COMMITMENTS, GUARANTEES AND CONTINGENCIES - Other Matters (Details) $ in Millions	Jun. 30, 2024 USD ($)
Guarantor Obligations [Line Items]
Production tax credits sold	$ 22.5
ALLETE, Inc. | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding	158.2
Regulated Operations | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding	28.2
ALLETE Clean Energy | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding	101.1
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