Loans - Credit Card Portfolio - Loan Modifications (Details) - Credit card - Credit card loans $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) payment	Sep. 30, 2014 USD ($)	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Fixed payment plan period			60 months
Weighted-average interest rate of loans – before TDR	15.09%	14.96%	15.13%	15.01%
Weighted-average interest rate of loans – after TDR	4.35%	4.40%	4.30%	4.37%
Loans that redefaulted within one year of modification	$ 23	$ 29	$ 65	$ 92
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			26.04%		27.91%
Total new enrollments
Financing Receivable, Impaired [Line Items]
New TDRs	$ 154	$ 196	$ 483	$ 622