Business Segments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) Segments	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments	2
Operating Revenue	$ 383.5	$ 339.2
Net Income (Loss) Attributable to ALLETE	66.3	51.8
Assets	$ 6,556.0		$ 6,435.0
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments	3
Operating Revenue	$ 329.0	293.0
Net Income (Loss) Attributable to ALLETE	51.5	45.0
Assets	4,297.1		4,289.4
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue	56.0	45.6
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue	48.3	39.7
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue	12.1	12.6
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue	147.8	129.0
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue	41.0	38.4
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	23.8	27.7
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	28.2	20.9
Net Income (Loss) Attributable to ALLETE	16.5	7.4
Assets	1,816.1		1,719.4
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue	25.4	18.1
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue	$ 2.8	2.8
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments	2
Operating Revenue	$ 26.3	25.3
Net Income (Loss) Attributable to ALLETE	(1.7)	(0.6)
Assets	442.8		$ 426.2
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue	22.6	21.7
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	$ 3.7	$ 3.6