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Insurance In Force (Summary Of Company's Reinsurance Agreements For U.S. Public Finance And Structured Finance And International Insurance Operations) (Detail) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Ceded Par Outstanding |  | $ 1,017 | $ 1,500
Letters of Credit/Trust Accounts |  | 15 |
Reinsurance Recoverable/ (Payable) | [1] | (48) |
Assured Guaranty Corp [Member] |  |  |
Ceded Par Outstanding |  | 463 |
Letters of Credit/Trust Accounts |  | 0 |
Reinsurance Recoverable/ (Payable) | [1] | (45) |
Assured Guaranty Re Ltd [Member] |  |  |
Ceded Par Outstanding | [2] | 503 |
Letters of Credit/Trust Accounts | [2] | 15 |
Reinsurance Recoverable/ (Payable) | [1],[2] | (2) |
Others [Member] |  |  |
Ceded Par Outstanding | [2] | 51 |
Letters of Credit/Trust Accounts | [2] | 0 |
Reinsurance Recoverable/ (Payable) | [1],[2] | $ (1) |
[1] | Total reinsurance recoverable/(payable) is primarily related to recoverables on unpaid losses net of (payables) on salvage received.
[2] | Represents a withdrawal of ratings.

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- Definition Aggregate amount of insured par outstanding ceded to reinsurers. + References No definition available. + Details
Name: | mbi_AggregateAmountOfInsuredParOutstandingCededToReinsurers
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Reinsurance Recoverables Secured By Letters Of Credit Or Trust Amount. + References No definition available. + Details
Name: | mbi_ReinsuranceRecoverablesSecuredByLettersOfCreditOrTrustAmount
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount as of the balance sheet date of the known and estimated amounts owed to insurers under reinsurance treaties or other arrangements. + References No definition available. + Details
Name: | us-gaap_ReinsurancePayable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_CededCreditRiskAxis=mbi_AssuredGuarantyCorpMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=mbi_AssuredGuarantyReLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=mbi_OtherReinsurersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |