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Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Dec. 31, 2014
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]      
Notional outstanding   $ 2,776,103invest_DerivativeNotionalAmount $ 1,529,759invest_DerivativeNotionalAmount
Other Contracts [Member]      
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]      
Notional outstanding 58,800invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
146,995invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
 
Weighted average life (WAL) in years 1 year 2 months 10 days 1 year  
Weighted average quotes 1.00%ambc_FairValueInputWeightedAverageCreditDerivativeQuoteAsPercentageOfNotional
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
1.00%ambc_FairValueInputWeightedAverageCreditDerivativeQuoteAsPercentageOfNotional
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
 
CVA percentage 2.21%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
8.70%ambc_PercentageOfCreditValueAdjustment
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
 
Weighted average credit rating A A+  
Fair value of derivative liabilities   $ 1,549us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember
$ 328us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherContractMember