Loans - Credit Card Portfolio - Loan Modifications (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) payment	Sep. 30, 2017 USD ($)	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	$ 215	$ 191	$ 640	$ 552
Weighted-average interest rate of loans – before TDR	18.25%	16.84%	17.82%	16.52%
Weighted-average interest rate of loans – after TDR	5.10%	4.95%	5.12%	4.84%
Loans that redefaulted within one year of modification	$ 31	$ 27	$ 82	$ 72
Modified loans, payment default, number of payments past due | payment			2
Rate of default for modified loans, estimated weighted average			32.78%		31.54%
New enrollments, percent of total retained credit card loans (less than)			1.00%