Business Segments (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2020 USD ($) a	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) a Segments	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				3
Operating Revenue		$ 293.9	$ 288.3	$ 848.7	$ 935.9
Net Income (Loss) Attributable to ALLETE		40.7	31.2	127.1	135.9
Gain on Sale of U.S. Water Services, After-tax						$ 13.2
Assets		5,878.5		$ 5,878.5		5,482.8
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		255.1	254.1	$ 721.2	786.1
Net Income (Loss) Attributable to ALLETE		42.4	32.4	111.0	114.2
Assets		4,134.4		4,134.4		4,130.8
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		34.2	29.8	102.5	104.2
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		37.1	36.8	103.2	110.6
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		11.2	11.8	30.5	39.4
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		109.6	115.6	316.4	357.5
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		30.9	37.3	96.6	111.9
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		29.7	20.0	69.6	59.7
Regulated Operations [Member] | CIP Financial Incentive [Member]
Business Segments [Line Items]
Operating Revenue	[1]	2.4	2.8	2.4	2.8
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		14.8	10.4	52.5	43.3
Net Income (Loss) Attributable to ALLETE		1.1	(1.2)	16.8	6.5
Assets		1,308.3		1,308.3		1,001.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		11.9	7.5	44.0	34.6
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.9	2.9	8.5	8.7
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	[2]	0.0	0.0	0.0	33.4
Net Income (Loss) Attributable to ALLETE	[2]	0.0	0.0	0.0	(1.1)
U.S. Water Services [Member] | Point-in-Time [Member]
Business Segments [Line Items]
Operating Revenue	[2]	0.0	0.0	0.0	19.0
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue	[2]	0.0	0.0	0.0	9.2
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue	[2]	$ 0.0	0.0	$ 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		4,000
Operating Revenue		$ 24.0	23.8	$ 75.0	73.1
Net Income (Loss) Attributable to ALLETE	[2]	(2.8)	0.0	(0.7)	16.3
Gain on Sale of U.S. Water Services, After-tax					11.1
Assets		435.8		435.8		$ 350.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		20.7	20.5	65.6	61.9
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		$ 3.3	$ 3.3	$ 9.4	$ 11.2

[1] See Note 2. Regulatory Matters.

[2] In March 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $11.1 million after-tax during the nine months ended September 30, 2019, which is reflected in Corporate and Other.