Loans - Consumer, Excluding Credit Card Loan Portfolio, Residential Real Estate (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Loans by origination year
Total retained loans	$ 1,264,093	$ 1,084,850	$ 1,280,870
Gross charge-offs by origination year
Total gross charge-offs	2,381	1,451
Consumer, excluding credit card
Loans by origination year
Total retained loans	389,592	300,447	397,275
Gross charge-offs by origination year
Total gross charge-offs	331	$ 235
Consumer, excluding credit card | Residential real estate
Loans by origination year
Term loans originated in 2024/2023	2,148		23,250
Term loans originated in 2023/2022	20,079		64,450
Term loans originated in 2022/2021	63,824		84,602
Term loans originated in 2021/2020	83,606		55,624
Term loans originated in 2020/2019	54,977		21,592
Term loans originated prior to 2020/2019	80,008		60,820
Revolving loans within the revolving period	7,103		7,533
Revolving loans converted to term loans	8,239		8,538
Total retained loans	$ 319,984		$ 326,409
% of 30+ days past due to total retained loans by origination year
% of 30 plus days past due to total retained loans, Term loans originated in 2024/2023	0.00%		0.15%
% of 30 plus days past due to total retained loans, Term loans originated in 2022/2022	0.10%		0.13%
% of 30 plus days past due to total retained loans, Term loans originated in 2022/2021	0.24%		0.13%
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	0.15%		0.14%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	0.15%		0.29%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2020/2019	1.61%		2.04%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.70%		0.72%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	4.78%		4.53%
% of 30 plus days past due to total retained loans	0.66%		0.63%
Gross charge-offs by origination year
Gross charge-offs originated in 2024/2023	$ 0		$ 0
Gross charge-offs originated in 2023/2022	0		0
Gross charge-offs originated in 2022/2021	0		0
Gross charge-offs originated in 2021/2020	0		0
Gross charge-offs originated in 2020/2019	0		4
Gross charge-offs originated prior to 2020/2019	55		167
Gross charge-offs originated within the revolving period	5		26
Gross charge-offs originated converted to term loans	3		7
Total gross charge-offs	$ 63		204
Consumer, excluding credit card | Residential real estate | Senior lien
Gross charge-offs by origination year
Percentage of total revolving loans that are senior lien loans	37.00%
Consumer, excluding credit card | Residential real estate | Current
Loans by origination year
Term loans originated in 2024/2023	$ 2,148		23,216
Term loans originated in 2023/2022	20,058		64,366
Term loans originated in 2022/2021	63,668		84,496
Term loans originated in 2021/2020	83,481		55,546
Term loans originated in 2020/2019	54,894		21,530
Term loans originated prior to 2020/2019	78,706		59,563
Revolving loans within the revolving period	7,053		7,479
Revolving loans converted to term loans	7,845		8,151
Total retained loans	317,853		324,347
Consumer, excluding credit card | Residential real estate | 30–149 days past due
Loans by origination year
Term loans originated in 2024/2023	0		33
Term loans originated in 2023/2022	19		74
Term loans originated in 2022/2021	114		89
Term loans originated in 2021/2020	95		70
Term loans originated in 2020/2019	49		41
Term loans originated prior to 2020/2019	762		801
Revolving loans within the revolving period	42		49
Revolving loans converted to term loans	230		223
Total retained loans	1,311		1,380
Consumer, excluding credit card | Residential real estate | 150 or more days past due
Loans by origination year
Term loans originated in 2024/2023	0		1
Term loans originated in 2023/2022	2		10
Term loans originated in 2022/2021	42		17
Term loans originated in 2021/2020	30		8
Term loans originated in 2020/2019	34		21
Term loans originated prior to 2020/2019	540		456
Revolving loans within the revolving period	8		5
Revolving loans converted to term loans	164		164
Total retained loans	$ 820		$ 682