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Schedule I - Summary Of Investments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Apr. 30, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Other investments $ 336,013us-gaap_OtherInvestments $ 240,969us-gaap_OtherInvestments  
Successor [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Summary of Investments, Other than Investments in Related Parties, Cost 5,296,341us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Available-for-sale Securities, Amortized Cost Basis 4,939,325us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
6,324,568us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 5,150,018us-gaap_AvailableForSaleSecurities
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
6,282,658us-gaap_AvailableForSaleSecurities
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Other Investments 357,016us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
241,069us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Other investments 357,016us-gaap_OtherInvestments
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Investments 5,507,034us-gaap_Investments
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
6,523,727us-gaap_Investments
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
6,457,264us-gaap_Investments
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
Successor [Member] | U S Government Obligations [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities 107,246us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_USGovernmentObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Successor [Member] | U.S. Government Obligations [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Summary of Investments, Other than Investments in Related Parties, Cost 106,706us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ambc_U.S.GovernmentObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Successor [Member] | Residential Mortgage-Backed Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 1,557,059us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Available-for-sale Securities 1,710,955us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Successor [Member] | Collateralized Debt Obligations [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 21,346us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Available-for-sale Securities 21,122us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Successor [Member] | Asset-backed Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 833,366us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Available-for-sale Securities 882,001us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
   
Successor [Member] | Corporate Debt Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities 2,743,050us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
2,475,182us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Successor [Member] | Short-term Investments [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 360,069us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
271,118us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 360,065us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
271,119us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 4,514,878us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
5,927,254us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 4,725,686us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
5,885,316us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | U.S. Agency Obligations [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 29,524us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
32,214us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 29,486us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
32,154us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Municipal Bonds [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 523,019us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,405,293us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 525,792us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,377,723us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Residential Mortgage-Backed Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 1,557,059us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,516,877us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 1,710,955us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,558,625us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Collateralized Debt Obligations [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 21,346us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
184,118us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 21,122us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
183,872us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Asset-backed Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 833,366us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,020,251us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 882,001us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
992,448us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Corporate Debt Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 1,382,195us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,508,377us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities 1,385,594us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
1,489,369us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Fixed Income Securities [Member] | Successor [Member] | Foreign Government Debt Securities [Member]      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 126,041us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
131,709us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
 
Available-for-sale Securities $ 127,757us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember
$ 124,877us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_SuccessorMember