Loans - Credit Card Loan Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 844,195	$ 832,792	$ 756,971
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	126,012	131,387	$ 120,835
Credit card loans | Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 126,012	$ 131,387
% of 30 days past due to total retained loans	1.45%	1.43%
% of 90 days past due to total retained loans	0.75%	0.72%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	84.60%	84.40%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	14.00%	13.10%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, No FICO available	1.40%	2.50%
Credit card loans | Credit card | California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 18,105	$ 18,802
Credit card loans | Credit card | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	11,517	11,847
Credit card loans | Credit card | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	10,871	11,360
Credit card loans | Credit card | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	7,564	7,806
Credit card loans | Credit card | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	7,319	7,655
Credit card loans | Credit card | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	5,603	5,879
Credit card loans | Credit card | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	4,445	4,700
Credit card loans | Credit card | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	4,296	4,533
Credit card loans | Credit card | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	3,365	3,562
Credit card loans | Credit card | Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	3,272	3,399
Credit card loans | Credit card | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	49,655	51,844
Credit card loans | Credit card | Current and less than 30 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	124,187	129,502
Credit card loans | Credit card | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	885	941
Credit card loans | Credit card | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 940	$ 944