Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 5,326	$ 4,226
Long-term borrowings	47,545	53,402
Total	52,871	57,628
Total assets	56,622	61,375
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	5,344	4,235
Long-term borrowings	47,900	53,680
Total	53,244	57,915
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	(18)	(9)
Long-term borrowings	(355)	(278)
Total	(373)	(287)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,225	3,634
Long-term borrowings	42,903	48,169
Total	47,128	51,803
Loans	47,798	53,591
Cash	2,904	1,937
Other assets	1,308	1,744
Total assets	52,010	57,272
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	159	59
Long-term borrowings	31,596	35,626
Total	31,755	35,685
Loans	31,166	35,935
Cash	2,414	1,441
Other assets	1,303	1,673
Total assets	34,883	39,049
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	719	435
Long-term borrowings	11,382	11,754
Total	12,101	12,189
Loans	13,084	13,396
Cash	342	350
Other assets	108	119
Total assets	13,534	13,865
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,600	1,854
Long-term borrowings	81	89
Total	1,681	1,943
Loans	1,613	1,897
Cash	101	77
Other assets	66	92
Total assets	1,780	2,066
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,747	1,286
Long-term borrowings	0	821
Total	1,747	2,107
Loans	1,935	2,363
Cash	47	69
Other assets	54	50
Total assets	2,036	2,482
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,225	3,634
Long-term borrowings	43,059	48,290
Total	47,284	51,924
Loans	47,798	53,591
Cash	2,904	1,937
Other assets	1,531	1,934
Total assets	52,233	57,462
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	0	0
Long-term borrowings	(156)	(121)
Total	(156)	(121)
Loans	0	0
Cash	0	0
Other assets	(223)	(190)
Total assets	$ (223)	$ (190)