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Business Segment - By Chanel (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Revenue	$ 364.8	$ 407.2	$ 354.5	$ 403.3	$ 402.7	$ 378.8	$ 533.4	$ 564.9	$ 425.8	$ 388.3	$ 373.1	$ 383.5	$ 1,529.8	$ 1,879.8	$ 1,570.7
Other													5.0	5.1	7.6
Net Income Attributable to ALLETE	$ 50.6	$ 45.0	$ 33.0	$ 50.7	$ 51.5	$ 85.9	$ 51.5	$ 58.2	$ 51.7	$ 33.7	$ 37.6	$ 66.3	179.3	247.1	189.3
New Energy
Segment Reporting Information [Line Items]
Acquisition related costs															2.7
Regulated Operations
Segment Reporting Information [Line Items]
Revenue													1,242.7	1,238.3	1,259.3
Net Income Attributable to ALLETE													160.9	147.2	149.9
ALLETE Clean Energy
Segment Reporting Information [Line Items]
Revenue													84.3	418.5	118.3
Net Income Attributable to ALLETE													17.8	71.7	16.3
Gain From Legal Settlements - after-tax														44.3
Corporate and Other
Segment Reporting Information [Line Items]
Revenue													202.8	223.0	193.1
Net Income Attributable to ALLETE													0.6	28.2	23.1
Purchase price accounting - after tax															8.3
Acquisition related costs													22.6
Corporate and Other | New Energy
Segment Reporting Information [Line Items]
Acquisition related costs															2.7
Residential | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													177.0	165.7	175.9
Commercial | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													189.8	184.6	187.2
Municipal | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													34.2	33.4	40.2
Industrial | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													610.8	593.6	589.0
Other Power Suppliers | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													137.2	146.1	165.8
Other | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													93.7	114.9	101.2
Long-term PSA | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Operating Revenue													56.4	65.0	77.2
Sale of Wind Energy Facilities | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Operating Revenue													22.9	348.4	33.5
Other | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Other													5.0	5.1	7.6
Long-term Contract | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													105.7	101.2	89.2
Sale of Renewable Development Projects | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													72.1	92.5	73.9
Other | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													$ 25.0	$ 29.3	$ 30.0

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