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Employment Benefit Plans - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Additions [Member]
Dec. 31, 2012
RSU and other stock [Member]
Dec. 31, 2011
RSU and other stock [Member]
Dec. 31, 2012
Unvested RSUs granted [Member]
Dec. 31, 2020
Subsequent Event [Member]
Postretirement And Other Employee Benefits [Line Items]              
Postretirement and postemployment benefits expense $ 2,099 $ 4,037          
Unfunded accumulated postretirement benefit obligation 17,485            
Assumed health care cost trend rates range 8.50%           5.00%
One percentage point increase on accumulated postretirement benefit obligation 3,476            
One percentage point decrease on accumulated postretirement benefit obligation 2,713            
One percentage point increase on accumulated postretirement benefit expense 454            
One percentage point decrease on accumulated postretirement benefit expense 345            
Projected plan benefit obligations 4.00% 4.50%          
Additional contribution by employer 100.00%   50.00%        
Additional to base compensation 3.00%   2.00%        
Employee Savings Incentive Plan cost 1,058 610          
Fair market value of Ambac's Common Stock on grant date 7 years            
Total unrecognized compensation costs 0         0  
Gross stock option expense 0 (12,576)          
Net income effect from stock options 0 (11,297)          
Vesting period       4 years      
Shares outstanding, Restricted stock units 104,389            
Shares outstanding, Equity instruments other than options nonvested 70,530            
Shares outstanding, Equity instruments restricted stock units vested 33,859            
Gross restricted stock units expense       0 (2,988)    
Restricted stock units expense, net of tax       0 (1,163)    
Fair value for RSUs vested $ 0 $ 93