Allowance for Loan Losses - Education Loans - 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, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | $ 187 | $ 142 | $ 241 | $ 345 |
| Interest Rate Concession [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | 145 | 95 | 177 | 242 |
| Combination Rate Reduction and Term Extension [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | 7 | 7 | 10 | 15 |
| More Than an Insignificant Payment Delay [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | $ 35 | $ 40 | $ 54 | $ 88 |