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Investments (Tables)
 12 Months Ended
Dec. 31, 2017
Investments [Abstract]  
Other Investments [Table Text Block]
Other Investments
 
 
As of December 31
2017

2016

Millions
 
 
ALLETE Properties

$26.4


$31.7

Available-for-sale Securities (a)
19.1

18.8

Cash Equivalents
3.8

1.3

Other
3.8

3.8

Total Other Investments

$53.1


$55.6


(a)
As of December 31, 2017, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.7 million, in one year to less than three years was $3.2 million, in three years to less than five years was $3.6 million, and in five or more years was $1.4 million.