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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 | 9 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 4 Months Ended
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Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2014 Successor Ambac [Member] | Apr. 30, 2013 Predecessor Ambac [Member] | Sep. 30, 2013 Random Samples [Member] Successor Ambac [Member] | Sep. 30, 2014 Random Samples [Member] Successor Ambac [Member] | Apr. 30, 2013 Random Samples [Member] Predecessor Ambac [Member] | Sep. 30, 2013 Adverse Samples [Member] Successor Ambac [Member] | Sep. 30, 2014 Adverse Samples [Member] Successor Ambac [Member] | Apr. 30, 2013 Adverse Samples [Member] Predecessor Ambac [Member]
Subrogation By Estimation Approach [Roll Forward] |  |  |  |  |  |  |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | $ 2,482,918 | $ 2,206,598 | $ 2,523,225 | $ 1,004,252 | $ 953,825 | $ 1,080,408 | $ 1,478,666 | $ 1,252,773 | $ 1,442,817
Changes recognized |  |  |  |  |  |  |  |  |
Additional Transactions Reviewed Changes in RMBS Subrogation | 2,426 | 24,565 | 0 | 2,426 | 24,565 |  |  |  |
Changes in Estimation Approach In Rmbs Subrogation | 0 | 53,851 | 0 | 0 | 1,272,532 | 0 | 0 | (1,218,681) | 0
Impact of sponsor actions | 0 | (90,000) | (54,195) | 0 | (90,000) | (54,195) | 0 | 0 | 0
All other changes | (123,488) | 19,416 | 13,888 | (48,910) | 53,508 | (21,961) | (74,578) | (34,092) | 35,849
Discounted RMBS subrogation (gross of reinsurance), ending balance | $ 2,361,856 | $ 2,214,430 | $ 2,482,918 | $ 957,768 | $ 2,214,430 | $ 1,004,252 | $ 1,404,088 | $ 0 | $ 1,478,666

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- 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

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- Definition This element represents the change in subrogation due to changes in estimation approach. + References No definition available. + Details
Name: | ambc_ChangesinEstimationApproachInRmbsSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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

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