Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,770,546
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,754,944
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,770,546	$ 1,834,387
Changes recognized
Impact of sponsor actions	0	0
Other Changes Rmbs Subrogation	(15,602)	(58,139)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,754,944	$ 1,776,248