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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (Successor [Member], USD $) In Thousands, unless otherwise specified | 4 Months Ended | 8 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 30, 2013 | Dec. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2012
Subrogation By Estimation Approach [Roll Forward] |  |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | $ 2,523,225 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | $ 2,482,918 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | $ 2,206,598 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | $ 2,720,266 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation
Changes recognized |  |  |  |
Additional Transactions Reviewed Changes in RMBS Subrogation | 0 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation | 2,451 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation | 24,565 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation | 18,981 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation
Changes in Estimation Approach In Rmbs Subrogation | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation | 198,875 ambc_ChangesinEstimationApproachInRmbsSubrogation | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation
Impact of sponsor actions | (54,195)ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation | 98 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation | (146,270)ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation
Other Changes Rmbs Subrogation | 13,888 ambc_OtherChangesRmbsSubrogation | (278,869)ambc_OtherChangesRmbsSubrogation | 239,772 ambc_OtherChangesRmbsSubrogation | (216,057)ambc_OtherChangesRmbsSubrogation
Discounted RMBS subrogation (gross of reinsurance), ending balance | 2,482,918 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | 2,206,598 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | 2,523,540 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation | 2,523,225 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation
Random Samples [Member] |  |  |  |
Subrogation By Estimation Approach [Roll Forward] |  |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | 1,080,408 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,004,252 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 953,825 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,262,794 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Changes recognized |  |  |  |
Additional Transactions Reviewed Changes in RMBS Subrogation | 0 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 2,451 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 24,565 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 54,467 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Changes in Estimation Approach In Rmbs Subrogation | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,417,556 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Impact of sponsor actions | (54,195)ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (146,270)ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Other Changes Rmbs Subrogation | (21,961)ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (52,878)ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 273,864 ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (236,853)ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Discounted RMBS subrogation (gross of reinsurance), ending balance | 1,004,252 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 953,825 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 2,523,540 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,080,408 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_RandomSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Adverse Samples [Member] |  |  |  |
Subrogation By Estimation Approach [Roll Forward] |  |  |  |
Discounted RMBS subrogation (gross of reinsurance), beginning balance | 1,442,817 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,478,666 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,252,773 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 1,457,472 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Changes recognized |  |  |  |
Additional Transactions Reviewed Changes in RMBS Subrogation | 0 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (35,486)ambc_AdditionalTransactionsReviewedChangesinRMBSSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Changes in Estimation Approach In Rmbs Subrogation | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (1,218,681)ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ChangesinEstimationApproachInRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Impact of sponsor actions | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 98 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 ambc_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Other Changes Rmbs Subrogation | 35,849 ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (225,991)ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | (34,092)ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 20,796 ambc_OtherChangesRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Discounted RMBS subrogation (gross of reinsurance), ending balance | $ 1,478,666 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | $ 1,252,773 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | $ 0 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | $ 1,442,817 ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation / us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis = ambc_AdverseSamplesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember

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
- 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

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_ImpactOfSponsorActionsInResidentialMortgageBackedSecuritiesSubrogation
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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 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

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

X
- Details
Name: | us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis=ambc_AdverseSamplesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis=ambc_RandomSamplesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |