Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Accounts Notes And Loans Receivable [Line Items]
Expected future recoveries of current period defaults	$ 13	$ 12
Change in balance during period	(5)	(8)
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Beginning of period expected future recoveries on previously fully charged-off loans	274	329
Expected future recoveries of current period defaults	13	12
Recoveries (cash collected)	(13)	(15)
Charge-offs (as a result of lower recovery expectations)	(6)	(5)
End of period expected future recoveries on previously fully charged-off loans	268	321
Change in balance during period	$ (5)	$ (8)