Loans - Credit Card Loan Portfolio, Delinquency Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Total retained loans	$ 960,506	$ 945,601	$ 969,415
Credit card
Loans and Leases Receivable Disclosure [Line Items]
Revolving loans within the revolving period	142,057
Revolving loans converted to term loans	1,375
Total retained loans	$ 143,432	$ 168,924
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	1.60%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	9.89%
% of 30 plus days past due to total retained loans	1.68%	1.87%
% of 90 plus days past due to total retained loans, Revolving loans within the revolving period	0.90%
% of 90 plus days past due to total retained loans, Revolving loans converted to term loans	3.05%
% of 90 plus days past due to total retained loans	0.92%	0.95%
Current and less than 30 days past due and still accruing | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Revolving loans within the revolving period	$ 139,783
Revolving loans converted to term loans	1,239
Total retained loans	141,022	$ 165,767
30–89 days past due and still accruing | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Revolving loans within the revolving period	997
Revolving loans converted to term loans	94
Total retained loans	1,091	1,550
90 or more days past due and still accruing | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Revolving loans within the revolving period	1,277
Revolving loans converted to term loans	42
Total retained loans	$ 1,319	$ 1,607