Equity Investments (Details) $ in Millions			3 Months Ended																12 Months Ended
		Jan. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) MW	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 ($)	Dec. 31, 2020 USD ($)		Sep. 30, 2020 USD ($)	[1]	Jun. 30, 2020 USD ($)	[1]	Mar. 31, 2020 USD ($)	[1]	Dec. 31, 2022 USD ($) MW	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance						$ 318.0													$ 318.0
Cash Investments																			5.9	$ 17.6	$ 99.1
Equity Earnings																			18.7	20.0	22.1
Equity Investment Ending Balance			$ 322.7				$ 318.0												322.7	318.0
Assets [Abstract]
Total Current Assets			718.0				291.3												718.0	291.3
Total Assets			6,845.6				6,422.3												6,845.6	6,422.3
Liabilities and Equity [Abstract]
Liabilities, Current			716.2				543.4												716.2	543.4
Members' Equity			3,348.3				2,937.5				$ 2,791.4								3,348.3	2,937.5	2,791.4
Total Liabilities and Equity			6,845.6				6,422.3												6,845.6	6,422.3
Income Statement Data [Abstract]
Revenue			425.8	$ 388.3	$ 373.1	383.5	399.0	$ 345.4	$ 335.6	$ 339.2	320.4	[1]	$ 293.9		$ 243.2		$ 311.6		1,570.7	1,419.2	1,169.1
Net Income																			131.3	137.8	152.8
ALLETE's Equity in Net Income																			18.7	20.0	22.1
ATC [Member]
Assets [Abstract]
Total Current Assets			89.6				89.8												89.6	89.8
Assets, Noncurrent			5,997.8				5,628.1												5,997.8	5,628.1
Total Assets			6,087.4				5,717.9												6,087.4	5,717.9
Liabilities and Equity [Abstract]
Liabilities, Current			511.9				436.9												511.9	436.9
Long-Term Debt			2,613.0				2,513.0												2,613.0	2,513.0
Other Non-Current Liabilities			485.8				422.0												485.8	422.0
Members' Equity			2,476.7				2,346.0												2,476.7	2,346.0
Total Liabilities and Equity			6,087.4				5,717.9												6,087.4	5,717.9
Income Statement Data [Abstract]
Revenue																			751.2	754.8	758.1
Operating Expense																			381.5	376.2	372.4
Other Expense																			122.9	113.9	110.9
Net Income																			246.8	264.7	274.8
Approximate Refund for Reversal of FERC ROE Order			31.0																31.0
Approximate Refund for Reversale of FERC ROE Order, ALLETE's Proportionate Share																			2.4
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share			$ 1.0																$ 1.0
FERC Proposed Reduction to Incentive Adder			0.50%																0.50%
Basis Point Incentive adder			50.00%																50.00%
ATC [Member]
Equity Investments [Line Items]
Ownership Percentage			8.00%																8.00%
Expected Additional Investment in 2022			$ 8.2																$ 8.2
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance						154.5				$ 149.0									154.5	149.0
Cash Investments																			5.9	0.0
Equity Earnings																			19.3	21.3	22.3
Distributed Earnings																			(15.5)	(17.2)
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments																			1.2	1.4
Equity Investment Ending Balance			$ 165.4				154.5				$ 149.0								165.4	154.5	149.0
Income Statement Data [Abstract]
ALLETE's Equity in Net Income																			$ 19.3	21.3	$ 22.3
Authorized Return on Equity			10.02%																10.02%
Authorized Return on Equity, Including Incentive Adder			10.52%																10.52%
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments		$ 0.8
Nobles 2 [Member]
Equity Investments [Line Items]
Ownership Percentage			49.00%																49.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance						$ 163.5													$ 163.5
Equity Earnings	[2]																		(0.6)
Distributed Earnings																			(5.6)
Equity Investment Ending Balance			$ 157.3				$ 163.5												157.3	$ 163.5
Income Statement Data [Abstract]
ALLETE's Equity in Net Income	[2]																		$ (0.6)
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member]
Equity Investments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW			250																250
Contract Term (Years)																			20 years

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

[2] The Company also recorded net loss attributable to non-controlling interest of $10.6 million related to its investment in Nobles 2