7
Available-for-sale Securities 19.6 [1]   18.8
Cash Equivalents 2.6   1.3
Other 3.9   3.8
Total Other Investments 57.7   $ 55.6
Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract]      
One Year or Less 0.2    
One Year to Less Than Three Years 3.0    
Three Years to Less Than Five Years 5.0    
Five or More Years 3.9    
Impairment of Land Inventory $ 0.0 $ 0.0  
[1] As of March 31, 2017, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $3.0 million, in three years to less than five years was $5.0 million and in five or more years was $3.9 million.