Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Reconciliation of Basic and Diluted Earnings Per Share [Abstract]
Antidilutive Shares Excluded from EPS Computation			0.2	0.4
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 29.4	$ 20.5	$ 68.2	$ 74.7
Average Common Shares	37.7	35.6	37.3	35.1
Earnings Per Share	$ 0.78	$ 0.57	$ 1.83	$ 2.13
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 29.4	$ 20.5	$ 68.2	$ 74.7
Average Common Shares	37.8	35.7	37.3	35.2
Earnings Per Share	$ 0.78	$ 0.57	$ 1.83	$ 2.12
Dilutive Securities (Shares)	0.1	0.1	0	0.1

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