Loans - Credit Card Portfolio - Loan Modifications (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) payment	Jun. 30, 2017 USD ($)	Dec. 31, 2017
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	$ 202	$ 176	$ 425	$ 361
Weighted-average interest rate of loans – before TDR	18.00%	16.55%	17.61%	16.35%
Weighted-average interest rate of loans – after TDR	5.06%	4.80%	5.13%	4.78%
Loans that redefaulted within one year of modification	$ 25	$ 24	$ 51	$ 45
Modified loans, payment default, number of payments past due | payment			2
Rate of default for modified loans, estimated weighted average			32.10%		31.54%