Loans - Credit Card Portfolio - Loan Modifications (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) payment	Mar. 31, 2015 USD ($)	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Number of years before payment default under a modified loan	1 year
Credit card | Credit card loans
Financing Receivable, Impaired [Line Items]
Fixed payment plan period	60 months
Weighted-average interest rate of loans – before TDR	15.48%	15.16%
Weighted-average interest rate of loans – after TDR	4.76%	4.29%
Loans that redefaulted within one year of modification	$ 19	$ 22
Number of payments past due for deemed payment | payment	2
Rate of default for modified loans, estimated weighted average	25.79%		25.61%
Credit card | Credit card loans | Total new enrollments
Financing Receivable, Impaired [Line Items]
New TDRs	$ 159	$ 178