Loans - Credit Card Portfolio - Loan Modifications (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) payment	Mar. 31, 2018 USD ($)	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	$ 249	$ 223
New enrollments, percent of total retained credit card loans (less than)	1.00%
Weighted-average interest rate of loans – before TDR	19.13%	17.25%
Weighted-average interest rate of loans – after TDR	5.03%	5.20%
Loans that redefaulted within one year of modification	$ 34	$ 26
Modified loans, payment default, number of payments past due | payment	2
Rate of default for modified loans, estimated weighted average	33.71%		33.38%