Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025
Debt Instrument [Line Items]
Short-term debt	$ 5,751		$ 5,751		$ 5,073
Long-term debt	37,429		37,429		40,633
FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Debt tranches default principal amount	948		948
Facilities [Member]
Debt Instrument [Line Items]
Gross issunaces	1,700	$ 870	3,300	$ 1,800
Gross paydowns	853	$ 675	2,500	$ 1,800
Long Term Senior Unsecured Debt [Member] \| FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Long-term debt	115		115		0
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term debt	5,759		5,759		5,074
Long-term debt	37,626		37,626		40,672
Total Before Hedge Accounting Adjustments [Member] \| FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term debt	0		0		109
Total Before Hedge Accounting Adjustments [Member] \| Private Education Loan Asset-backed Securitization Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term debt	$ 384		$ 384		$ 469