Allowance for Credit Losses and Unfunded Loan Commitments - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | 12 Months Ended | |||||
|---|---|---|---|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
Dec. 31, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
Dec. 31, 2024 |
|
| Allowance For Credit Losses [Abstract] | ||||||||
| Allowance for loan losses, beginning balance | $ 1,430,318 | |||||||
| Loan sale reduction to provision | $ (235,000) | |||||||
| Net charge-offs: | ||||||||
| Allowance for loan losses, ending balance | $ 1,449,614 | 1,449,614 | $ 1,430,318 | |||||
| Allowance | ||||||||
| Beginning balance | 66,015 | $ 52,541 | 77,132 | 84,568 | 84,568 | |||
| Provision | 153,230 | 154,758 | 269,616 | 255,743 | ||||
| Transfer to allowance for loan losses | (125,541) | (109,422) | (253,044) | (242,434) | ||||
| Ending balance | 93,704 | 97,877 | 93,704 | 97,877 | 77,132 | |||
| Accrued interest to be capitalized | ||||||||
| Unfunded loan commitments for loans held for investment | 3,215,644 | 2,673,369 | 3,215,644 | 2,673,369 | 2,437,035 | $ 1,728,478 | $ 1,358,163 | $ 2,311,660 |
| Total accrued interest receivable | 1,680,595 | 1,680,595 | 1,562,811 | |||||
| Private Education Loans | ||||||||
| Allowance For Credit Losses [Abstract] | ||||||||
| Allowance for loan losses, beginning balance | 1,354,014 | 1,469,509 | 1,430,318 | 1,435,920 | 1,435,920 | |||
| Transfer from allowance for unfunded loan commitments | 125,541 | 109,422 | 253,044 | 242,434 | ||||
| Provision for current period | 103,063 | 143,498 | 244,589 | 330,976 | ||||
| Loan sale reduction to provision | (8,984) | (118,794) | (139,896) | (235,253) | ||||
| Loans transferred to held for sale | (33,634) | (46,437) | ||||||
| Total provisions | 60,445 | 24,704 | 58,256 | 95,723 | ||||
| Net charge-offs: | ||||||||
| Charge-offs | (105,676) | (91,072) | (330,725) | (284,841) | ||||
| Recoveries | 15,290 | 13,541 | 38,721 | 36,868 | ||||
| Net charge-offs | (90,386) | (77,531) | (292,004) | (247,973) | ||||
| Allowance for loan losses, ending balance | 1,449,614 | 1,526,104 | 1,449,614 | 1,526,104 | 1,430,318 | |||
| Allowance | ||||||||
| Beginning balance | 66,015 | 52,541 | 77,132 | 84,568 | 84,568 | |||
| Provision | 153,230 | 154,758 | 269,616 | 255,743 | ||||
| Transfer to allowance for loan losses | (125,541) | (109,422) | (253,044) | (242,434) | ||||
| Ending balance | 93,704 | 97,877 | 93,704 | 97,877 | $ 77,132 | |||
| Total allowance for credit losses, ending balance | $ 1,543,318 | $ 1,623,981 | $ 1,543,318 | $ 1,623,981 | ||||
| Accrued interest to be capitalized | ||||||||
| Net charge-offs as a percentage of average loans in repayment (annualized) | 2.32% | 1.95% | 2.47% | 2.06% | ||||
| Allowance coverage of net charge-offs (annualized) | 4.01 | 4.92 | 3.72 | 4.62 | ||||
| Total Allowance Percentage of Private Education Loan Exposure | 5.89% | 5.93% | 5.89% | 5.93% | ||||
| Ending total loans, gross | $ 21,966,017 | $ 23,029,999 | $ 21,966,017 | $ 23,029,999 | ||||
| Average loans in repayment | 15,607,233 | 15,870,798 | 15,756,540 | 16,027,567 | ||||
| Ending loans in repayment | 16,043,441 | 16,292,470 | 16,043,441 | 16,292,470 | ||||
| Unfunded loan commitments for loans held for investment | 2,544,083 | 2,673,369 | 2,544,083 | 2,673,369 | ||||
| Total accrued interest receivable | 1,674,856 | 1,693,268 | 1,674,856 | 1,693,268 | ||||
| Provisions for loan losses | 60,445 | 24,704 | 58,256 | 95,723 | ||||
| Provisions for unfunded loan commitments | 153,230 | 154,758 | 269,616 | 255,743 | ||||
| Provisions for credit losses reported in consolidated statements of operations | 213,675 | $ 179,462 | 327,872 | $ 351,466 | ||||
| Loans purchased, previously charged off | 19,100 | 19,100 | ||||||
| Unfunded loan commitments, classified as held for sale | $ 672,000 | $ 672,000 | ||||||