Loans - Credit Card Portfolio - Loan Modifications (Details) - Credit card - Consumer Credit Card Financing Receivable $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) payment	Jun. 30, 2014 USD ($)	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Fixed payment plan period			60 months
Weighted-average interest rate of loans – before TDR	15.15%	15.05%	15.16%	15.04%
Weighted-average interest rate of loans – after TDR	4.27%	4.33%	4.28%	4.36%
Modifications, subsequent default, recorded investment	$ 20	$ 29	$ 42	$ 63
Number of years before payment default under a modified loan			1 year
Number of payments past due for deemed payment | payment			2
Rate of default for modified loans, estimated weighted average			26.45%		27.91%
Total new enrollments
Financing Receivable, Impaired [Line Items]
New TDRs	$ 151	$ 193	$ 329	$ 426