Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 4,855	$ 5,134
Long-term borrowings	42,872	43,184
Total	47,727	48,318
Total assets	50,950	51,789
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,857	5,139
Long-term borrowings	43,098	43,526
Total	47,955	48,665
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	(2)	(5)
Long-term borrowings	(226)	(342)
Total	(228)	(347)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,296	4,532
Long-term borrowings	38,126	38,497
Total	42,422	43,029
Loans	44,565	45,258
Cash	1,411	1,364
Other assets	1,232	1,224
Total assets	47,208	47,846
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	39	41
Long-term borrowings	27,644	28,268
Total	27,683	28,309
Loans	28,365	28,983
Cash	914	901
Other assets	1,169	1,211
Total assets	30,448	31,095
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	595	631
Long-term borrowings	10,340	10,338
Total	10,935	10,969
Loans	12,085	12,054
Cash	353	335
Other assets	114	113
Total assets	12,552	12,502
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,557	1,586
Long-term borrowings	268	74
Total	1,825	1,660
Loans	1,814	1,637
Cash	58	53
Other assets	80	78
Total assets	1,952	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	2,105	2,274
Long-term borrowings	0	0
Total	2,105	2,274
Loans	2,301	2,584
Cash	86	75
Other assets	59	66
Total assets	2,446	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	4,296	4,532
Long-term borrowings	38,252	38,680
Total	42,548	43,212
Loans	44,565	45,258
Cash	1,411	1,364
Other assets	1,422	1,468
Total assets	47,398	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	(126)	(183)
Total	(126)	(183)
Loans	0	0
Cash	0	0
Other assets	(190)	(244)
Total assets	$ (190)	$ (244)