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 | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | $ 232 | $ 109 | $ 278 | $ 179 |
| Interest Rate Concession [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | 179 | 71 | 208 | 119 |
| Combination Rate Reduction and Term Extension [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | 10 | 5 | 12 | 8 |
| More Than an Insignificant Payment Delay [Member] | ||||
| Accounts Notes And Loans Receivable [Line Items] | ||||
| Modified Loans (Amortized Cost) | $ 43 | $ 33 | $ 58 | $ 52 |