Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Antidilutive Shares Excluded from Diluted Earnings Per Share Computation			0.0	0.0
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 40.3	$ 60.4	$ 111.0	$ 122.8
Average Common Shares	49.4	48.8	49.3	48.0
Earnings Per Share	$ 0.82	$ 1.24	$ 2.25	$ 2.56
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 40.3	$ 60.4	$ 111.0	$ 122.8
Average Common Shares	49.5	48.9	49.4	48.1
Earnings Per Share	$ 0.81	$ 1.23	$ 2.25	$ 2.55
Dilutive Securities (Shares)	0.1	0.1	0.1	0.1