Loans - Net Gains and Losses on Sale (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Line Items]
Total net gains/(losses) on sales of loans (including lower of cost or fair value adjustments)	$ (93)	$ 245	$ 340
Consumer, excluding credit card
Loans and Leases Receivable Disclosure [Line Items]
Total net gains/(losses) on sales of loans (including lower of cost or fair value adjustments)	(126)	231	305
Credit card
Loans and Leases Receivable Disclosure [Line Items]
Total net gains/(losses) on sales of loans (including lower of cost or fair value adjustments)	(8)	(12)	1
Wholesale
Loans and Leases Receivable Disclosure [Line Items]
Total net gains/(losses) on sales of loans (including lower of cost or fair value adjustments)	$ 41	$ 26	$ 34