Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounts Notes And Loans Receivable [Line Items]
Expected future recoveries of current period defaults	$ 43	$ 47	$ 57
Gross charge-offs	(391)	(383)	(410)
Change in balance during period	(47)	(48)	(55)
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Expected future recoveries of current period defaults	43	47	57
Gross charge-offs	(355)	(320)	(370)
Change in balance during period	(47)	(48)	(55)
Private Education Loans [Member] | Loans Receivable For Previously Fully Charged Off Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Beginning of period expected future recoveries on previously fully charged-off loans	226	274	329
Expected future recoveries of current period defaults	43	47	57
Recoveries (cash collected)	(41)	(46)	(56)
Gross charge-offs	(49)	(49)	(56)
Charge offs related to resolution of loans in bankruptcy	28
End of period expected future recoveries on previously fully charged-off loans	179	226	274
Change in balance during period	$ (47)	$ 48	$ (55)