Loans - Credit Card Loan Portfolio, Delinquency Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 974,260	$ 960,506	$ 945,537
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	142,083	142,057
Revolving loans converted to term loans	1,083	1,375
Total retained loans	$ 143,166	$ 143,432
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.95%	1.60%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	7.57%	9.89%
% of 30 plus days past due to total retained loans	1.00%	1.68%
% of 90 plus days past due to total retained loans, Revolving loans within the revolving period	0.47%	0.90%
% of 90 plus days past due to total retained loans, Revolving loans converted to term loans	2.31%	3.05%
% of 90 plus days past due to total retained loans	0.49%	0.92%
Credit card | Current and less than 30 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	$ 140,730	$ 139,783
Revolving loans converted to term loans	1,001	1,239
Total retained loans	141,731	141,022
Credit card | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	679	997
Revolving loans converted to term loans	57	94
Total retained loans	736	1,091
Credit card | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving loans within the revolving period	674	1,277
Revolving loans converted to term loans	25	42
Total retained loans	$ 699	$ 1,319