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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Subrogation By Estimation Approach [Roll Forward] |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | $ 1,834,387 |  |
Changes recognized |  |  |
Discounted RMBS subrogation (gross of reinsurance), ending balance | 1,770,546 | $ 1,834,387 |
Random Samples [Member] |  |  |
Subrogation By Estimation Approach [Roll Forward] |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | 1,834,387 | 1,907,035 | $ 2,829,575
Changes recognized |  |  |
Impact of sponsor actions | 0 | 0 | (995,000)
Other Changes Rmbs Subrogation | (63,841) | (72,648) | 72,460
Discounted RMBS subrogation (gross of reinsurance), ending balance | $ 1,770,546 | $ 1,834,387 | $ 1,907,035

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- Definition Changes Recognized in Residential Mortgage Backed Security [Abstract] + References No definition available. + Details
Name: | ambc_ChangesRecognizedInResidentialMortgageBackedSecurityAbstract
Namespace Prefix: | ambc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The change in RMBS subrogation due to the impact of sponsor actions which includes loan repurchases, direct payments, and other contributions from sponsors. + References No definition available. + Details
Name: | ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This element represents the changes in RMBS subrogation for reasons other than additional transactions being reviewed, adverse loans being reviewed or due to adverse loans being repurchased by the sponsor. + References No definition available. + Details
Name: | ambc_OtherChangesRmbsSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This element represents the reduction to gross claim liability for subrogation on insured RMBS financial obligations with credit deterioration. + References No definition available. + Details
Name: | ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Subrogation By Estimation Approach [Roll Forward] + References No definition available. + Details
Name: | ambc_SubrogationByEstimationApproachRollForward
Namespace Prefix: | ambc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis=ambc_RandomSamplesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |