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Fair Value Measurements - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Credit valuation adjustment included in fair value of credit derivative liabilities $ 18,020us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities $ 15,910us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
Credit valuation adjustment included in fair value of derivative liabilities other than credit derivatives 77,191ambc_DerivativesCreditRiskValuationAdjustmentDerivativeLiabilitiesOtherThanCreditDerivatives 64,547ambc_DerivativesCreditRiskValuationAdjustmentDerivativeLiabilitiesOtherThanCreditDerivatives
Percentage of CDS gross par outstanding used in the determination of CDS fair values 86.00%ambc_PercentageOfCreditDerivativesGrossParOutstandingUsingThirdPartyReferenceObligationQuotesInDeterminationOfFairValue  
Percentage of CDS derivative liability used in the determination of CDS fair values 65.00%ambc_PercentageOfCreditDerivativesLiabilityUsingThirdPartyReferenceObligationQuotesInDeterminationOfFairValue  
Percentage of CDS gross par outstanding using internally estimated reference obligation prices in determination of fair value 14.00%ambc_PercentageOfCreditDerivativeGrossParOutstandingUsingInternallyEstimatedReferenceObligationPricesInDeterminationOfFairValue  
Percentage of CDS liability using internally estimated reference obligation prices in determination of fair value 35.00%ambc_PercentageOfCreditDerivativeLiabilityUsingInternallyEstimatedReferenceObligationPricesInDeterminationOfFairValue  
Additional basis point fee need to receive for issuing a CDS on obligation 0.05%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesHypotheticalIncrementalFee  
Additional basis points fee increase on change of reference obligation spread 0.15%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesHypotheticalIncrementalFeeWithCreditDeterioration  
Revised relative change ratio of CDS fee to cash market spread 73.20%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesRevisedRelativeChangeRatio  
Estimated remaining life 8 years  
Notional outstanding 1,594,571invest_DerivativeNotionalAmount 1,529,759invest_DerivativeNotionalAmount
Derivative liabilities 76,377us-gaap_DerivativeFairValueOfDerivativeLiability 73,459us-gaap_DerivativeFairValueOfDerivativeLiability
Percentage of profit margin 17.00%us-gaap_FairValueInputsLongTermNetOfTaxOperatingIncomeMarginPercent 17.00%us-gaap_FairValueInputsLongTermNetOfTaxOperatingIncomeMarginPercent
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs 2.50%ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate 2.70%ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
Credit Derivatives [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
CVA percentage 19.10%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_CreditDerivativesByContractTypeAxis
= ambc_CreditDerivativesMember
17.80%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_CreditDerivativesByContractTypeAxis
= ambc_CreditDerivativesMember
European ABS Transactions [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value of consolidated VIE debt obligations valued using unobservable inputs by asset class 1,841,412ambc_FairValueOfConsolidatedVariableInterestEntityDebtObligationsValuedUsingUnobservableInputsByAssetClass
/ us-gaap_FairValueByAssetClassAxis
= ambc_EuropeanTransactionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
1,070,439ambc_FairValueOfConsolidatedVariableInterestEntityDebtObligationsValuedUsingUnobservableInputsByAssetClass
/ us-gaap_FairValueByAssetClassAxis
= ambc_EuropeanTransactionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
US Commercial ABS Transaction [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair value of consolidated VIE debt obligations valued using unobservable inputs by asset class 190,687ambc_FairValueOfConsolidatedVariableInterestEntityDebtObligationsValuedUsingUnobservableInputsByAssetClass
/ us-gaap_FairValueByAssetClassAxis
= ambc_UnitedStatesTransactionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
193,225ambc_FairValueOfConsolidatedVariableInterestEntityDebtObligationsValuedUsingUnobservableInputsByAssetClass
/ us-gaap_FairValueByAssetClassAxis
= ambc_UnitedStatesTransactionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Minimum [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Basis point fee received for issuing a CDS on reference obligation base fee 0.20%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesBaseFee
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Credit spread of reference obligation of a base transaction 0.80%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesBaseReferenceObligationSpread
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Relative change ratio of CDS fee to cash market spread 25.00%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesBaseRelativeChangeRatio
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Maximum [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Basis point fee received for issuing a CDS on reference obligation hypothetical fee 0.25%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesRevisedHypotheticalFee
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
Relative change ratio of CDS fee to cash market spread 35.00%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesRevisedHypotheticalFeeWithCreditDownGrade
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
Credit spread of reference obligation of revised transaction 1.00%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesRevisedHypotheticalReferenceObligationSpreadTwo
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
Internal Credit Rating [Member] | Minimum [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Revised relative change ratio of CDS fee to cash market spread 33.00%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesRevisedRelativeChangeRatio
/ invest_CreditRatingSourceAxis
= us-gaap_InternalCreditAssessmentDomain
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Internal Credit Rating [Member] | Maximum [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Full relative change ratio of CDS fee to cash market spread 100.00%ambc_FairValueMeasurementProcessDescriptionForCreditDerivativesFullRelativeChangeRatio
/ invest_CreditRatingSourceAxis
= us-gaap_InternalCreditAssessmentDomain
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
Successor [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 5,151,543us-gaap_AvailableForSaleSecurities
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
5,150,018us-gaap_AvailableForSaleSecurities
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | Fixed Income Securities [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Percentage of investment portfolio valued using dealer quotes 6.00%ambc_PercentageOfInvestmentPortfolioValuedUsingDealerQuotes
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
10.00%ambc_PercentageOfInvestmentPortfolioValuedUsingDealerQuotes
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Percentage of investment portfolio valued using external pricing services 89.00%ambc_PercentageOfInvestmentPortfolioValuedUsingExternalPricingServices
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
86.00%ambc_PercentageOfInvestmentPortfolioValuedUsingExternalPricingServices
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Percentage of investment portfolio valued using internal valuation models 5.00%ambc_PercentageOfInvestmentPortfolioValuedUsingInternalValuationModels
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
4.00%ambc_PercentageOfInvestmentPortfolioValuedUsingInternalValuationModels
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | U.S. Agency Obligations [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | Asset-backed Securities [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 262,732us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
194,393us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | Corporate Obligations [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Coupon rate 0.345%ambc_FairValueInputsCouponRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fair Value Inputs Maturity 19 years 1 month 21 days  
Fair Value Inputs Yield 4.93%ambc_FairValueInputsYield
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Successor [Member] | Corporate Obligations [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
3,808us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | Collateralized Debt Obligations [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Variable Interest Entities [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs 3.50%ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
/ dei_LegalEntityAxis
= ambc_VariableInterestEntityMember
4.10%ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
/ dei_LegalEntityAxis
= ambc_VariableInterestEntityMember
Other Contracts [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Fair Value Input Weighted Average Credit Derivative Quote As Percentage Of Notional 1.00%ambc_FairValueInputWeightedAverageCreditDerivativeQuoteAsPercentageOfNotional
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
1.00%ambc_FairValueInputWeightedAverageCreditDerivativeQuoteAsPercentageOfNotional
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
Fair Value Input Weighted Average Credit Rating A A
CVA percentage 2.02%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
2.21%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
Notional outstanding 58,801invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
58,800invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
Fair Value Input Weighted Average Life 0 years 11 months 1 year 2 months 12 days
Variable Interest Entity, Primary Beneficiary [Member] | Successor [Member] | Corporate Obligations [Member] | Level 3 [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Available-for-sale Securities $ 2,704,657us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
$ 2,743,050us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity, Primary Beneficiary [Member] | Successor [Member] | European ABS Transactions [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Coupon rate 0.57%ambc_FairValueInputsCouponRate
/ us-gaap_FairValueByAssetClassAxis
= ambc_EuropeanTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0.30%ambc_FairValueInputsCouponRate
/ us-gaap_FairValueByAssetClassAxis
= ambc_EuropeanTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Fair Value Inputs Maturity 17 years 11 months 21 years 2 months 30 days
Fair Value Inputs Yield 7.30%ambc_FairValueInputsYield
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
9.52%ambc_FairValueInputsYield
/ us-gaap_FairValueByAssetClassAxis
= ambc_EuropeanTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity, Primary Beneficiary [Member] | Successor [Member] | US Commercial ABS Transaction [Member]    
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]    
Coupon rate 5.88%ambc_FairValueInputsCouponRate
/ us-gaap_FairValueByAssetClassAxis
= ambc_UnitedStatesTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
5.88%ambc_FairValueInputsCouponRate
/ us-gaap_FairValueByAssetClassAxis
= ambc_UnitedStatesTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Fair Value Inputs Maturity 15 years 8 months 20 days 15 years 10 months 4 days
Fair Value Inputs Yield 7.50%ambc_FairValueInputsYield
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
7.37%ambc_FairValueInputsYield
/ us-gaap_FairValueByAssetClassAxis
= ambc_UnitedStatesTransactionsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
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