Noninterest Revenue and Noninterest Expense - Card Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Non interest Revenue [Line Items]
Credit cost amortization period		12 months
Direct loan origination costs amortization period		12 months
Total card income		$ 4,989	$ 4,433	$ 4,779
Interchange and merchant processing income
Schedule of Non interest Revenue [Line Items]
Total card income		18,808	17,080	15,367
Reward costs and partner payments
Schedule of Non interest Revenue [Line Items]
Total card income		(13,074)	(10,820)	(9,480)
Adjustment to credit card rewards liability	$ 330
Other card income
Schedule of Non interest Revenue [Line Items]
Total card income		$ (745)	$ (1,827)	$ (1,108)
Deferred revenues, recognition period		12 months
Minimum
Schedule of Non interest Revenue [Line Items]
Credit card revenue sharing agreement terms		5 years
Maximum
Schedule of Non interest Revenue [Line Items]
Credit card revenue sharing agreement terms		10 years