Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 18.3	$ 60.4	$ 22.5	$ 39.9	$ 32.9	$ 41.6	$ 16.8	$ 33.5	$ 141.1	$ 124.8	$ 104.7
Average Common Shares									48.3	42.9	39.7
Earnings Per Share	$ 0.37	$ 1.24	$ 0.46	$ 0.85	$ 0.73	$ 0.97	$ 0.40	$ 0.81	$ 2.92	$ 2.91	$ 2.64
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE									$ 141.1	$ 124.8	$ 104.7
Average Common Shares									48.4	43.1	39.8
Earnings Per Share	$ 0.37	$ 1.23	$ 0.46	$ 0.85	$ 0.73	$ 0.97	$ 0.40	$ 0.80	$ 2.92	$ 2.90	$ 2.63
Dilutive Securities (Shares)									0.1	0.2	0.1
Common Stock [Member]
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share [Line Items]
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share									0.0	0.0	0.0