Loans - Credit Card Loan Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 804,293	$ 747,508	$ 735,304
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	125,634	128,027	$ 126,564
Credit card | Credit card loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 125,634	$ 128,027
% of 30 days past due to total retained loans	1.38%	1.44%
% of 90 days past due to total retained loans	0.66%	0.70%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	85.10%	85.70%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	14.90%	14.30%
Credit card | Credit card loans | California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 17,830	$ 17,940
Credit card | Credit card loans | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	11,270	11,088
Credit card | Credit card loans | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	10,965	10,940
Credit card | Credit card loans | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	7,389	7,497
Credit card | Credit card loans | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	7,375	7,398
Credit card | Credit card loans | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	5,668	5,750
Credit card | Credit card loans | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	4,504	4,707
Credit card | Credit card loans | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	4,324	4,489
Credit card | Credit card loans | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	3,448	3,552
Credit card | Credit card loans | Virginia
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	3,068	3,263
Credit card | Credit card loans | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	49,793	51,403
Credit card | Credit card loans | Current and less than 30 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	123,901	126,189
Credit card | Credit card loans | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	908	943
Credit card | Credit card loans | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	825	895
Credit card | Credit card loans | Nonaccrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 0	$ 0