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Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 73 | $ 85 | $ 162
United States [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 55 | 63 | 94
Other Americas [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 16 | 17 | 68
Other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 2 | $ 5 | $ 0

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- Definition Premiums earned net financial guarantee insurance contracts and insured derivatives. + References No definition available. + Details
Name: | mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=mbi_OtherAmericasMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=mbi_OtherLocationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |