Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Adjustment resulting from the change in the charge-off rate	$ 30	$ 16	$ 23
Expected future recoveries of current period defaults	57	22	32
Change in balance during period	(55)	(150)	(109)
Sale of private education loan		1,600
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Adjustment resulting from the change in the charge-off rate	30	16	23
Beginning of period expected future recoveries on previously fully charged-off loans	329	479	588
Expected future recoveries of current period defaults	57	22	32
Recoveries (cash collected)	(56)	(87)	(107)
Charge-offs (as a result of lower recovery expectations)	(56)	(35)	(34)
Reduction in expected recoveries related to regulatory settlement		(50)
End of period expected future recoveries on previously fully charged-off loans	274	329	479
Change in balance during period	(55)	(150)	(109)
Private Education Loans [Member] | Loans Receivable For Previously Fully Charged Off Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Adjustment resulting from the change in the charge-off rate	$ 30	$ 16	$ 23