Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
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	1,764
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,727	$ 1,771
Changes recognized
Other Changes Rmbs Subrogation	36	(43)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,764	$ 1,727