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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 Transactions
Rollforward: |
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | $ 2,523,225
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | 49
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | 49
Changes recognized in 2012: |
Additional transactions reviewed |
Additional adverse sample loans reviewed |
Impact of sponsor actions | (54,195)
All other changes | 18,339
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | 2,487,369
Random samples [Member] |
Rollforward: |
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | 1,080,408
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | 22
Changes recognized in 2013: Additional transactions reviewed | 0
Additional adverse sample loans reviewed | 0
Impact of sponsor actions | (2)
All other changes | 0
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | 20
Changes recognized in 2012: |
Additional transactions reviewed |
Additional adverse sample loans reviewed |
Impact of sponsor actions | (54,195)
All other changes | (22,570)
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | 1,003,643
Adverse samples [Member] |
Rollforward: |
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | 1,442,817
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2012 | 27
Changes recognized in 2013: Additional transactions reviewed | 0
Additional adverse sample loans reviewed | 0
Impact of sponsor actions | 0
All other changes | 2
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | 29
Changes recognized in 2012: |
Additional transactions reviewed |
Additional adverse sample loans reviewed |
All other changes | 40,909
Discounted RMBS subrogation (gross of reinsurance) at March 31, 2013 | $ 1,483,726

X
- Definition This element represents the change in RMBS subrogation due to additional adverse sample loans reviewed. + References No definition available. + Details
Name: | ambc_AdditionalAdverseSampleLoansReviewedChangeInRmbsSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Additional Adverse Sample Loans Reviewed Number Of Transactions + References No definition available. + Details
Name: | ambc_AdditionalAdverseSampleLoansReviewedNumberOfTransactions
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents the change in subrogation due to additional transactions reviewed. + References No definition available. + Details
Name: | ambc_AdditionalTransactionsReviewedChangesInRmbsSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

X
- 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_ImpactOfSponsorActions
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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

X
- Definition Number Of Additional Rmbs Transactions Reviewed With Potential Recoveries + References No definition available. + Details
Name: | ambc_NumberOfAdditionalRmbsTransactionsReviewedWithPotentialRecoveries
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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
- 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

X
- Definition Other Changes Rmbs Subrogation Number Of Transactions + References No definition available. + Details
Name: | ambc_OtherChangesRmbsSubrogationNumberOfTransactions
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Subrogation Rmbs + References No definition available. + Details
Name: | ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilitySubrogationRmbs
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant