Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Earnings Per Share [Abstract]
Options to Purchase Shares of Common Stock Excluded (Shares)			0.4
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 20.5	$ 19.6	$ 74.7	$ 62.0
Common Shares	35.6	34.4	35.1	34.1
Earnings Per Share	$ 0.57	$ 0.57	$ 2.13	$ 1.82
Dilutive Securities (Shares)	0.1	0.1	0.1	0.1
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 20.5	$ 19.6	$ 74.7	$ 62.0
Common Shares	35.7	34.5	35.2	34.2
Earnings Per Share	$ 0.57	$ 0.56	$ 2.12	$ 1.81

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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