Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,727
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,751	$ 1,727
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,727	1,771	$ 1,834
Changes recognized
Other Changes Rmbs Subrogation	23	(43)	(64)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,751	$ 1,727	$ 1,771