Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Investments [Abstract]
ALLETE Properties	$ 90.1		$ 89.9
Available-for-sale Securities	19.8	[1]	17.7	[1]
Cash Equivalents	3.9	[2]	34.2	[2]
Other	2.6		4.5
Total Other Investments	116.4		146.3
Available-for-sale Corporate Debt Securities, Maturities, One Year or Less	0.4
Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years	4.0
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years	1.3
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More	4.3
ALLETE Properties [Rollforward]
Land Inventory Beginning Balance	85.4		86.5
Cost of Sales	0		(1.5)
Other	0.2		0.4
Land Inventory Ending Balance	85.6		85.4
Long-Term Finance Reveivables (net of allowances of $0.6 and $0.6)	1.4		1.4
Other	3.1		3.1
Total Real Estate Assets	90.1		89.9
Impairment of Real Estate	0		0
Long-Term Finance Receivables, Allowance	$ 0.6		$ 0.6

[1] As of March 31, 2014, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.4 million, in one year to less than three years was $4.0 million, in three years to less than five years was $1.3 million, and in five or more years was $4.3 million.

[2] During the quarter ended March 31, 2014, cash included in Other Investments was transferred to Cash and Cash Equivalents.