COMMITMENTS, GUARANTEES AND CONTINGENCIES - Other Matters (Details) $ in Millions	1 Months Ended
	Aug. 06, 2024 complaint	Jun. 30, 2025 USD ($)
Guarantor Obligations [Line Items]
Production tax credits sold		$ 88.1
Number of complaints | complaint	3
ALLETE, Inc. | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding		139.6
ALLETE, Inc. | Surety Bonds
Guarantor Obligations [Line Items]
Letters of credit outstanding		133.2
Regulated Operations
Guarantor Obligations [Line Items]
Standby letters of credit and surety bonds outstanding		33.9
ALLETE Clean Energy
Guarantor Obligations [Line Items]
Standby letters of credit and surety bonds outstanding		86.7
New Energy | Standby Letters of Credit
Guarantor Obligations [Line Items]
Collateral		12.7
BNI Energy Reclamation Liability
Guarantor Obligations [Line Items]
Estimated obligation		99.5
BNI Energy Reclamation Liability | Surety Bonds
Guarantor Obligations [Line Items]
Collateral		99.7
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