Loans - Credit Card Portfolio - Loan Modifications (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) payment	Jun. 30, 2018 USD ($)	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Number of years before payment default under a modified loan			1 year
Credit card
Financing Receivable, Impaired [Line Items]
Fixed payment plan period			60 months
New TDRs	$ 226	$ 202	$ 475	$ 425
New enrollments, percent of total retained credit card loans (less than)			1.00%
Weighted-average interest rate of loans – before TDR	19.38%	18.00%	19.25%	17.61%
Weighted-average interest rate of loans – after TDR	4.71%	5.06%	4.88%	5.13%
Loans that redefaulted within one year of modification	$ 32	$ 25	$ 66	$ 51
Modified loans, payment default, number of payments past due | payment			2
Rate of default for modified loans, estimated weighted average			34.04%		33.38%