Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value			$ 13.5	$ 0.0
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 44.9	$ 40.3	$ 130.8	$ 111.0
Average Common Shares	51.0	49.4	50.7	49.3
Earnings Per Share	$ 0.88	$ 0.82	$ 2.58	$ 2.25
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 44.9	$ 40.3	$ 130.8	$ 111.0
Average Common Shares	51.2	49.5	50.9	49.4
Earnings Per Share	$ 0.88	$ 0.81	$ 2.57	$ 2.25
Dilutive Securities (Shares)	0.2	0.1	0.2	0.1
Common Stock [Member]
Class of Stock [Line Items]
Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share			0.0	0.0
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares			0.2	0.0
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value			$ 13.5