Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Collateral pledged under repurchase agreements) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase Agreements	$ 1,405	$ 1,404
Assets pledged under repurchase agreements	1,457	1,459
Securities lending collateral	1,142	1,046
Securities lending payable	1,143	$ 1,047
U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	239
Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	1,218
Repurchase Agreements [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	52
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	(1)
Overnight and Continuous [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	447
Overnight and Continuous [Member] | Corporate [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	11
Overnight and Continuous [Member] | U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	71
Maturity Less than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	563
Maturity Less than 30 Days [Member] | U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	234
Maturity Less than 30 Days [Member] | Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	329
Maturity Greater than 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	894
Maturity Greater than 90 Days [Member] | U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	5
Maturity Greater than 90 Days [Member] | Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	$ 889