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Derivative Instruments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 3 Months Ended 3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Call Option on Long-Term Debt [Member]
Mar. 31, 2013
Call Option on Long-Term Debt [Member]
Mar. 31, 2013
Credit derivatives [Member]
Contract
Dec. 31, 2012
Credit derivatives [Member]
Contract
Mar. 31, 2013
CLO [Member]
Derivative
Derivative Instruments [Line Items]              
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets" $ 5,197 $ 5,472          
Value of obligation to return cash collateral, recorded in "Other liabilities" 0 0          
Gross principal notional outstanding, credit derivatives not written on pay-as-you-go basis             57,154
Number of credit derivatives transactions not written on a pay-as-you-go basis             2
Net credit derivative liabilities not written on pay-as-you-go basis at fair value             76
Number of outstanding credit derivative transactions include ratings based collateral-posting triggers None of our outstanding credit derivative transactions            
Number of credit derivative contracts         4 4  
Net liability fair value of credit derivative contracts         60,791 67,219  
Notional value of credit derivative contracts         291,822 291,690  
Gains in change in fair value of the call options     61,680        
Amount of surplus notes that may be repurchased under stand-alone derivatives       500,000      
Aggregate fair value of all derivative instruments linked to Ambac's own credit risk in a net liability position 147,925 180,113          
Fair value of posted assets as collateral $ 233,425 $ 271,251