Allowance for Loan Losses - Summary of Performance and Related Loan Status of Loans Modified (Details) - Private Education Loans [Member] - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 2,388	$ 2,729	$ 2,380
Financing Asset, Not Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	1,969	2,199	2,039
Financial Asset, 31 to 60 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	137	160	133
Financial Asset, 61 to 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	64	96	61
Financial Asset, 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	83	159	64
School/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	14	22	16
Forbearance [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 121	$ 93	$ 67