Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015		Sep. 30, 2015		Dec. 31, 2014
Investments [Abstract]
ALLETE Properties	$ 88.3		$ 88.3		$ 88.2
Available-for-sale Securities	18.5	[1]	18.5	[1]	18.9
Cash Equivalents	2.6		2.6		2.9
Other	4.1		4.1		4.4
Total Other Investments	113.5		113.5		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	1.6		1.6
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years	3.0		3.0
Available-for-sale Corporate Debt Securities, Maturities, Five or More Years	6.4		6.4
Impairment of Land Inventory	$ 0.0		$ 0.0		$ 0.0

[1] As of September 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 $1.6 million, in three years to less than five years was $3.0 million, and in five or more years was $6.4 million.