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Schedule II - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Mar. 15, 2012
Amount shall have been paid or paid into escrow by the company     $ 30,000
Reduction in NOLs rate (1.10%) 0.20%  
Beneficial ownership of company common stock held by external investor shares 13,500,000    
Percentage of stock value of reorganized company 4.50%    
Ownership of senior debts 56,930    
Discontinued Liabilities subject to compromise 211,896    
Liabilities subject to compromise contractual interest on debt 85,371 110,093  
5.1% junior surplus notes, segregated account, due 2020 [Member]
      
Segregated Account issue of junior surplus notes 350,000    
Internal Revenue Service (IRS) [Member]
      
Minimum payment to IRS 100,000    
Reduction in NOLs rate 10.00%    
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member]
      
Amount shall have been paid or paid into escrow by the company 30,000    
Shareholders ownership interest 5.00%    
Increase in ownership percentage, by value 50.00%    
Increase in ownership percentage, rolling period beginning with the last Ownership Change Three-year    
Beneficial ownership of company common stock held by external investor shares 13,500,000    
Percentage of stock value of reorganized company 4.50%    
Ownership of senior debts 56,930    
Discontinued Liabilities subject to compromise 211,896 126,524  
Liabilities subject to compromise contractual interest on debt 85,372 110,093  
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member] | 5.1% junior surplus notes, segregated account, due 2020 [Member]
      
Segregated Account issue of junior surplus notes 350,000    
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member] | Internal Revenue Service (IRS) [Member]
      
Minimum payment to IRS 100,000    
Reduction in NOLs rate 10.00%    
First five years [Member] | AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member]
      
Reimbursement amount 5,000    
All later years [Member] | AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member]
      
Reimbursement amount $ 4,000