Loans - Credit Card Portfolio, Loan Modifications (Details) - Credit card $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) loan_payment	Mar. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items]
Fixed payment plan period	60 months
New enrollments, percent of total retained credit card loans (less than)	1.00%
Balance of new TDRs	$ 277	$ 249
Weighted-average interest rate of loans – before TDR	18.82%	19.13%
Weighted-average interest rate of loans – after TDR	4.02%	5.03%
Balance of loans that redefaulted within one year of modification	$ 36	$ 34
Number of years before payment default under a modified loan	1 year
Modified loans, payment default, number of payments past due | loan_payment	2
Rate of default for modified loans, estimated weighted average	35.15%	32.89%