Allowance for Loan Losses - Amount of Loans Modified for Borrowers Financial Difficulty and Payment Default (Parenthetical) (Details) - Private Education Loans [Member] $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 272
Interest Rate Reduction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans (Amortized Cost)	175
Combination Rate Reduction and Term Extension [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans (Amortized Cost)	14
More Than an Insignificant Payment Delay [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 83