Loans - Credit Card Loan Portfolio, Delinquency Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 965,448	$ 945,601	$ 947,728
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	140,234
Revolving loans converted to term loans	1,422
Total retained loans	$ 141,656	$ 168,924
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	1.63%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	9.63%
% of 30 plus days past due to total retained loans	1.71%	1.87%
% of 90 plus days past due to total retained loans, Revolving loans within the revolving period	0.91%
% of 90 plus days past due to total retained loans, Revolving loans converted to term loans	3.23%
% of 90 plus days past due to total retained loans	0.93%	0.95%
Current and less than 30 days past due and still accruing | Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	$ 137,953
Revolving loans converted to term loans	1,285
Total retained loans	139,238	$ 165,767
30–89 days past due and still accruing | Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	1,004
Revolving loans converted to term loans	91
Total retained loans	1,095	1,550
90 or more days past due and still accruing | Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	1,277
Revolving loans converted to term loans	46
Total retained loans	$ 1,323	$ 1,607