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COMMITMENTS, GUARANTEES AND CONTINGENCIES - Other Matters (Details) $ in Millions	1 Months Ended	3 Months Ended
	Aug. 06, 2024 complaint	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)
Guarantor Obligations [Line Items]
Production tax credits sold			$ 61.5
Number of complaints | complaint	2
Gross proceeds from arbitration		$ 68.3
Prejudgment interest from legal settlements		5.1
Arbitration costs recovery		3.6
Pre tax gain from legal settlements		58.4
Proceeds from arbitration ruling		$ 60.0
ALLETE, Inc. | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding			140.0
ALLETE, Inc. | Surety Bonds
Guarantor Obligations [Line Items]
Letters of credit outstanding			105.1
Regulated Operations
Guarantor Obligations [Line Items]
Standby letters of credit and surety bonds outstanding			31.3
ALLETE Clean Energy
Guarantor Obligations [Line Items]
Standby letters of credit and surety bonds outstanding			81.6
BNI Energy Reclamation Liability
Guarantor Obligations [Line Items]
Estimated obligation			82.1
BNI Energy Reclamation Liability | Surety Bonds
Guarantor Obligations [Line Items]
Collateral			82.4
ALLETE South Wind | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding			10.1
South Shore Energy | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding			29.7
New Energy | Surety Bonds
Guarantor Obligations [Line Items]
Collateral			$ 10.0

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