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Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance $ (1,749,808)   $ (1,806,736)  
Successor [Member]        
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Subrogation recoveries 1,776,248   1,834,387  
Successor [Member] | Random Samples [Member]        
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Gross loss reserve before subrogation recoveries 131,245   1,366,483  
Subrogation recoveries 1,776,248   1,834,387  
Gross loss reserve after subrogation recoveries (1,645,003)   (467,904)  
Random Samples [Member] | Successor [Member]        
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Subrogation recoveries 1,776,248 $ 1,844,116 $ 1,834,387 $ 1,907,035
Other Changes Rmbs Subrogation $ (58,139) $ (62,919)