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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Regulated Operations [Member] | Standby Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Letters of Credit Outstanding, Amount | $ 4.9 |
ALLETE Clean Energy [Member] | Standby Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Letters of Credit Outstanding, Amount | 81.9 |
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 [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | $ 2.0 |
Town Center District Capital Improvement Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Ownership Percentage of Benefited Property | 48.00% | 48.00%
Annual Assessment | $ 1.8 |
ALLETE South Wind [Member] | Standby Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Letters of Credit Outstanding, Amount | 13.2 |
ALLETE, Inc. [Member] | Standby Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Letters of Credit Outstanding, Amount | 123.9 |
South Shore Energy | Standby Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Letters of Credit Outstanding, Amount | $ 23.9 |

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- Definition Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
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- Definition Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
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- Definition Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=124440162&loc=d3e12069-110248 + Details
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