Investments (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
	Sep. 22, 2016	Sep. 30, 2016		Sep. 30, 2016		Dec. 31, 2015
Investments [Abstract]
ALLETE Properties		$ 32.9	[1]	$ 32.9	[1]	$ 50.1
Available-for-sale Securities		19.4	[2]	19.4	[2]	18.5
Cash Equivalents		2.0		2.0		2.0
Other		3.8		3.8		4.0
Total Other Investments		58.1		58.1		$ 74.6
Real Estate Sale Consideration [Abstract]
Total Consideration for Land Inventory	$ 21.0
Down Payment of ALLETE Common Stock, Shares	0.1
Down Payment of ALLETE Common Stock, Value	$ 8.0			8.0
Available-for-sale Securities Corporate Debt Securities, Maturities [Abstract]
One Year or Less		0.2		0.2
One Year to Less Than Three Years		2.7		2.7
Three Years to Less Than Five Years		5.5		5.5
Five or More Years		3.3		3.3
Impairment of Land Inventory		$ 0.0		$ 0.0

[1] On September 22, 2016, ALLETE Properties sold its Ormond Crossings project and Lake Swamp wetland mitigation bank for consideration of approximately $21 million. The consideration included a down payment in the form of 0.1 million shares of ALLETE common stock with a value of $8.0 million, with the remaining purchase price to be paid under the terms of a finance receivable due over a five-year period which bears interest at market rates. The finance receivable is collateralized by the property sold.

[2] As of September 30, 2016, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $2.7 million, in three years to less than five years was $5.5 million, and in five or more years was $3.3 million.