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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Subrogation By Estimation Approach [Roll Forward]      
Discounted RMBS subrogation (gross of reinsurance), beginning balance $ 1,834,387    
Changes recognized      
Discounted RMBS subrogation (gross of reinsurance), ending balance 1,770,546 $ 1,834,387  
Random Samples [Member]      
Subrogation By Estimation Approach [Roll Forward]      
Discounted RMBS subrogation (gross of reinsurance), beginning balance 1,834,387 1,907,035 $ 2,829,575
Changes recognized      
Impact of sponsor actions 0 0 (995,000)
Other Changes Rmbs Subrogation (63,841) (72,648) 72,460
Discounted RMBS subrogation (gross of reinsurance), ending balance $ 1,770,546 $ 1,834,387 $ 1,907,035