Business Segment - Table (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	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, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 402.7	$ 378.8	$ 533.4	$ 564.9	$ 425.8	$ 388.3	$ 373.1	$ 383.5	$ 399.0	$ 345.4	$ 335.6	$ 339.2	$ 1,879.8	$ 1,570.7	$ 1,419.2
Other – Non-utility													5.1	7.6	11.4
Net Income Attributable to ALLETE	51.5	$ 85.9	$ 51.5	$ 58.2	51.7	$ 33.7	$ 37.6	$ 66.3	$ 61.9	$ 27.6	$ 27.9	$ 51.8	247.1	189.3	169.2
Total Depreciation and Amortization													251.8	242.2	231.7
Total Interest Expense													80.8	75.2	69.1
Equity Earnings													21.7	18.7	20.0
Income Tax Expense (Benefit)													27.9	(31.2)	(26.9)
Total Assets	6,656.4				6,845.6								6,656.4	6,845.6
Total Capital Expenditures													256.0	208.1
New Energy
Segment Reporting Information [Line Items]
Acquisition related costs														2.7
Operating Segments
Segment Reporting Information [Line Items]
Total Depreciation and Amortization													251.8	242.2	231.7
Eliminations
Segment Reporting Information [Line Items]
Total Interest Expense													(6.4)	(4.8)	(2.9)
Regulated Operations
Segment Reporting Information [Line Items]
Revenue													1,238.3	1,259.3	1,227.9
Net Income Attributable to ALLETE													147.2	149.9	129.1
Equity Earnings													23.1	19.3	21.3
Income Tax Expense (Benefit)													22.4	(10.4)	(16.6)
Total Assets	4,335.0				4,291.4								4,335.0	4,291.4
Total Capital Expenditures													236.3	158.3
Regulated Operations | Operating Segments
Segment Reporting Information [Line Items]
Total Depreciation and Amortization													179.2	171.9	170.7
Total Interest Expense													63.9	58.1	57.3
ALLETE Clean Energy
Segment Reporting Information [Line Items]
Revenue													418.5	118.3	86.9
Net Income Attributable to ALLETE													71.7	16.3	26.3
Income Tax Expense (Benefit)													2.7	(15.4)	(16.6)
Total Assets	1,594.1				1,873.3								1,594.1	1,873.3
Total Capital Expenditures													(5.3)	2.2
Gain From Legal Settlements - after-tax													44.3
ALLETE Clean Energy | Operating Segments
Segment Reporting Information [Line Items]
Total Depreciation and Amortization													57.5	58.6	49.2
Total Interest Expense													0.8	2.3	1.5
Corporate and Other
Segment Reporting Information [Line Items]
Revenue													223.0	193.1	104.4
Net Income Attributable to ALLETE													28.2	23.1	13.8
Purchase price accounting - after tax														8.3
Total Assets	$ 727.3				$ 680.9								727.3	680.9
Total Capital Expenditures													25.0	47.6
Corporate and Other | New Energy
Segment Reporting Information [Line Items]
Acquisition related costs														2.7
Corporate and Other | Nemadji Trail Energy Center | South Shore Energy
Segment Reporting Information [Line Items]
Gain on sale after tax														8.5
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Total Depreciation and Amortization													15.1	11.7	11.8
Corporate and Other | Corporate and Other
Segment Reporting Information [Line Items]
Total Interest Expense													22.5	19.6	13.2
Equity Earnings													(1.4)	(0.6)	(1.3)
Income Tax Expense (Benefit)													2.8	(5.4)	6.3
Residential | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													165.7	175.9	160.8
Commercial | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													184.6	187.2	168.6
Municipal | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													33.4	40.2	52.0
Industrial | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													593.6	589.0	565.5
Other Power Suppliers | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													146.1	165.8	168.7
Other | Regulated Operations
Segment Reporting Information [Line Items]
Operating Revenue													114.9	101.2	112.3
Long-term PSA | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Operating Revenue													65.0	77.2	75.5
Sale of Wind Energy Facilities | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Operating Revenue													348.4	33.5	0.0
Other | ALLETE Clean Energy
Segment Reporting Information [Line Items]
Other – Non-utility													5.1	7.6	11.4
Long-term Contract | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													101.2	89.2	84.4
Sale of Renewable Development Projects | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													92.5	73.9	0.0
Other | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenue													$ 29.3	$ 30.0	$ 20.0