Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share and Common Stock [Abstract]
Antidilutive Shares Excluded from EPS Computation	0.2	0.4
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 24.4	$ 37.2
Common Shares	36.8	34.6
Earnings Per Share	$ 0.66	$ 1.07
Dilutive Securities (Shares)	0.1	0.1
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 24.4	$ 37.2
Common Shares	36.9	34.7
Earnings Per Share	$ 0.66	$ 1.07