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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 162 | $ 202 | $ 304
United States [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 94 | 169 | 238
United Kingdom [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 0 | 2 | 27
Other Americas [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 68 | 25 | 26
Other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 0 | $ 6 | $ 13

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- Definition Premiums Earned Net Financial Guarantee Insurance Contracts And Insured Derivatives + References No definition available. + Details
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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=country_GB
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: |