Borrowings - Company's Borrowings (Detail) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Instrument [Line Items]
Short-term borrowings	$ 4,214	$ 5,073
Long-term borrowings	40,123	40,633
Total	44,337	45,706
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	4,225	5,074
Long-term borrowings	40,208	40,672
Total	44,433	45,746
Total Before Hedge Accounting Adjustments [Member] \| Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Short-term borrowings	720	525
Long-term borrowings	4,580	4,782
Total	5,300	5,307
Total Before Hedge Accounting Adjustments [Member] \| Total Unsecured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	720	525
Long-term borrowings	4,580	4,782
Total	5,300	5,307
Total Before Hedge Accounting Adjustments [Member] \| FFELP Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	0	109
Long-term borrowings	23,670	25,302
Total	23,670	25,411
Total Before Hedge Accounting Adjustments [Member] \| Private Education Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	408	469
Long-term borrowings	10,847	10,250
Total	11,255	10,719
Total Before Hedge Accounting Adjustments [Member] \| FFELP Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,752	1,869
Long-term borrowings	499	299
Total	2,251	2,168
Total Before Hedge Accounting Adjustments [Member] \| Private Education Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,159	1,942
Long-term borrowings	574	0
Total	1,733	1,942
Total Before Hedge Accounting Adjustments [Member] \| Other [Member]
Debt Instrument [Line Items]
Short-term borrowings	186	160
Long-term borrowings	38	39
Total	224	199
Total Before Hedge Accounting Adjustments [Member] \| Secured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	3,505	4,549
Long-term borrowings	35,628	35,890
Total	39,133	40,439
Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	(11)	(1)
Long-term borrowings	(85)	(39)
Total	$ (96)	$ (40)