Loans - Credit Card Portfolio - Loan Modifications (Details) - Credit card $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) payment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financing Receivable, Impaired [Line Items]
Fixed payment plan period	60 months
New TDRs	$ 866	$ 756	$ 636
New enrollments, percent of total retained credit card loans (less than)	1.00%
Weighted-average interest rate of loans – before TDR	17.98%	16.58%	15.56%
Weighted-average interest rate of loans – after TDR	5.16%	4.88%	4.76%
Loans that redefaulted within one year of modification	$ 116	$ 93	$ 74
Number of years before payment default under a modified loan	1 year
Modified loans, payment default, number of payments past due | payment	2
Rate of default for modified loans, estimated weighted-average	33.38%	31.54%	28.87%