Loans - Consumer, Excluding Credit Card Loan Portfolio, Auto and Other (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 948,642	$ 960,506	$ 1,003,089
Consumer, excluding credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	302,392	302,127	$ 293,779
Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	17,751	46,266
Term loans originated in 2020/2019	39,211	12,936
Term loans originated in 2019/2018	11,470	7,444
Term loans originated in 2018/2017	6,397	4,575
Term loans originated in 2017/2016	3,738	2,100
Term loans originated prior to 2017/2016	2,079	766
Revolving loans within the revolving period	2,402	2,555
Revolving loans converted to term loans	171	183
Total retained loans	$ 83,219	$ 76,825
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	0.18%	0.21%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	0.13%	0.83%
% of 30 plus days past due to total retained loans, Term loans originated in 2019/2018	0.57%	1.03%
% of 30 plus days past due to total retained loans, Term loans originated in 2018/2017	0.75%	1.18%
% of 30 plus days past due to total retained loans, Term loans originated in 2017/2016	0.86%	2.00%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2017/2016	1.64%	3.13%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	1.37%	1.49%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	12.87%	13.66%
% of 30 plus days past due to total retained loans	0.38%	0.60%
Consumer, excluding credit card | Auto and other | Paycheck Protection Program (PPP)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 23,400	$ 19,200
Current | Consumer, excluding credit card | Auto and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	17,719	46,169
Term loans originated in 2020/2019	39,161	12,829
Term loans originated in 2019/2018	11,405	7,367
Term loans originated in 2018/2017	6,349	4,521
Term loans originated in 2017/2016	3,706	2,058
Term loans originated prior to 2017/2016	2,045	742
Revolving loans within the revolving period	2,369	2,517
Revolving loans converted to term loans	149	158
Total retained loans	82,903	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	8,700	19,200
Term loans originated in 2020/2019	14,700
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	32	97
Term loans originated in 2020/2019	50	107
Term loans originated in 2019/2018	65	77
Term loans originated in 2018/2017	47	53
Term loans originated in 2017/2016	31	42
Term loans originated prior to 2017/2016	33	23
Revolving loans within the revolving period	21	30
Revolving loans converted to term loans	12	17
Total retained loans	291	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	0	0
Term loans originated in 2019/2018	0	0
Term loans originated in 2018/2017	1	1
Term loans originated in 2017/2016	1	0
Term loans originated prior to 2017/2016	1	1
Revolving loans within the revolving period	12	8
Revolving loans converted to term loans	10	8
Total retained loans	$ 25	$ 18