Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 4,752	$ 5,134
Long-term borrowings	42,345	43,184
Total	47,097	48,318
Total assets	50,222	51,789
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,755	5,139
Long-term borrowings	42,408	43,526
Total	47,163	48,665
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	(3)	(5)
Long-term borrowings	(63)	(342)
Total	(66)	(347)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,153	4,532
Long-term borrowings	37,552	38,497
Total	41,705	43,029
Loans	43,786	45,258
Cash	1,363	1,364
Other assets	1,366	1,224
Total assets	46,515	47,846
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	117	41
Long-term borrowings	26,948	28,268
Total	27,065	28,309
Loans	27,749	28,983
Cash	871	901
Other assets	1,187	1,211
Total assets	29,807	31,095
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	562	631
Long-term borrowings	10,322	10,338
Total	10,884	10,969
Loans	12,103	12,054
Cash	342	335
Other assets	130	113
Total assets	12,575	12,502
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,531	1,586
Long-term borrowings	301	74
Total	1,832	1,660
Loans	1,816	1,637
Cash	65	53
Other assets	86	78
Total assets	1,967	1,768
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,943	2,274
Long-term borrowings	0	0
Total	1,943	2,274
Loans	2,118	2,584
Cash	85	75
Other assets	38	66
Total assets	2,241	2,725
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,153	4,532
Long-term borrowings	37,571	38,680
Total	41,724	43,212
Loans	43,786	45,258
Cash	1,363	1,364
Other assets	1,441	1,468
Total assets	46,590	48,090
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	(19)	(183)
Total	(19)	(183)
Loans	0	0
Cash	0	0
Other assets	(75)	(244)
Total assets	$ (75)	$ (244)