Loans - Consumer, Excluding Credit Card Loan Portfolio, Auto and Other (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
Consumer, excluding credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	298,308	302,127	$ 305,106
Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	35,489	46,266
Term loans originated in 2020/2019	22,114	12,936
Term loans originated in 2019/2018	8,718	7,444
Term loans originated in 2018/2017	4,516	4,575
Term loans originated in 2017/2016	2,373	2,100
Term loans originated prior to 2017/2016	1,019	766
Revolving loans within the revolving period	2,263	2,555
Revolving loans converted to term loans	145	183
Total retained loans	$ 76,637	$ 76,825
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	0.31%	0.21%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	0.32%	0.83%
% of 30 plus days past due to total retained loans, Term loans originated in 2019/2018	0.88%	1.03%
% of 30 plus days past due to total retained loans, Term loans originated in 2018/2017	1.11%	1.18%
% of 30 plus days past due to total retained loans, Term loans originated in 2017/2016	1.26%	2.00%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2017/2016	2.55%	3.13%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.75%	1.49%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	11.03%	13.66%
% of 30 plus days past due to total retained loans	0.68%	0.60%
Consumer, excluding credit card | Auto and other | Paycheck Protection Program (PPP)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 11,100	$ 19,200
Current | Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	35,379	46,169
Term loans originated in 2020/2019	21,917	12,829
Term loans originated in 2019/2018	8,641	7,367
Term loans originated in 2018/2017	4,466	4,521
Term loans originated in 2017/2016	2,343	2,058
Term loans originated prior to 2017/2016	993	742
Revolving loans within the revolving period	2,246	2,517
Revolving loans converted to term loans	129	158
Total retained loans	76,114	76,361
Current | Consumer, excluding credit card | Auto and other | Paycheck Protection Program (PPP)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	9,100	19,200
Term loans originated in 2020/2019	2,000
30 or more days past due | Consumer, excluding credit card | Auto and other | Paycheck Protection Program (PPP)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	126
30–119 days past due | Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	110	97
Term loans originated in 2020/2019	193	107
Term loans originated in 2019/2018	77	77
Term loans originated in 2018/2017	49	53
Term loans originated in 2017/2016	30	42
Term loans originated prior to 2017/2016	25	23
Revolving loans within the revolving period	12	30
Revolving loans converted to term loans	8	17
Total retained loans	504	446
120 or more days past due | Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	0	0
Term loans originated in 2020/2019	4	0
Term loans originated in 2019/2018	0	0
Term loans originated in 2018/2017	1	1
Term loans originated in 2017/2016	0	0
Term loans originated prior to 2017/2016	1	1
Revolving loans within the revolving period	5	8
Revolving loans converted to term loans	8	8
Total retained loans	$ 19	$ 18