Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Investments [Abstract]
ALLETE Properties	$ 50.1	[1]	$ 88.2
Available-for-sale Securities	18.5	[2]	18.9	$ 17.7
Cash Equivalents	2.0		2.9
Other	4.0		4.4
Total Other Investments	74.6		114.4
Impairment of Real Estate	36.3		$ 0.0	$ 0.0
Available-for-sale Corporate Debt Securities, Maturities [Abstract]
One Year or Less	0.0
One Year to Less Than Three Years	1.0
Three Years to Less Than Five Years	3.2
Five or More Years	$ 6.7

[1] In 2015, ALLETE Properties recorded a $36.3 million non-cash impairment charge related to its real estate assets. (See Note 1. Operations and Significant Accounting Policies.)

[2] As of December 31, 2015, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was none, in one year to less than three years was $1.0 million, in three years to less than five years was $3.2 million, and in five or more years was $6.7 million.