Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,751
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,748
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,751	$ 1,727
Changes recognized
Other Changes Rmbs Subrogation	(3)	36
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,748	$ 1,764