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Investments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended 12 Months Ended 8 Months Ended 4 Months Ended 12 Months Ended 0 Months Ended
Sep. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Apr. 30, 2013
Dec. 31, 2012
Dec. 06, 2013
Investment [Line Items]            
Payment of permitted policy claim (percent) 25.00%ambc_PaymentAsPercentageOfPermittedPolicyClaim 25.00%ambc_PaymentAsPercentageOfPermittedPolicyClaim        
Future payment of permitted policy claim (percent) 45.00%ambc_CurrentPaymentasaPercentageofPermittedPolicyClaim 45.00%ambc_CurrentPaymentasaPercentageofPermittedPolicyClaim        
Securities fair value   $ 6,790ambc_FairValueOfSecuritiesDepositedWithGovernmentalAuthorities $ 6,799ambc_FairValueOfSecuritiesDepositedWithGovernmentalAuthorities      
Fair value of securities held by bankruptcy remote trust   0ambc_SecuritiesHeldToCollateralizeAndFundRepaymentOfDebtIssuedThroughResecuritizationTransactionFairValue 240,150ambc_SecuritiesHeldToCollateralizeAndFundRepaymentOfDebtIssuedThroughResecuritizationTransactionFairValue      
Successor [Member]            
Investment [Line Items]            
Unrealized losses on securities   20,081us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
116,634us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Other investments   8,108ambc_OtherInvestmentsIncome
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
3,580ambc_OtherInvestmentsIncome
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments   32,269us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
116,634us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Predecessor [Member]            
Investment [Line Items]            
Other investments       369ambc_OtherInvestmentsIncome
/ us-gaap_StatementScenarioAxis
= us-gaap_PredecessorMember
0ambc_OtherInvestmentsIncome
/ us-gaap_StatementScenarioAxis
= us-gaap_PredecessorMember
 
Below Investment Grade Securities and Non-Rated Securities [Member] | Successor [Member]            
Investment [Line Items]            
Fair value of below investment grade securities and non-rated securities   473,656ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
821,308ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Gross unrealized loss   16,001ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesContinuousUnrealizedLossPositionAggregateLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
36,900ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesContinuousUnrealizedLossPositionAggregateLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_BelowInvestmentGradeSecuritiesAndNonRatedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Ambac Wrapped Securities [Member] | Successor [Member]            
Investment [Line Items]            
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments   11,358us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_AmbacWrappedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
       
Gains (losses) on securities held as of reporting date [Member] | Successor [Member]            
Investment [Line Items]            
Other investments   7,200ambc_OtherInvestmentsIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_GainslossesonsecuritiesheldasofreportingdateMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,878ambc_OtherInvestmentsIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_GainslossesonsecuritiesheldasofreportingdateMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Gains (losses) on securities held as of reporting date [Member] | Predecessor [Member]            
Investment [Line Items]            
Other investments       369ambc_OtherInvestmentsIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_GainslossesonsecuritiesheldasofreportingdateMember
/ us-gaap_StatementScenarioAxis
= us-gaap_PredecessorMember
0ambc_OtherInvestmentsIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_GainslossesonsecuritiesheldasofreportingdateMember
/ us-gaap_StatementScenarioAxis
= us-gaap_PredecessorMember
 
National Century Financial Enterprises, Inc. [Member]            
Investment [Line Items]            
Cash recoveries   35us-gaap_LitigationSettlementAmount
/ dei_LegalEntityAxis
= ambc_NationalCenturyFinancialEnterprisesIncMember
1,441us-gaap_LitigationSettlementAmount
/ dei_LegalEntityAxis
= ambc_NationalCenturyFinancialEnterprisesIncMember
39,978us-gaap_LitigationSettlementAmount
/ dei_LegalEntityAxis
= ambc_NationalCenturyFinancialEnterprisesIncMember
  0us-gaap_LitigationSettlementAmount
/ dei_LegalEntityAxis
= ambc_NationalCenturyFinancialEnterprisesIncMember
Fixed Income Securities [Member] | Successor [Member]            
Investment [Line Items]            
Unrealized losses on securities   19,966us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
116,634us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] | Successor [Member]            
Investment [Line Items]            
Unrealized losses on securities   12,391us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
18,105us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
     
Fixed Income Securities [Member] | Asset-backed Securities [Member] | Successor [Member]            
Investment [Line Items]            
Unrealized losses on securities   $ 155us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
$ 36,598us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember