Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 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	1,757
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	30	(16)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,757	$ 1,755