Allowance for Loan Losses - Summary of Performance and Related Loan Status of Loans Modified (Details) - Private Education Loans (Details) - Private Education Loans [Member] $ in Millions	Dec. 31, 2023 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	$ 2,729
School/Deferment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	22
Forbearance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	93
Financing Asset, Not Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	2,199
Financial Asset, 31 to 60 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	160
Financial Asset, 61 to 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	96
Financial Asset, 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment status (Amortized Cost)	$ 159