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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 | Mar. 31, 2013 Transactions | Dec. 31, 2012 Transactions
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Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 49 | 49
Gross loss reserve before subrogation recoveries | $ 3,439,641 | $ 3,563,344
Subrogation recoveries | (2,487,369) | (2,523,225)
Gross loss reserve after subrogation recoveries | 952,272 | 1,040,119
Adverse samples [Member] |  |
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 29 | 27
Gross loss reserve before subrogation recoveries | 2,424,630 | 2,331,878
Subrogation recoveries | (1,483,726) | (1,442,817)
Gross loss reserve after subrogation recoveries | 940,904 | 889,061
Random samples [Member] |  |
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 20 | 22
Gross loss reserve before subrogation recoveries | 1,015,011 | 1,231,466
Subrogation recoveries | (1,003,643) | (1,080,408)
Gross loss reserve after subrogation recoveries | $ 11,368 | $ 151,058

X
- 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

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

X
- 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

X
- 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