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Financial Guarantee Insurance - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($)
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] |
Percentage ceded Par | 100.00%
Net unsecured reinsurance recoverable | $ 34
Assured Guaranty Re Ltd [Member] |
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] |
Percentage ceded Par | 46.00%
Net unsecured reinsurance recoverable | $ 0
Sompo Japan Nipponkoa Holdings, Inc [Member] |
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] |
Percentage ceded Par | 3.00%
Net unsecured reinsurance recoverable | $ 0
Assured Guaranty Corporation [Member] |
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] |
Percentage ceded Par | 8.00%
Net unsecured reinsurance recoverable | $ 4
Build America Mutual Assurance Company [Domain] |
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] |
Percentage ceded Par | 43.00%
Net unsecured reinsurance recoverable | $ 30

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- Definition Percentage of the cedant's total ceded financial guarantee principal outstanding. + References No definition available. + Details
Name: | ambc_PercentageOfCededFinancialGuaranteePrincipalOutstanding
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items] + References No definition available. + Details
Name: | ambc_SummaryOfCededParAndNetUnsecuredReinsuranceRecoverableLineItems
Namespace Prefix: | ambc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents reinsurance recoverables on paid and unpaid losses and deferred ceded premiums, net of ceded premiums payables due to reinsurers, letters of credit, and collateral posted for the benefit of the cedant. + References No definition available. + Details
Name: | ambc_UnsecuredReinsuranceRecoverables
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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

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

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