Investments (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
		Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Investments [Abstract]
ALLETE Properties		$ 88.6	$ 88.6	$ 88.2
Available-for-sale Securities	[1]	19.3	19.3	18.9
Cash Equivalents		2.8	2.8	2.9
Other		4.3	4.3	4.4
Total Other Investments		115.0	115.0	114.4
Available-for-sale Corporate Debt Securities, Maturities, One Year or Less		0.2	0.2
Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years		1.7	1.7
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years		3.1	3.1
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More		6.2	6.2
Impairment of Real Estate		$ 0.0	$ 0.0	$ 0.0

[1] As of June 30, 2015, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $1.7 million, in three years to less than five years was $3.1 million, and in five or more years was $6.2 million.