Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Reconciliation of Basic and Diluted Earnings Per Share [Abstract]
Antidilutive Shares Excluded from EPS Computation			0.3	0.3
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 14.4	$ 17.0	$ 38.8	$ 54.2
Common Shares	37.3	35.0	37.0	34.8
Earnings Per Share	$ 0.39	$ 0.49	$ 1.05	$ 1.56
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 14.4	$ 17.0	$ 38.8	$ 54.2
Common Shares	37.4	35.1	37.1	34.9
Earnings Per Share	$ 0.39	$ 0.48	$ 1.05	$ 1.55
Dilutive Securities (Shares)	0.1	0.1	0.1	0.1