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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 128	$ 36	$ 163	$ 38
Interest Rate Concession [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	101	22	121	24
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	22	12	35	13
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 5	$ 2	$ 7	$ 2