Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Debt Instrument [Line Items]
Short-term debt	$ 4,214		$ 4,214		$ 5,073
Long-term debt	40,123		40,123		40,633
FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Debt tranches default principal amount	1,400		1,400
Facilities [Member]
Debt Instrument [Line Items]
Gross issunaces	806	$ 432	1,500	$ 964
Gross paydowns	1,100	$ 587	1,700	$ 1,100
Long Term Senior Unsecured Debt [Member] \| FFELP Loan Securitizations ABS Repurchase Facilities [Member]
Debt Instrument [Line Items]
Long-term debt	118		118		0
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term debt	4,225		4,225		5,074
Long-term debt	40,208		40,208		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	$ 408		$ 408		$ 469