Commitments, Guarantees and Contingencies - Other Matters (Details) $ in Millions	Mar. 31, 2022 USD ($)
Regulated Operations [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Letters of Credit Outstanding, Amount	$ 18.1
ALLETE Clean Energy [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Letters of Credit Outstanding, Amount	93.4
BNI Energy Reclamation Liability [Member]
Guarantor Obligations [Line Items]
Estimated Obligation	82.1
BNI Energy Reclamation Liability [Member] | Surety Bonds [Member]
Guarantor Obligations [Line Items]
Collateral	82.4
ALLETE South Wind [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Letters of Credit Outstanding, Amount	11.8
ALLETE, Inc. [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Letters of Credit Outstanding, Amount	147.2
South Shore Energy | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Letters of Credit Outstanding, Amount	$ 23.9