Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions					3 Months Ended		6 Months Ended
	Feb. 04, 2015	Sep. 05, 2014	Mar. 04, 2014	Feb. 26, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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					$ 22.5	$ 16.8	$ 62.4	$ 50.3
Average Common Shares					48.6	42.1	47.7	41.7
Earnings Per Share					$ 0.46	$ 0.40	$ 1.31	$ 1.21
Earnings Per Share - Diluted [Abstract]
Net Income to Attributable ALLETE					$ 22.5	$ 16.8	$ 62.4	$ 50.3
Average Common Shares					48.7	42.3	47.8	41.9
Earnings Per Share					$ 0.46	$ 0.40	$ 1.30	$ 1.20
Dilutive Securities (in shares)					0.1	0.2	0.1	0.2
Forward Contract Indexed to Issuer's Equity [Line Items]
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
Common Stock Issued - Forward Contract (in shares)	1.4	1.4
Proceeds from Issuance of Common Stock - Forward Contract	$ 65.4	$ 65.0
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share				$ 48.01
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