Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Earnings Per Share and Common Stock [Abstract]
Common Shares Excluded (shares)	0.3	0.6	0.3	0.6
Net Income (Loss) Attributable to ALLETE	$ 17.0	$ 19.4	$ 54.2	$ 42.4
Common Shares (shares)	35.0	34.1	34.8	34.0
Earnings Per Share - Basic (in dollars per share)	$ 0.49	$ 0.57	$ 1.56	$ 1.25
Dilutive Securities (shares)	0.1	0.1	0.1	0.1
Diluted Common Shares (shares)	35.1	34.2	34.9	34.1
Earnings Per Share -Diluted (in dollars per share)	$ 0.48	$ 0.57	$ 1.55	$ 1.25