Operations and Significant Accounting Policies - Immaterial Revision of Prior Periods (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	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, 2020		Sep. 30, 2020	[1]	Jun. 30, 2020	[1]	Mar. 31, 2020	[1]	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Impairment Charge - After Tax																			$ 8.8
Error Corrections and Prior Period Adjustments, Description																	We have identified an immaterial prior period error with respect to the recognition of a non-cash impairment expense for our Northern Wind operating assets in the fourth quarter of 2020 prior to the execution of the purchase and sales agreement for a repower and sale of the wind energy facility. Specifically, the remaining net book value of the property, plant and equipment used in the recoverability test was calculated incorrectly.
Loss on Impairment of Assets																	$ 0.0	$ 0.0	12.7
Income Tax Benefit																	(31.2)	(26.9)	(43.4)
Net Income Attributable to ALLETE	$ 51.7	$ 33.7	$ 37.6	$ 66.3	$ 61.9	$ 27.6	$ 27.9	$ 51.8	$ 38.3	[1]	$ 40.7		$ 20.1		$ 66.3		189.3	169.2	165.4
Comprehensive Income Attributable to ALLETE																	$ 188.7	$ 176.5	$ 157.9
Earnings Per Share	$ 0.90	$ 0.59	$ 0.67	$ 1.24	$ 1.18	$ 0.53	$ 0.53	$ 0.99	$ 0.73	[1]	$ 0.78		$ 0.39		$ 1.28		$ 3.38	$ 3.23	$ 3.18
Property, Plant and Equipment – Net	$ 5,004.0				$ 5,087.5												$ 5,004.0	$ 5,087.5
Assets	6,845.6				6,422.3												6,845.6	6,422.3
Deferred Income Taxes	158.1				181.8												158.1	181.8
Liabilities	3,497.3				3,484.8												3,497.3	3,484.8
Retained Earnings	934.8				891.4												934.8	891.4
Members' Equity	$ 3,348.3				2,937.5				$ 2,791.4								$ 3,348.3	2,937.5	$ 2,791.4
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Loss on Impairment of Assets																			0.0
Income Tax Benefit																			(39.5)
Net Income Attributable to ALLETE																			174.2
Comprehensive Income Attributable to ALLETE																			$ 166.7
Earnings Per Share																			$ 3.35
Property, Plant and Equipment – Net					5,100.2													5,100.2
Assets					6,435.0													6,435.0
Deferred Income Taxes					185.7													185.7
Liabilities					3,488.7													3,488.7
Retained Earnings					900.2													900.2
Members' Equity					2,946.3													2,946.3
Revision of Prior Period - Adjusted Balance
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Loss on Impairment of Assets																			$ 12.7
Income Tax Benefit																			(43.4)
Net Income Attributable to ALLETE																			165.4
Comprehensive Income Attributable to ALLETE																			$ 157.9
Earnings Per Share																			$ 3.18
Property, Plant and Equipment – Net					5,087.5													5,087.5
Assets					6,422.3													6,422.3
Deferred Income Taxes					181.8													181.8
Liabilities					3,484.8													3,484.8
Retained Earnings					891.4													891.4
Members' Equity					2,937.5													2,937.5
Revision of Prior Period, Error Correction, Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Loss on Impairment of Assets																			$ 12.7
Income Tax Benefit																			(3.9)
Net Income Attributable to ALLETE																			(8.8)
Comprehensive Income Attributable to ALLETE																			$ (8.8)
Earnings Per Share																			$ (0.17)
Property, Plant and Equipment – Net					(12.7)													(12.7)
Assets					(12.7)													(12.7)
Deferred Income Taxes					(3.9)													(3.9)
Liabilities					(3.9)													(3.9)
Retained Earnings					(8.8)													(8.8)
Members' Equity					$ (8.8)													$ (8.8)

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