Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) a	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) a Segments	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Segments [Line Items]
Number of Reportable Segments			3
Operating Revenue	$ 353.3	$ 314.8	$ 718.9	$ 648.6
Net Income (Loss) Attributable to ALLETE	36.9	24.8	85.9	70.7
Assets	4,931.5		$ 4,931.5		$ 4,876.9
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			3
Operating Revenue	264.9	234.9	$ 546.5	487.2
Net Income (Loss) Attributable to ALLETE	32.4	22.6	75.9	65.0
Assets	3,824.8		3,824.8		3,823.9
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	19.6	18.8	43.3	42.4
Net Income (Loss) Attributable to ALLETE	3.8	2.6	10.5	8.7
Assets	559.1		559.1		566.0
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	38.4	34.3	70.5	66.7
Net Income (Loss) Attributable to ALLETE	0.6	1.0	0.3	0.5
Assets	$ 267.0		$ 267.0		264.1
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			2
Land in Minnesota (Acres) | a	5,000		5,000
Operating Revenue	$ 30.4	26.8	$ 58.6	52.3
Net Income (Loss) Attributable to ALLETE	0.1	$ (1.4)	(0.8)	$ (3.5)
Assets	$ 280.6		$ 280.6		$ 222.9