Investments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Investments [Abstract]
ALLETE Properties	$ 91.2		$ 91.1
Available-for-sale Securities	16.1	[1]	26.8
Cash Equivalents	31.2		20.7
Other	4.4		4.9
Total Other Investments	142.9		143.5
Available-for-sale Corporate Debt Securities, Maturities, One Year or Less	0.8
Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years	4.8
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years	2.0
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More	1.3
ALLETE Properties [Rollforward]
Land Inventory Beginning Balance	86.5		86.0
Cost of Sales	0		(0.2)
Other	0.2		0.7
Land Inventory Ending Balance	86.7		86.5
Long-Term Finance Reveivables (net of allowances of $0.6 and $0.6)	1.4		1.4
Other	3.1		3.2
Total Real Estate Assets	91.2		91.1
Impairment of Real Estate	0		0
Long-Term Finance Receivables, Allowance	$ 0.6		$ 0.6

[1] As of September 30, 2013, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.8 million, in one year to less than three years was $4.8 million, in three years to less than five years was $2.0 million, and in five or more years was $1.3 million.