Allowance for Loan Losses - Summary of Performance and Related Loan Status of Loans Modified (Details) - Private Education Loans [Member] $ in Millions	Sep. 30, 2023 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 2,380
Financing Asset, Not Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	2,039
Financial Asset, 31 to 60 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	133
Financial Asset, 61 to 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	61
Financial Asset, 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	64
School/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	16
Forbearance [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 67