Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Collateral pledged under repurchase agreements) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	$ 1,472	$ 1,461
Repurchase agreements	1,408	1,403
Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0	1
U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	246	240
Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	1,226	1,220
Repurchase Agreements [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	64	58
Maturity 30 to 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	501
Maturity 30 to 90 Days [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0
Maturity 30 to 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	6
Maturity 30 to 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	495
Maturity Less than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements		569
Maturity Less than 30 Days [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements		0
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		230
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		339
Maturity Greater than 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	971	892
Maturity Greater than 90 Days [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0	1
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	240	10
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	$ 731	$ 881