Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Litigation Settlement, Amount Awarded from Other Party			$ 995,000
Subrogation recoveries	$ 1,770,546	$ 1,834,387
Random Samples [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,770,546	1,834,387	1,907,035	$ 2,829,575
Impact Of Sponsor Actions In Residential Mortgage Backed Securities Subrogation	0	0	(995,000)
Other Changes Rmbs Subrogation	$ (63,841)	$ (72,648)	$ 72,460