Loans - Credit Card Portfolio - Loan Modifications (Details) - Credit card - Credit card $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) payment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Financing Receivable, Impaired [Line Items]
Fixed payment plan period	60 months
Weighted-average interest rate of loans – before TDR	15.08%	14.96%	15.37%
Weighted-average interest rate of loans – after TDR	4.40%	4.40%	4.38%
Loans that redefaulted within one year of modification	$ 85	$ 119	$ 167
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	25.61%	27.91%	30.72%
Total new enrollments
Financing Receivable, Impaired [Line Items]
New TDRs	$ 638	$ 807	$ 1,200