Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Accounts Notes And Loans Receivable [Line Items]
Expected future recoveries of current period defaults	$ 10	$ 11	$ 21	$ 24
Charge-offs (as a result of lower recovery expectations)	(87)	(92)	(207)	(198)
Change in balance during period	(6)	(7)	(15)	(12)
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Expected future recoveries of current period defaults	10	11	21	24
Charge-offs (as a result of lower recovery expectations)	(77)	(73)	(187)	(161)
Change in balance during period	(6)	(7)	(15)	(12)
Private Education Loans [Member] | Loans Receivable For Previously Fully Charged Off Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Charge offs related to resolution of loans in bankruptcy			28
Beginning of period expected future recoveries on previously fully charged-off loans	217	268	226	274
Expected future recoveries of current period defaults	10	11	21	24
Recoveries (cash collected)	(10)	(11)	(21)	(24)
Charge-offs (as a result of lower recovery expectations)	(6)	(6)	(15)	(12)
End of period expected future recoveries on previously fully charged-off loans	211	262	211	262
Change in balance during period	$ (6)	$ (7)	$ (15)	$ (12)