Business Segment (Details) $ in Millions		3 Months Ended												12 Months Ended
		Dec. 31, 2020 USD ($) a	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2020 USD ($) a	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments														3	3
Revenue		$ 320.4	$ 293.9	$ 243.2	$ 311.6	$ 304.6	$ 288.3	$ 290.4	$ 357.2	$ 448.3	$ 348.0	$ 344.1	$ 358.2	$ 1,169.1	$ 1,240.5	$ 1,498.6
Other – Non-utility														11.3	11.6	23.6
Net Income Attributable to ALLETE		47.1	$ 40.7	$ 20.1	$ 66.3	49.7	$ 31.2	$ 34.2	$ 70.5	$ 61.1	$ 30.7	$ 31.3	$ 51.0	174.2	185.6	174.1
Depreciation, Depletion and Amortization														217.8	202.0	205.6
Operating Expenses – Other														0.0	0.0	(2.0)
Interest Expense														65.6	64.9	67.9
Equity Earnings														22.1	21.7	17.5
Income Tax Expense (Benefit)														(39.5)	(6.6)	(15.5)
Assets		6,084.6				5,482.8								6,084.6	5,482.8
Property, Plant and Equipment, Additions														657.2	626.6
Income Tax Expense (Benefit), Due to Sale of Subsidiary														10.4
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Interest Expense	[1]													$ (8.3)	(5.0)	(4.7)
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														3
Depreciation, Depletion and Amortization														$ 166.9	159.4	158.0
Interest Expense	[1]													58.5	58.9	60.2
Equity Earnings														22.3	21.7	17.5
Income Tax Expense (Benefit)														(19.4)	(7.1)	(15.5)
Property, Plant and Equipment, Additions														133.7	230.9
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue														987.3	1,042.4	1,059.5
Net Income Attributable to ALLETE	[2]													136.3	154.4	131.0
Assets		$ 4,196.8				4,130.8								$ 4,196.8	4,130.8
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														2
Land in Minnesota (Acres) | a		4,000												4,000
Revenue														$ 102.2	105.1	107.1
Net Income Attributable to ALLETE	[2],[3]													8.0	19.9	6.2
Depreciation, Depletion and Amortization														13.0	13.5	13.0
Operating Expenses – Other	[4]													0.0	0.0	(2.0)
Interest Expense	[1]													13.2	8.0	7.3
Equity Earnings														(0.2)	0.0	0.0
Income Tax Expense (Benefit)	[5]													(1.0)	12.8	0.0
Assets		$ 404.5				350.5								404.5	350.5
Property, Plant and Equipment, Additions														15.7	10.1
ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue														79.6	59.6	159.9
Net Income Attributable to ALLETE	[2],[6]													29.9	12.4	33.7
Depreciation, Depletion and Amortization														37.9	26.8	24.4
Interest Expense	[1]													2.2	2.8	3.6
Income Tax Expense (Benefit)														(19.1)	(11.9)	(1.0)
Assets		$ 1,483.3				$ 1,001.5								1,483.3	1,001.5
Property, Plant and Equipment, Additions														507.8	385.6
U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue	[3]													0.0	33.4	172.1
Net Income Attributable to ALLETE	[2]													0.0	(1.1)	3.2
Depreciation, Depletion and Amortization														0.0	2.3	10.2
Interest Expense	[1]													0.0	0.2	1.5
Income Tax Expense (Benefit)														0.0	(0.4)	1.0
Gain on Sale of Affiliate, After-tax														13.2
Residential [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														140.7	139.6	139.7
Commercial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														139.5	145.7	147.9
Municipal [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														41.2	48.6	54.9
Industrial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														432.8	476.4	469.5
Other Power Suppliers [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														138.8	153.7	170.3
CIP Financial Incentive [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[7]													2.4	2.8	3.0
Other [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														91.9	75.6	74.2
Long-term PSA [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														68.3	48.0	55.2
Sale of Wind Energy Facility [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Sale of Wind Energy Facility														0.0	0.0	81.1
ALLETE Clean Energy Other [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Other – Non-utility														11.3	11.6	23.6
Point-in-Time [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]													0.0	19.0	100.3
Contract [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]													0.0	9.2	38.3
Capital Project [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]													0.0	5.2	33.5
Long-term Contract [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														86.0	82.8	85.5
Corporate & Other - Other [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														$ 16.2	$ 22.3	$ 21.6

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

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

[3] In March 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $13.2 million after-tax which is reflected in Corporate and Other. (See Note 1. Operations and Significant Accounting Policies.)

[4] See Note 1. Operations and Significant Accounting Policies.

[5] In March 2019, ALLETE sold U.S. Water Services. The Company recognized income tax expense of $10.4 million for the gain on sale of U.S. Water Services which is reflected in Corporate and Other. (See Note 1. Operations and Significant Accounting Policies.)

[6] Net income in 2018 includes the recognition of profit for the sale of a wind energy facility to Montana-Dakota Utilities. 

[7] See Note 4. Regulatory Matters.