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Background and Business Description - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Jun. 30, 2018
Feb. 12, 2018
May 01, 2013
Apr. 30, 2013
Background And Basis Of Presentation [Line Items]                
Entity Incorporation, Date of Incorporation Apr. 29, 1991              
Minimum surplus amount $ 100,000              
Gain Contingency, Unrecorded Amount         $ 1,400,000      
Amount of Senior Surplus Notes Outstanding on June 8, 2020   25.00%            
Tier 2 Notes           $ 240,000    
Tier 2 Note Security in RMBS Litigation Settlement Proceeds           $ 1,600,000    
Accrued rate of interest on outstanding policy obligations (percent)   5.10%            
Common stock, shares authorized 130,000,000 130,000,000            
Common Stock, Par or Stated Value Per Share $ 0.01 $ 0.01           $ 0.01
Preferred stock, shares authorized 20,000,000 20,000,000            
Preferred Stock, Par or Stated Value Per Share $ 0.01 $ 0.01            
Class of Warrant or Right, Exercise Price of Warrants or Rights             $ 16.67  
Junior surplus note of Ambac Assurance Segregated Account             $ 350,000  
Common Stock Voting Restrictions       10.00%        
Common Stock Voting Restriction Less One Vote       10.00%        
Common Stock Transfer Restrictions       5.00%        
Common Stock Additional Transfer Restrictions       5.00%        
Litigation Settlement, Amount Awarded from Other Party     $ 995,000          
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount $ 32,482 $ 64,494 $ 52,416          
Ambac UK [Member]                
Background And Basis Of Presentation [Line Items]                
Total Benefit Related to Ballantyne Settlement   $ 144,600