Investments (Details) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments [Abstract]
ALLETE Properties	[1]	$ 31.7	$ 50.1
Available-for-sale Securities	[2]	18.8	18.5
Cash Equivalents		1.3	2.0
Other		3.8	4.0
Total Other Investments		55.6	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		$ 8.0
Finance Receivable Term (Years)		5 years
Available-for-sale Corporate Debt Securities, Maturities [Abstract]
One Year or Less		$ 0.2
One Year to Less Than Three Years		3.2
Three Years to Less Than Five Years		5.0
Five or More Years		3.3
Impairment of Land Inventory	[3]	$ 0.0	$ 36.3	$ 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 December 31, 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 $3.2 million, in three years to less than five years was $5.0 million, and in five or more years was $3.3 million.

[3] See Impairment of Long-Lived Assets.