Allowance for Loan Losses - Summary of Performance and Related Loan Status of Loans Modified (Details) - Private Education Loans [Member] - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 1,818	$ 2,729	$ 1,791
Financing Asset, Not Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	1,656	2,199	1,646
Financial Asset, 31 to 60 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	57	160	51
Financial Asset, 61 to 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	25	96	21
Financial Asset, 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	22	159	20
School/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	3	22	5
Forbearance [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 55	$ 93	$ 48