Loans - Credit Card Portfolio - Loan Modifications (Details) - Credit card $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) payment	Sep. 30, 2018 USD ($)	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Fixed payment plan period			60 months
New TDRs	$ 242	$ 215	$ 717	$ 640
New enrollments, percent of total retained credit card loans (less than)			1.00%
Weighted-average interest rate of loans – before TDR	19.18%	18.25%	19.23%	17.82%
Weighted-average interest rate of loans – after TDR	4.65%	5.10%	4.80%	5.12%
Loans that redefaulted within one year of modification	$ 42	$ 31	$ 108	$ 82
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			34.30%		33.38%