Securitizations and Variable Interest Entities (Schedule of Quantitative Information and Net Credit Losses about Securitized and Other Financial Assets Managed Together) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Loans managed and securitized, principal amount outstanding	$ 2,249		$ 2,249		$ 3,363
Loans managed and securitized, delinquent amount at end of period	15		15		20
Loans managed and securitized, net credit losses	3	$ 4	9	$ 12
Off-balance sheet loans | Consumer portfolio segment | Automobile loan
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Loans managed and securitized, principal amount outstanding	1,462		1,462		1,964
Loans managed and securitized, delinquent amount at end of period	12		12		16
Loans managed and securitized, net credit losses	2	3	7	9
Whole-loan sales
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Loans managed and securitized, principal amount outstanding	787		787		1,399
Loans managed and securitized, delinquent amount at end of period	3		3		$ 4
Loans managed and securitized, net credit losses	$ 1	$ 1	$ 2	$ 3