Allowance for Loan Losses - Amount of Loans Modified for Borrowers Financial Difficulty and Payment Default (Parenthetical) (Details) - Private Education Loans [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 65	$ 123
Interest Rate Reduction [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	44	78
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	18	39
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 3	$ 6