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Commitments and Contingencies - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Jun. 30, 2012
Jun. 04, 2012
Jun. 14, 2011
May 31, 2011
Dec. 31, 2010
Contingencies And Commitments [Line Items]              
Rent expense $ 5,252,000 $ 6,980,000          
Total par value of repurchased surplus notes     789,179,000 789,179,000      
Principal amount of surplus notes       188,446,000      
IRS claim in the Bankruptcy Court         807,244,000 807,244,000  
Loss carry-forwards 3,400,000           3,400,000,000
Payment by subsidiary associated with NOL Usage Tier C 12.50%            
Payment by subsidiary associated with NOL Usage Tier D 17.50%            
Ambac Assurance's payment under a swap termination surety bond 4,524,000            
Ambac [Member]
              
Contingencies And Commitments [Line Items]              
Payment to settle the IRS dispute and related proceedings 1,900,000            
Bonds redeemed 2,700,000            
Ambac Assurance [Member]
              
Contingencies And Commitments [Line Items]              
Payment to settle the IRS dispute and related proceedings 100,000,000            
Ambac Assurance's payment under a swap termination surety bond $ 800,000,000            
IRS [Member]
              
Contingencies And Commitments [Line Items]              
Payment by subsidiary associated with NOL Usage Tier C 12.50%            
Payment by subsidiary associated with NOL Usage Tier D 17.50%