Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share - Basic [Abstract]
Net Income	$ 30.7	$ 44.9	$ 113.0	$ 130.8
Average Common Shares	51.4	51.0	51.3	50.7
Earnings Per Share	$ 0.59	$ 0.88	$ 2.20	$ 2.58
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 30.7	$ 44.9	$ 113.0	$ 130.8
Average Common Shares	51.6	51.2	51.5	50.9
Earnings Per Share	$ 0.59	$ 0.88	$ 2.19	$ 2.57
Dilutive Securities (Shares)	0.2	0.2	0.2	0.2
Common Stock [Member]
Class of Stock [Line Items]
Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share			0.0	0.0