Loans - Consumer, Excluding Credit Card Loan Portfolio, Residential Real Estate (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 | 30 or more days past due | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	31	40
Consumer, excluding credit card | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	50,911	56,588
Term loans originated in 2020/2019	51,590	31,859
Term loans originated in 2019/2018	21,082	13,930
Term loans originated in 2018/2017	8,980	20,450
Term loans originated in 2017/2016	13,254	28,025
Term loans originated prior to 2017/2016	56,097	50,736
Revolving loans within the revolving period	6,728	7,945
Revolving loans converted to term loans	13,029	15,769
Total retained loans	$ 221,671	$ 225,302
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	0.02%	0.02%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	0.03%	0.12%
% of 30 plus days past due to total retained loans, Term loans originated in 2019/2018	0.11%	0.22%
% of 30 plus days past due to total retained loans, Term loans originated in 2018/2017	0.24%	0.20%
% of 30 plus days past due to total retained loans, Term loans originated in 2017/2016	0.17%	0.17%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2017/2016	2.45%	2.91%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.31%	0.54%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	3.33%	3.23%
% of 30 plus days past due to total retained loans	0.87%	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	36.00%
Consumer, excluding credit card | Residential real estate | U.S. government-guaranteed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total retained loans	$ 72	$ 76
Consumer, excluding credit card | Residential real estate | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term loans originated in 2021/2020	50,900	56,576
Term loans originated in 2020/2019	51,577	31,820
Term loans originated in 2019/2018	21,058	13,900
Term loans originated in 2018/2017	8,958	20,410
Term loans originated in 2017/2016	13,231	27,978
Term loans originated prior to 2017/2016	54,691	49,218
Revolving loans within the revolving period	6,707	7,902
Revolving loans converted to term loans	12,595	15,260
Total retained loans	219,717	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	41	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	11	9
Term loans originated in 2020/2019	13	25
Term loans originated in 2019/2018	17	20
Term loans originated in 2018/2017	17	22
Term loans originated in 2017/2016	10	29
Term loans originated prior to 2017/2016	479	674
Revolving loans within the revolving period	16	21
Revolving loans converted to term loans	172	245
Total retained loans	735	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	11	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	0	14
Term loans originated in 2019/2018	7	10
Term loans originated in 2018/2017	5	18
Term loans originated in 2017/2016	13	18
Term loans originated prior to 2017/2016	927	844
Revolving loans within the revolving period	5	22
Revolving loans converted to term loans	262	264
Total retained loans	1,219	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	$ 20	$ 24