XML 61 R43.htm IDEA: XBRL DOCUMENT v3.8.0.1
Investments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Investments [Abstract]      
ALLETE Properties $ 26.0   $ 26.4
Available-for-sale Securities 21.1 [1]   19.1
Cash Equivalents 2.0   3.8
Other 3.7   3.8
Total Other Investments 52.8   $ 53.1
Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract]      
One Year or Less 0.9    
One Year to Less Than Three Years 3.4    
Three Years to Less Than Five Years 3.3    
Five or More Years 1.1    
Impairment of Land Inventory $ 0.0 $ 0.0  
[1] As of March 31, 2018, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.9 million, in one year to less than three years was $3.4 million, in three years to less than five years was $3.3 million and in five or more years was $1.1 million.