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Background and Business Description - Additional Information (Detail)
 9 Months Ended 12 Months Ended
Sep. 30, 2015
USD ($)
Segment
$ / shares
shares
Sep. 30, 2014
USD ($)
Dec. 31, 2014
$ / shares
shares
Apr. 30, 2013
$ / shares
shares
Background And Basis Of Presentation [Line Items]        
Entity Incorporation, Date of Incorporation Apr. 29, 1991      
Number of reportable segments | Segment 2      
Investment Warrants Expiration Date     Apr. 30, 2023  
Payment of permitted policy claim (percent)     25.00%  
Common stock, shares authorized 130,000,000   130,000,000  
Common Stock, Par or Stated Value Per Share | $ / shares $ 0.01   $ 0.01 $ 0.01
Preferred stock, shares authorized 20,000,000   20,000,000  
Preferred Stock, Par or Stated Value Per Share | $ / shares $ 0.01   $ 0.01  
Common stock, shares issued 45,036,587   45,005,932  
Warrants Distributed In Bankruptcy       5,047,138
Warrants issued exercise price | $ / shares       $ 16.67
Ambac Assurance [Member]        
Background And Basis Of Presentation [Line Items]        
Net par exposure for policies allocated to the Segregated Account | $ $ 16,595,339,000      
Minimum surplus amount | $ 100,000,000      
Statutory Accounting Practices, Statutory Capital and Surplus, Balance | $ 350,559,000      
Successor [Member]        
Background And Basis Of Presentation [Line Items]        
Proceeds from the sale of Junior Surplus Notes of the Segregated Account | $ $ 0 $ 224,262,000    
Common stock, shares issued       45,000,000
Warrants Distributed In Bankruptcy       5,047,138