Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 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	1,762,217
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	(8,329)	(18,002)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,762,217	$ 1,816,385