Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) Segments	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments			2
Operating Revenue	$ 533.4	$ 373.1	$ 1,098.3	$ 756.6
Net Income Attributable to ALLETE	51.5	37.6	109.7	103.9
Assets	6,567.9		$ 6,567.9		$ 6,845.6
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			3
Operating Revenue	292.2	308.7	$ 604.8	637.7
Net Income Attributable to ALLETE	37.8	29.6	78.4	81.1
Assets	4,259.8		4,259.8		4,291.4
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue	36.0	40.6	85.4	96.6
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue	44.0	44.4	91.7	92.7
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue	7.6	10.0	16.5	22.1
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue	140.2	150.6	285.1	298.4
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue	36.3	36.9	72.2	77.9
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	28.1	26.2	53.9	50.0
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	181.1	21.2	382.6	49.4
Net Income Attributable to ALLETE	3.1	5.8	11.6	22.3
Assets	1,557.9		1,557.9		1,873.3
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue	13.2	18.9	31.6	44.3
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue	1.3	2.3	2.6	5.1
ALLETE Clean Energy [Member] | Sale of Wind Energy Facility
Business Segments [Line Items]
Operating Revenue	166.6	0.0	348.4	0.0
Corporate and Other [Member]
Business Segments [Line Items]
Operating Revenue	60.1	43.2	110.9	69.5
Net Income Attributable to ALLETE	10.6	2.2	19.7	0.5
Assets	750.2		750.2		$ 680.9
Corporate and Other [Member] | New Energy
Business Segments [Line Items]
Business Combination, Acquisition Related Costs				3.0
Purchase Price Accounting Impact			4.0
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue	23.9	21.4	49.4	44.0
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	3.6	7.3	9.1	11.0
Corporate and Other [Member] | Sale of Renewable Development Projects
Business Segments [Line Items]
Operating Revenue	$ 32.6	$ 14.5	$ 52.4	$ 14.5