Business Segment (Details) $ in Millions		3 Months Ended																12 Months Ended
		Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	[1]	Sep. 30, 2020 USD ($)	[1]	Jun. 30, 2020 USD ($)	[1]	Mar. 31, 2020 USD ($)	[1]	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments																		2	3
Revenue		$ 425.8	$ 388.3	$ 373.1	$ 383.5	$ 399.0	$ 345.4	$ 335.6	$ 339.2	$ 320.4		$ 293.9		$ 243.2		$ 311.6		$ 1,570.7	$ 1,419.2	$ 1,169.1
Other – Non-utility																		7.6	11.4	11.3
Net Income Attributable to ALLETE		51.7	$ 33.7	$ 37.6	$ 66.3	61.9	$ 27.6	$ 27.9	$ 51.8	$ 38.3		$ 40.7		$ 20.1		$ 66.3		189.3	169.2	165.4
Depreciation, Depletion and Amortization																		242.2	231.7	217.8
Impairment of Property, Plant and Equipment																		0.0	0.0	12.7
Interest Expense																		75.2	69.1	65.6
Equity Earnings																		18.7	20.0	22.1
Income Tax Expense (Benefit)																		(31.2)	(26.9)	(43.4)
Assets		6,845.6				6,422.3												6,845.6	6,422.3
Property, Plant and Equipment, Additions																		208.1	476.4
New Energy
Segment Reporting Information [Line Items]
Business Combination, Acquisition Related Costs																		2.7
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Interest Expense	[2]																	$ (4.8)	(2.9)	(8.3)
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments																		3
Depreciation, Depletion and Amortization																		$ 171.9	170.7	166.9
Interest Expense	[2]																	58.1	57.3	58.5
Equity Earnings																		19.3	21.3	22.3
Income Tax Expense (Benefit)																		(10.4)	(16.6)	(19.4)
Property, Plant and Equipment, Additions																		158.3	166.8
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue																		1,259.3	1,227.9	987.3
Net Income Attributable to ALLETE	[3]																	149.9	129.1	136.3
Assets		4,291.4				4,289.4												$ 4,291.4	4,289.4
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments																		3
Revenue																		$ 193.1	104.4	102.2
Net Income Attributable to ALLETE	[3],[4],[5]																	23.1	13.8	8.0
Depreciation, Depletion and Amortization																		11.7	11.8	13.0
Interest Expense	[2]																	19.6	13.2	13.2
Equity Earnings																		(0.6)	(1.3)	(0.2)
Income Tax Expense (Benefit)																		(5.4)	6.3	(1.0)
Assets		680.9				426.2												680.9	426.2
Property, Plant and Equipment, Additions																		47.6	39.7
Purchase Price Accounting - After Tax																		8.3
Corporate and Other [Member] | New Energy
Segment Reporting Information [Line Items]
Business Combination, Acquisition Related Costs																		2.7
Corporate and Other [Member] | Nemadji Trail Energy Center [Member] | South Shore Energy
Segment Reporting Information [Line Items]
Gain on Sale After Tax																			8.5
ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue																		118.3	86.9	79.6
Net Income Attributable to ALLETE	[3]																	16.3	26.3	21.1
Depreciation, Depletion and Amortization																		58.6	49.2	37.9
Impairment of Property, Plant and Equipment																		0.0	0.0	12.7
Interest Expense	[2]																	2.3	1.5	2.2
Income Tax Expense (Benefit)																		(15.4)	(16.6)	(23.0)
Assets		$ 1,873.3				$ 1,706.7												1,873.3	1,706.7
Property, Plant and Equipment, Additions																		2.2	269.9
Residential [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		175.9	160.8	140.7
Commercial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		187.2	168.6	139.5
Municipal [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		40.2	52.0	41.2
Industrial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		589.0	565.5	432.8
Other Power Suppliers [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		165.8	168.7	138.8
Other [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		101.2	112.3	94.3
Long-term PSA [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		77.2	75.5	68.3
ALLETE Clean Energy Other [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Other – Non-utility																		7.6	11.4	11.3
Long-term Contract [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		89.2	84.4	86.0
Sale of Renewable Development Projects | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		73.9	0.0	0.0
Corporate & Other - Other [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		30.0	20.0	16.2
Sale of Wind Energy Facility [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Sale of Wind Energy Facility																		$ 33.5	$ 0.0	$ 0.0

[1] See Immaterial Revision to Prior Period in Note 1. Operations and Significant Accounting Policies.

[2] Includes interest expense resulting from intercompany loan agreements and allocated to certain subsidiaries. The amounts are eliminated in consolidation.    

[3] Includes interest expense resulting from intercompany loan agreements and allocated to certain subsidiaries. The amounts are eliminated in consolidation. 

[4] In 2021, South Shore Energy sold a portion of its undivided ownership interest in NTEC to Basin. The closing of the transaction resulted in the recognition of an approximately $8.5 million after-tax gain which is reflected in Corporate and Other. (See Note 1. Operations and Significant Accounting Policies.)

[5] Net Income in 2022 includes a $8.3 million after-tax expense as a result of purchase price accounting related to projects under development at the time of acquisition and $2.7 million after-tax of transaction costs related to the acquisition of New Energy.