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, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance	$ (1,728,879)		$ (1,744,243)
Subrogation recoveries	1,754,944		1,770,546
Random Samples [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,754,944	$ 1,776,248	$ 1,770,546	$ 1,834,387
Other Changes Rmbs Subrogation	$ (15,602)	$ (58,139)