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Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Transactions accounted for as secured borrowings) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral $ 994 $ 1,926
Collateral held under securities lending agreements 994 1,926
Maturity Overnight [Member] | Cash    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 346 756
Maturity Overnight [Member] | U.S. Treasury and agency    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 6 64
Maturity Overnight [Member] | Foreign [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 595 795
Maturity Overnight [Member] | Corporate securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 5 15
Maturity Overnight [Member] | Mortgage-backed securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 18 45
Maturity Overnight [Member] | Equity securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral $ 24 $ 251