Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 36.9	$ 24.8	$ 85.9	$ 70.7
Average Common Shares	50.9	49.3	50.5	49.2
Earnings Per Share	$ 0.73	$ 0.50	$ 1.70	$ 1.44
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 36.9	$ 24.8	$ 85.9	$ 70.7
Average Common Shares	51.1	49.5	50.7	49.3
Earnings Per Share	$ 0.72	$ 0.50	$ 1.69	$ 1.43
Dilutive Securities (Shares)	0.2	0.2	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