Loans - Consumer, Excluding Credit Card Loan Portfolio, Residential Real Estate (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 | 30 or more days past due | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	36	40
Consumer, excluding credit card | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	13,779	56,588
Term loans originated in 2020/2019	57,116	31,859
Term loans originated in 2019/2018	27,279	13,930
Term loans originated in 2018/2017	11,891	20,450
Term loans originated in 2017/2016	17,419	28,025
Term loans originated prior to 2017/2016	69,494	50,736
Revolving loans within the revolving period	6,198	7,413
Revolving loans converted to term loans	15,997	16,301
Total retained loans	$ 219,173	$ 225,302
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	0.00%	0.02%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	0.01%	0.12%
% of 30 plus days past due to total retained loans, Term loans originated in 2019/2018	0.05%	0.22%
% of 30 plus days past due to total retained loans, Term loans originated in 2018/2017	0.19%	0.20%
% of 30 plus days past due to total retained loans, Term loans originated in 2017/2016	0.18%	0.17%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2017/2016	2.01%	2.91%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.53%	0.58%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	2.84%	3.12%
% of 30 plus days past due to total retained loans	0.90%	0.98%
Consumer, excluding credit card | Residential real estate | Senior lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total revolving loans that are senior lien loans	35.00%
Consumer, excluding credit card | Residential real estate | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 76	$ 76
Consumer, excluding credit card | Residential real estate | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	13,779	56,576
Term loans originated in 2020/2019	57,109	31,820
Term loans originated in 2019/2018	27,264	13,900
Term loans originated in 2018/2017	11,868	20,410
Term loans originated in 2017/2016	17,387	27,978
Term loans originated prior to 2017/2016	68,061	49,218
Revolving loans within the revolving period	6,165	7,370
Revolving loans converted to term loans	15,542	15,792
Total retained loans	217,175	223,064
Consumer, excluding credit card | Residential real estate | Current | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	40	36
Consumer, excluding credit card | Residential real estate | 30–149 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	0	9
Term loans originated in 2020/2019	5	25
Term loans originated in 2019/2018	11	20
Term loans originated in 2018/2017	15	22
Term loans originated in 2017/2016	12	29
Term loans originated prior to 2017/2016	595	674
Revolving loans within the revolving period	17	21
Revolving loans converted to term loans	193	245
Total retained loans	848	1,045
Consumer, excluding credit card | Residential real estate | 30–149 days past due | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	13	16
Consumer, excluding credit card | Residential real estate | 150 or more days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	0	3
Term loans originated in 2020/2019	2	14
Term loans originated in 2019/2018	4	10
Term loans originated in 2018/2017	8	18
Term loans originated in 2017/2016	20	18
Term loans originated prior to 2017/2016	838	844
Revolving loans within the revolving period	16	22
Revolving loans converted to term loans	262	264
Total retained loans	1,150	1,193
Consumer, excluding credit card | Residential real estate | 150 or more days past due | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 23	$ 24