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Derivative Instruments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Derivative Instruments [Line Items]    
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets" $ 5,472 $ 47,421
Value of obligation to return cash collateral, recorded in "Other liabilities" 0 0
Number of outstanding credit derivative transactions include ratings based collateral-posting triggers None of our outstanding credit derivative transactions  
Aggregate fair value of all derivative instruments linked to Ambac's own credit risk in a net liability position 180,113 276,043
Fair value of posted assets as collateral 271,251 310,951
Call Option on Long-Term Debt [Member]
    
Derivative Instruments [Line Items]    
Gains in change in fair value of the call options 100,710 6,055
Amount of surplus notes that may be repurchased under stand-alone derivatives 500,000 500,000
Credit derivatives [Member]
    
Derivative Instruments [Line Items]    
Number of credit derivative contracts 4  
Net liability fair value of credit derivative contracts 67,219 46,056
Notional value of credit derivative contracts 291,690 290,007
CLO [Member]
    
Derivative Instruments [Line Items]    
Gross principal notional outstanding, credit derivatives not written on pay-as-you-go basis 88,540  
Number of credit derivatives transactions not written on a pay-as-you-go basis 4  
Net credit derivative liabilities not written on pay-as-you-go basis at fair value $ 130