Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Investments [Abstract]
ALLETE Properties	$ 88.3		$ 88.2
Available-for-sale Securities	19.3	[1]	18.9	[1]
Cash Equivalents	2.8		2.9
Other	4.3		4.4
Total Other Investments	114.7		114.4
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	1.7
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years	3.0
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More	5.9
ALLETE Properties [Rollforward]
Land Inventory Beginning Balance	83.8		85.4
Cost of Sales	0		(2.2)
Other	0.1		0.6
Land Inventory Ending Balance	83.9		83.8
Long-Term Finance Receivables (net of allowances of $0.6 and $0.6)	1.2		1.2
Other	3.2		3.2
Impairment of Real Estate	0		0
Long-Term Finance Receivable, Allowance	$ 0.6		$ 0.6

[1] As of March 31, 2015, 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 $1.7 million, in three years to less than five years was $3.0 million, and in five or more years was $5.9 million.