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Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 Successor Ambac [Member] Transactions | Apr. 30, 2013 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] Adverse Samples [Member] Transactions | Sep. 30, 2013 Successor Ambac [Member] Random Samples [Member] Transactions | Apr. 30, 2013 Predecessor Ambac [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Transactions | Dec. 31, 2011 Predecessor Ambac [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Adverse Samples [Member] Transactions | Dec. 31, 2012 Predecessor Ambac [Member] Random Samples [Member] Transactions
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Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |  |  |  |  |  |  |  |
Count | 48 |  | 27 | 21 |  | 49 |  | 27 | 22
Gross loss reserve before subrogation recoveries | $ 3,331,107 |  | $ 2,235,026 | $ 1,096,081 |  | $ 3,563,344 |  | $ 2,331,878 | $ 1,231,466
Subrogation recoveries | (2,361,856) | (2,482,918) | (1,404,088) | (957,768) | (2,482,918) | (2,523,225) | (2,720,266) | (1,442,817) | (1,080,408)
Gross loss reserve after subrogation recoveries | $ 969,251 |  | $ 830,938 | $ 138,313 |  | $ 1,040,119 |  | $ 889,061 | $ 151,058

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- Definition Number of total remediation transactions. + References No definition available. + Details
Name: | ambc_NumberOfRemediationTransactions
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | ambc_ScheduleOfBalanceOfRmbsSubrogationRecoveriesAndRelatedClaimLiabilitiesByEstimationApproachLineItems
Namespace Prefix: | ambc_
Data Type: | xbrli:stringItemType
Balance Type: | na
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: | debit
Period Type: | instant

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- Definition This element represents the related gross claim liability net of subrogation recoveries for RMBS financial guarantee insurance contracts with credit deterioration that have estimated subrogation recoveries. + References No definition available. + Details
Name: | ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationRmbsGrossClaimLiabilitiesAfterSubrogationRecoveries
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This element represents the related gross claim liability for RMBS financial guarantee insurance contracts with credit deterioration that have estimated subrogation recoveries. + References No definition available. + Details
Name: | ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationRmbsGrossClaimLiabilitiesBeforeSubrogationRecoveries
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant