Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 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	1,741
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	(10)	30
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,741	$ 1,757