Equity Investments (Details) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2020 USD ($) MW	Sep. 30, 2020 USD ($)	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 ($) MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance				$ 197.6									$ 197.6
Cash Investments													99.1	$ 37.9	$ 39.2
Equity Earnings													22.1	21.7	17.5
Equity Investment Ending Balance	$ 301.2				$ 197.6								301.2	197.6
Assets [Abstract]
Total Current Assets	254.9				269.5								254.9	269.5
Total Assets	6,084.6				5,482.8								6,084.6	5,482.8
Liabilities and Equity [Abstract]
Liabilities, Current	459.6				507.4								459.6	507.4
Members' Equity	2,800.2				2,335.6				$ 2,155.8				2,800.2	2,335.6	2,155.8
Total Liabilities and Equity	6,084.6				5,482.8								6,084.6	5,482.8
Income Statement Data [Abstract]
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
Net Income													161.6	185.5	174.1
ALLETE's Equity in Net Income													$ 22.1	21.7	17.5
ATC [Member]
Equity Investments [Line Items]
Ownership Percentage	8.00%												8.00%
Expected Additional Investment in 2021	$ 2.0												$ 2.0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance				141.6				$ 128.1					141.6	128.1
Cash Investments													2.7	6.6
Equity Earnings													22.3	21.7
Distributed ATC Earnings													(18.9)	(16.1)
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments													1.3	1.3
Equity Investment Ending Balance	$ 149.0				141.6				$ 128.1				149.0	141.6	128.1
Income Statement Data [Abstract]
ALLETE's Equity in Net Income													$ 22.3	21.7
Authorized Return on Equity	10.02%												10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%												10.52%
ATC [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees
ALLETE's Investment in ATC [Roll Forward]
Equity Earnings													$ 22.3	21.7	17.5
Assets [Abstract]
Total Current Assets	$ 92.8				84.6								92.8	84.6
Assets, Noncurrent	5,400.5				5,244.3								5,400.5	5,244.3
Total Assets	5,493.3				5,328.9								5,493.3	5,328.9
Liabilities and Equity [Abstract]
Liabilities, Current	310.8				502.6								310.8	502.6
Long-Term Debt	2,512.2				2,312.8								2,512.2	2,312.8
Other Non-Current Liabilities	378.2				298.9								378.2	298.9
Members' Equity	2,292.1				2,214.6								2,292.1	2,214.6
Total Liabilities and Equity	$ 5,493.3				5,328.9								5,493.3	5,328.9
Income Statement Data [Abstract]
Revenue													758.1	744.4	690.5
Operating Expense													372.4	373.5	358.7
Other Expense													110.9	110.5	108.3
Net Income													274.8	260.4	223.5
ALLETE's Equity in Net Income													$ 22.3	21.7	$ 17.5
Nobles 2 [Member]
Equity Investments [Line Items]
Ownership Percentage	49.00%												49.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance				$ 56.0									$ 56.0
Cash Investments													96.4
Equity Investment Ending Balance	$ 152.2				$ 56.0								$ 152.2	$ 56.0
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member] | MPUC [Member] | Resource Package [Member]
Equity Investments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW	250												250
Contract Term (Years)													20 years