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Significant Accounting Policies - Allowance for FFELP Loans and Loan Interest Income (Details) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Percentage reimbursement on all qualifying default claims period two (as a percentage) |  | 97.00% |
Percentage reimbursement on all qualifying default claims period one (as a percentage) |  | 98.00% |
Variable rate |  | 100.00% |
Maximum loss exposure due to federal guarantee |  | 3.00% |
Basis points |  | 0.0105 |
Loans in nonaccrual status |  | $ 0 | $ 0
Percent of private education loans related to borrowers in school, grace or deferment |  | 26.00% | 25.00%
Credit Cards |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Gain (loss) on sale of financing receivable | $ (8,000,000) |  | $ (4,000,000)
Credit Card |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing receivable allowance for credit loss, transfer to held-to-sale |  | $ 2,400,000 |

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