Loans - Credit Card Portfolio, Loan Modifications (Details) - Credit card $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) loan_payment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 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%	1.00%	1.00%
Balance of new TDRs	$ 818	$ 961	$ 866
Weighted-average interest rate of loans – before TDR	18.04%	19.07%	17.98%
Weighted-average interest rate of loans – after TDR	4.64%	4.70%	5.16%
Loans that redefaulted within one year of modification	$ 110	$ 148	$ 116
Number of years before payment default under a modified loan	1 year
Modified loans, payment default, number of payments past due | loan_payment	2