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Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance $ (1,715)   $ (1,725)  
Subrogation recoveries 1,741   1,751  
Random Samples [Member]        
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]        
Subrogation recoveries 1,741 $ 1,757 $ 1,751 $ 1,727
Other Changes Rmbs Subrogation $ (10) $ 30