Investments (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Investments [Abstract]
ALLETE Properties		$ 24.4	$ 26.4
Available-for-sale Securities	[1]	20.2	19.1
Cash Equivalents		1.0	3.8
Other		3.5	3.8
Total Other Investments		49.1	$ 53.1
Available-for-sale Corporate Debt Securities, Maturities [Abstract]
One Year or Less		2.0
One Year to Less Than Three Years		2.9
Three Years to Less Than Five Years		2.6
Five or More Years		$ 0.5

[1] As of December 31, 2018, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $2.0 million, in one year to less than three years was $2.9 million, in three years to less than five years was $2.6 million, and in five or more years was $0.5 million.