Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Class of Stock [Line Items]
Employer Contributions to Defined Benefit Pension Plan - Common Stock Value	$ 13.5	$ 0.0
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 49.0	$ 45.9
Average Common Shares	50.2	49.2
Earnings Per Share	$ 0.97	$ 0.93
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 49.0	$ 45.9
Average Common Shares	50.4	49.2
Earnings Per Share	$ 0.97	$ 0.93
Dilutive Securities (Shares)	0.2	0.0
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 Plan - Common Stock Shares	0.2	0.0
Employer Contributions to Defined Benefit Pension Plan - Common Stock Value	$ 13.5

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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