Commitments, Guarantees and Contingencies - Other Matters (Details) $ in Millions	3 Months Ended
	Sep. 30, 2023 USD ($)
Guarantor Obligations [Line Items]
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	163.6
Regulated Operations | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding	24.2
ALLETE Clean Energy | Standby Letters of Credit
Guarantor Obligations [Line Items]
Letters of credit outstanding	105.3
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	$ 23.9