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Financial Guarantees in Force - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Guarantor Obligations [Line Items]    
Gross par amount of financial guarantees outstanding 159,688,000ambc_FinancialGuaranteeGrossOutstandingPrincipal $ 197,691,000ambc_FinancialGuaranteeGrossOutstandingPrincipal
Net par amount of financial guarantees 144,734,000us-gaap_ParticipatingPoliciesAmountInForce 179,092,000us-gaap_ParticipatingPoliciesAmountInForce
Gross financial guarantees in force 253,383,000ambc_FinancialGuaranteeGrossOutstandingPrincipalAndInterest 312,710,000ambc_FinancialGuaranteeGrossOutstandingPrincipalAndInterest
Net financial guarantees in force 228,890,000ambc_NetFinancialGuaranteePrincipalAndInterestInForce $ 282,210,000ambc_NetFinancialGuaranteePrincipalAndInterestInForce
Highest single insured risk of aggregate net par amount guaranteed 1.40%ambc_PercentageOfHighestSingleInsuredRiskOfAggregateNetParAmountGuaranteed  
California [Member]    
Guarantor Obligations [Line Items]    
Aggregate net par amounts in force 14.30%ambc_PercentageOfAggregateGuaranteedInsuredRiskByNetPar
/ us-gaap_StatementGeographicalAxis
= stpr_CA		  
New York [Member]    
Guarantor Obligations [Line Items]    
Aggregate net par amounts in force 6.40%ambc_PercentageOfAggregateGuaranteedInsuredRiskByNetPar
/ us-gaap_StatementGeographicalAxis
= stpr_NY		  
No other state [Member] | Maximum [Member]    
Guarantor Obligations [Line Items]    
Aggregate net par amounts in force 5.00%ambc_PercentageOfAggregateGuaranteedInsuredRiskByNetPar
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= ambc_UnitedStatesNonCaliforniaAndNonNewYorkMember