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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ in Millions | 3 Months Ended
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Mar. 31, 2019 USD ($) segments
Business Acquisition [Line Items] |
Number of operating segments | segments | 3
Puerto Rico [Member] |
Business Acquisition [Line Items] |
Claims payments | $ 65
Outstanding bonds | 3,100
Zohar [Member] |
Business Acquisition [Line Items] |
Aggregate claim amount | 919
CABs [Member] | Puerto Rico [Member] |
Business Acquisition [Line Items] |
Outstanding bonds | $ 3,400

X
- Definition Thus element represents the aggregate amounts of claim paid to date + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the gross amount of claim payments during the period + References No definition available. + Details
Name: | mbi_ClaimPayments
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of outstanding bonds for a specific issuer + References No definition available. + Details
Name: | mbi_OutstandingBonds
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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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Data Type: | xbrli:stringItemType
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Period Type: | duration

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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration

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

X
- Details
Name: | srt_MajorCustomersAxis=mbi_ZoharMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=mbi_CABsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |