Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Debt Instrument [Line Items]
Short-term debt	$ 4,752		$ 4,752		$ 5,134
Long-term debt	42,345		42,345		43,184
FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Debt tranches default principal amount	1,200		1,200
Facilities [Member]
Debt Instrument [Line Items]
Gross issunaces	432	$ 121	964	$ 522
Gross paydowns	587	$ 944	1,100	$ 1,100
Long Term Senior Unsecured Debt [Member] | FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Long-term debt	0		0		87
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term debt	4,755		4,755		5,139
Long-term debt	42,408		42,408		43,526
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term debt	117		117		41
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Asset-backed Securitization Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term debt	$ 562		$ 562		$ 631