SHORT-TERM AND LONG-TERM DEBT - Narrative (Details)	Jul. 31, 2024 USD ($)	Apr. 23, 2024 USD ($)	Sep. 30, 2024 USD ($)	Sep. 05, 2024 USD ($)	Dec. 31, 2023 USD ($)
Minimum
Debt Instrument [Line Items]
Required ratio of indebtedness to total capitalization			0.65
Actual ratio of indebtedness to total capitalization			0.36
Maximum
Debt Instrument [Line Items]
Required ratio of indebtedness to total capitalization			1.00
Actual ratio of indebtedness to total capitalization			1.00
ALLETE Bonds 5.72% Due April 2039
Debt Instrument [Line Items]
Interest rate		5.72%
Proceeds from issuance of first mortgage bond		$ 100,000,000
Line of Credit | Credit Agreement
Debt Instrument [Line Items]
Letters of credit outstanding			$ 23,400,000		$ 19,400,000
Line of credit draw outstanding			$ 0		$ 34,100,000
Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt				$ 150,000,000
Senior Notes | 5.94% Senior Unsecured Notes Due September 2029
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt				$ 100,000,000
Interest rate				5.94%
Senior Notes | 6.18% Senior Unsecured Notes Due September 2034
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt				$ 50,000,000
Interest rate				6.18%
Senior Notes | 2.65% Senior Notes Due September 10, 2025
Debt Instrument [Line Items]
Interest rate	2.65%
Repayments of senior debt	$ 150,000,000
Percentage of principal amount prepaid	100.00%