Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 32.9	$ 41.6	$ 16.8	$ 33.5	$ 33.0	$ 25.2	$ 14.0	$ 32.5	$ 124.8	$ 104.7	$ 97.1
Average Common Shares									42.9	39.7	37.6
Earnings Per Share	$ 0.73	$ 0.97	$ 0.40	$ 0.81	$ 0.82	$ 0.63	$ 0.36	$ 0.83	$ 2.91	$ 2.64	$ 2.59
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE									$ 124.8	$ 104.7	$ 97.1
Average Common Shares									43.1	39.8	37.6
Earnings Per Share	$ 0.73	$ 0.97	$ 0.40	$ 0.80	$ 0.82	$ 0.63	$ 0.35	$ 0.83	$ 2.90	$ 2.63	$ 2.58
Dilutive Securities (Shares)									0.2	0.1	0