Loans - Credit Card Portfolio - Loan Modifications (Details) (Credit card, Consumer Credit Card Financing Receivable, USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015 payment	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Fixed payment plan period	60 months
Weighted-average interest rate of loans – before TDR	15.16%	15.03%
Weighted-average interest rate of loans – after TDR	4.29%	4.43%
Modifications, subsequent default, recorded investment	$ 22	$ 34
Number of years before payment default under a modified loan	1 year
Number of payments past due for deemed payment	2
Rate of default for modified loans, estimated weighted average	27.34%		27.91%
Total new enrollments
Financing Receivable, Impaired [Line Items]
New TDRs	$ 178	$ 233