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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020
BNI Energy Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | $ 70.7 |
BNI Energy Reclamation Liability [Member] | Surety Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 71.2 |
ALLETE Properties Development and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 1.0 |
ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 2.0 |
Town Center District Capital Improvement Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Bonds | $ 26.4 |
Bond Term (Years) | 31 years |
Ownership Percentage of Benefited Property | 30.00% | 48.00%
Annual Assessment | $ 1.8 |
Bond Interest Rate | 6.00% |
ALLETE, Inc. [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Standby Letters of Credit | $ 134.7 |
Regulated Operations [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Standby Letters of Credit | 10.9 |
ALLETE Clean Energy [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Standby Letters of Credit | 87.4 |
ALLETE South Wind [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Standby Letters of Credit | 12.5 |
South Shore Energy | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Standby Letters of Credit | $ 23.9 |

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