Business Segments (Details) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2020 USD ($) a Segments	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments		3
Operating Revenue		$ 311.6	$ 357.2
Net Income (Loss) Attributable to ALLETE		66.3	70.5
Gain on Sale of U.S. Water Services, After-tax			9.9	$ 13.2
Assets		$ 5,619.5		5,482.8
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments		3
Operating Revenue		$ 265.3	282.2
Net Income (Loss) Attributable to ALLETE		57.5	51.5
Assets		4,145.5		4,130.8
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		40.9	45.2
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		37.3	38.9
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		10.3	15.4
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		118.8	121.6
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		38.3	39.4
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		19.7	21.7
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		20.1	17.5
Net Income (Loss) Attributable to ALLETE		11.7	5.8
Assets		1,099.6		1,001.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		17.3	14.6
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.8	2.9
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	33.4
Net Income (Loss) Attributable to ALLETE	[1]	0.0	(1.1)
U.S. Water Services [Member] | Point-in-Time [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	19.0
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	9.2
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue	[1]	$ 0.0	5.2
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments		2
Land in Minnesota (Acres) | a		4,000
Operating Revenue		$ 26.2	24.1
Net Income (Loss) Attributable to ALLETE	[1]	(2.9)	14.3
Gain on Sale of U.S. Water Services, After-tax			9.9
Assets		374.4		$ 350.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		22.4	20.2
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		$ 3.8	$ 3.9

[1] In March 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $9.9 million after-tax during the three months ended March 31, 2019, which is reflected in Corporate and Other.