Loans - Consumer, Excluding Credit Card Loan Portfolio, Auto and Other (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 | Auto and other
Loans by origination year
Term loans originated in 2024/2023	7,230		30,605
Term loans originated in 2023/2022	26,562		15,077
Term loans originated in 2022/2021	13,590		13,063
Term loans originated in 2021/2020	11,491		6,624
Term loans originated in 2020/2019	5,608		1,820
Term loans originated prior to 2020/2019	1,763		528
Revolving loans within the revolving period	3,214		3,006
Revolving loans converted to term loans	150		143
Total retained loans	$ 69,608		$ 70,866
% 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	0.76%		0.91%
% of 30 plus days past due to total retained loans, Term loans originated in 2022/2022	0.88%		1.86%
% of 30 plus days past due to total retained loans, Term loans originated in 2022/2021	1.69%		1.75%
% of 30 plus days past due to total retained loans, Term loans originated in 2021/2020	1.56%		1.15%
% of 30 plus days past due to total retained loans, Term loans originated in 2020/2019	1.07%		2.36%
% of 30 plus days past due to total retained loans, Term loans originated prior to 2020/2019	2.21%		3.22%
% of 30 plus days past due to total retained loans, Revolving loans within the revolving period	0.75%		0.73%
% of 30 plus days past due to total retained loans, Revolving loans converted to term loans	31.33%		28.67%
% of 30 plus days past due to total retained loans	1.24%		1.39%
Gross charge-offs by origination year
Gross charge-offs originated in 2024/2023	$ 22		$ 333
Gross charge-offs originated in 2023/2022	111		297
Gross charge-offs originated in 2022/2021	63		161
Gross charge-offs originated in 2021/2020	36		53
Gross charge-offs originated in 2020/2019	11		35
Gross charge-offs originated prior to 2020/2019	24		64
Gross charge-offs originated within the revolving period	0		0
Gross charge-offs originated converted to term loans	1		4
Total gross charge-offs	268		947
Current | Consumer, excluding credit card | Auto and other
Loans by origination year
Term loans originated in 2024/2023	7,175		30,328
Term loans originated in 2023/2022	26,329		14,797
Term loans originated in 2022/2021	13,361		12,825
Term loans originated in 2021/2020	11,306		6,538
Term loans originated in 2020/2019	5,538		1,777
Term loans originated prior to 2020/2019	1,724		511
Revolving loans within the revolving period	3,190		2,984
Revolving loans converted to term loans	103		102
Total retained loans	68,726		69,862
30–119 days past due | Consumer, excluding credit card | Auto and other
Loans by origination year
Term loans originated in 2024/2023	55		276
Term loans originated in 2023/2022	232		279
Term loans originated in 2022/2021	228		231
Term loans originated in 2021/2020	180		78
Term loans originated in 2020/2019	62		43
Term loans originated prior to 2020/2019	39		17
Revolving loans within the revolving period	23		19
Revolving loans converted to term loans	28		24
Total retained loans	847		967
120 or more days past due | Consumer, excluding credit card | Auto and other
Loans by origination year
Term loans originated in 2024/2023	0		1
Term loans originated in 2023/2022	1		1
Term loans originated in 2022/2021	1		7
Term loans originated in 2021/2020	5		8
Term loans originated in 2020/2019	8		0
Term loans originated prior to 2020/2019	0		0
Revolving loans within the revolving period	1		3
Revolving loans converted to term loans	19		17
Total retained loans	$ 35		$ 37