Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions					3 Months Ended		9 Months Ended
	Feb. 04, 2015	Sep. 05, 2014	Mar. 04, 2014	Feb. 26, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share and Common Stock [Abstract]
Antidilutive Shares Excluded from Diluted EPS Computation							0.0	0.0
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE					$ 60.4	$ 41.6	$ 122.8	$ 91.9
Average Common Shares					48.8	42.9	48.0	42.1
Earnings Per Share					$ 1.24	$ 0.97	$ 2.56	$ 2.18
Earnings Per Share - Diluted [Abstract]
Net Income to Attributable ALLETE					$ 60.4	$ 41.6	$ 122.8	$ 91.9
Average Common Shares					48.9	42.9	48.1	42.3
Earnings Per Share					$ 1.23	$ 0.97	$ 2.55	$ 2.17
Dilutive Securities (in shares)					0.1	0.0	0.1	0.2
Proceeds from Issuance of Common Stock - Pension Plan							$ 0.0	$ 19.5
Common Stock Issued - Pension Plan (in shares)								0.4
Forward Contracts [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward Contract Indexed to Issuer's Equity, Indexed Shares				2.8
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share				$ 48.01
Common Stock Issued - Forward Contract (in shares)	1.4	1.4
Proceeds from Issuance of Common Stock - Forward Contract	$ 65.4	$ 65.0
Options Held [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Common Stock Issued - Forward Contract (in shares)			0.4
Proceeds from Issuance of Common Stock - Forward Contract			$ 20.2