Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) a	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) a Segments	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments			2
Operating Revenue	$ 335.6	$ 243.2	$ 674.8	$ 554.8
Net Income Attributable to ALLETE	27.9	20.1	79.7	86.4
Assets	6,293.3		$ 6,293.3		$ 6,084.6
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			3
Operating Revenue	290.4	200.8	$ 583.4	466.1
Net Income Attributable to ALLETE	21.5	11.1	66.5	68.6
Assets	4,200.1		4,200.1		4,196.8
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue	35.4	27.4	81.0	68.3
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue	40.7	28.8	80.4	66.1
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue	11.5	9.0	24.1	19.3
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue	139.1	88.0	268.1	206.8
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue	37.3	27.4	75.7	65.7
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	26.4	20.2	54.1	39.9
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	21.6	17.6	42.5	37.7
Net Income Attributable to ALLETE	5.1	4.0	12.5	15.7
Assets	1,653.5		1,653.5		1,483.3
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue	18.7	14.8	36.8	32.1
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue	$ 2.9	2.8	$ 5.7	5.6
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	$ 23.6	24.8	$ 48.9	51.0
Net Income Attributable to ALLETE	1.3	5.0	0.7	2.1
Assets	439.7		439.7		$ 404.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue	21.4	22.5	43.1	44.9
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	$ 2.2	$ 2.3	$ 5.8	$ 6.1