Borrowings - Company's Borrowings (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Instrument [Line Items]
Short-term borrowings	$ 5,751	$ 5,073
Long-term borrowings	37,429	40,633
Total	43,180	45,706
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	5,759	5,074
Long-term borrowings	37,626	40,672
Total	43,385	45,746
Total Before Hedge Accounting Adjustments [Member] \| Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Short-term borrowings	721	525
Long-term borrowings	4,581	4,782
Total	5,302	5,307
Total Before Hedge Accounting Adjustments [Member] \| Total Unsecured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	721	525
Long-term borrowings	4,581	4,782
Total	5,302	5,307
Total Before Hedge Accounting Adjustments [Member] \| FFELP Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	0	109
Long-term borrowings	22,637	25,302
Total	22,637	25,411
Total Before Hedge Accounting Adjustments [Member] \| Private Education Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	384	469
Long-term borrowings	9,953	10,250
Total	10,337	10,719
Total Before Hedge Accounting Adjustments [Member] \| FFELP Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	2,349	1,869
Long-term borrowings	376	299
Total	2,725	2,168
Total Before Hedge Accounting Adjustments [Member] \| Private Education Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	2,109	1,942
Long-term borrowings	0	0
Total	2,109	1,942
Total Before Hedge Accounting Adjustments [Member] \| Other [Member]
Debt Instrument [Line Items]
Short-term borrowings	196	160
Long-term borrowings	79	39
Total	275	199
Total Before Hedge Accounting Adjustments [Member] \| Secured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	5,038	4,549
Long-term borrowings	33,045	35,890
Total	38,083	40,439
Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	(8)	(1)
Long-term borrowings	(197)	(39)
Total	$ (205)	$ (40)