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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified | 5 Months Ended | 4 Months Ended | 12 Months Ended | 5 Months Ended | 4 Months Ended | 12 Months Ended | 5 Months Ended | 4 Months Ended | 12 Months Ended
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Sep. 30, 2013 Successor Ambac [Member] Transactions | Apr. 30, 2013 Predecessor Ambac [Member] Transactions | Dec. 31, 2012 Predecessor Ambac [Member] Transactions | Sep. 30, 2013 Random Samples [Member] Successor Ambac [Member] Transactions | Apr. 30, 2013 Random Samples [Member] Predecessor Ambac [Member] Transactions | Dec. 31, 2012 Random Samples [Member] Predecessor Ambac [Member] Transactions | Sep. 30, 2013 Adverse Samples [Member] Successor Ambac [Member] Transactions | Apr. 30, 2013 Adverse Samples [Member] Predecessor Ambac [Member] Transactions | Dec. 31, 2012 Adverse Samples [Member] Predecessor Ambac [Member] Transactions
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items] |  |  |  |  |  |  |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | $ 2,482,918 | $ 2,523,225 | $ 2,720,266 | $ 1,004,252 | $ 1,080,408 | $ 1,262,794 | $ 1,478,666 | $ 1,442,817 | $ 1,457,472
Changes recognized |  |  |  |  |  |  |  |  |
Additional transactions reviewed | 2,426 |  | 18,981 | 2,426 |  | 54,467 |  |  | (35,486)
Additional adverse sample loans reviewed |  |  |  |  |  |  |  |  |
Impact of sponsor actions |  | (54,195) | 35 |  | (54,195) |  |  |  | 35
All other changes | (123,488) | 13,888 | (216,057) | (48,910) | (21,961) | (236,853) | (74,578) | 35,849 | 20,796
Discounted RMBS subrogation (gross of reinsurance), ending balance | $ 2,361,856 | $ 2,482,918 | $ 2,523,225 | $ 957,768 | $ 1,004,252 | $ 1,080,408 | $ 1,404,088 | $ 1,478,666 | $ 1,442,817
Discounted RMBS subrogation (gross of reinsurance), beginning balance |  | 49 |  | 20 | 22 | 16 | 29 | 27 | 30
Changes recognized: Additional transactions reviewed |  |  |  | 1 |  | 6 |  |  | (1)
Additional adverse sample loans reviewed |  |  |  |  |  |  |  |  |
Impact of sponsor actions |  |  |  |  |  |  |  |  |
All other changes |  |  |  |  | (2) |  | (2) | 2 | (2)
Discounted RMBS subrogation (gross of reinsurance), ending balance | 48 |  | 49 | 21 | 20 | 22 | 27 | 29 | 27

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