Business Segments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) a Segments	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Segments [Line Items]
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments	We present three reportable segments: Regulated Operations, ALLETE Clean Energy and U.S. Water Services. We measure performance of our operations through budgeting and monitoring of contributions to consolidated net income by each business segment.
Number of Reportable Segments	3
Operating Revenue	$ 365.6	$ 333.8
Net Income (Loss) Attributable to ALLETE	49.0	45.9
Assets	$ 4,941.9		$ 4,876.9
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments	3
Operating Revenue	$ 281.6	252.3
Net Income (Loss) Attributable to ALLETE	43.5	42.4
Assets	3,828.2		3,823.9
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	23.7	23.6
Net Income (Loss) Attributable to ALLETE	6.7	6.1
Assets	571.7		566.0
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	32.1	32.4
Net Income (Loss) Attributable to ALLETE	(0.3)	(0.5)
Assets	$ 265.1		264.1
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments	2
Land in Minnesota (Acres) | a	5,000
Operating Revenue	$ 28.2	25.5
Net Income (Loss) Attributable to ALLETE	(0.9)	$ (2.1)
Assets	$ 276.9		$ 222.9