Borrowings - Company's Borrowings (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Short-term borrowings	$ 5,326	$ 4,226
Long-term borrowings	47,545	53,402
Total	52,871	57,628
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	5,344	4,235
Long-term borrowings	47,900	53,680
Total	53,244	57,915
Total Before Hedge Accounting Adjustments [Member] | Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,052	506
Long-term borrowings	4,802	5,351
Total	5,854	5,857
Total Before Hedge Accounting Adjustments [Member] | Total Unsecured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,052	506
Long-term borrowings	4,802	5,351
Total	5,854	5,857
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	159	59
Long-term borrowings	31,596	35,626
Total	31,755	35,685
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Securitizations [Member]
Debt Instrument [Line Items]
Short-term borrowings	719	435
Long-term borrowings	11,382	11,754
Total	12,101	12,189
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,600	1,854
Long-term borrowings	81	89
Total	1,681	1,943
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan ABCP Facilities [Member]
Debt Instrument [Line Items]
Short-term borrowings	1,747	1,286
Long-term borrowings	0	821
Total	1,747	2,107
Total Before Hedge Accounting Adjustments [Member] | Other [Member]
Debt Instrument [Line Items]
Short-term borrowings	67	95
Long-term borrowings	39	39
Total	106	134
Total Before Hedge Accounting Adjustments [Member] | Secured Borrowings [Member]
Debt Instrument [Line Items]
Short-term borrowings	4,292	3,729
Long-term borrowings	43,098	48,329
Total	47,390	52,058
Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term borrowings	(18)	(9)
Long-term borrowings	(355)	(278)
Total	$ (373)	$ (287)