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Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Transactions accounted for as secured borrowings) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral $ 1,757 $ 1,092
Securities lending payable 1,757 1,093
Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Secured Borrowings, Gross, Difference, Amount 0 (1) [1]
Cash and Cash Equivalents [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 1,109 423
U.S. Treasury And Agency [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 38 54
Foreign Government Debt [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 491 578
Corporate [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 0 37
Collateralized Mortgage Backed Securities [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral 46 0
Equity Securities [Member] | Overnight and Continuous [Member]    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Securities lending collateral $ 73 $ 0
[1] (1) The carrying value of the securities lending collateral held is $1 million lower than the securities lending payable at December 31, 2016 due to accrued interest recorded in the securities lending payable.