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Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Mar. 31, 2025 | Dec. 31, 2024 | Mar. 31, 2024 | Dec. 31, 2023
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,443,715) | $ (1,435,920) | $ (1,350,058) | $ (1,339,772)
Private Education Loans, net | $ 21,091,204 | $ 20,902,158 |  |
Percentage of Private Education Loans in repayment, Year 1 | 53.60% | 56.50% |  |
Percentage of Private Education Loans in repayment, Year 2 | 57.20% | 63.50% |  |
Percentage of Private Education Loans in repayment, Year 3 | 64.20% | 73.70% |  |
Percentage of Private Education Loans in repayment, Year 4 | 73.70% | 80.30% |  |
Percentage of Private Education Loans in repayment, Year 5 | 79.70% | 84.30% |  |
Percentage of Private Education Loans in repayment, After Year 5 | 88.30% | 89.70% |  |
Percentage of Private Education Loans in repayment | 70.90% | 72.40% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.20% | 0.70% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 0.60% | 2.00% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.00% | 3.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 4.20% | 4.30% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.50% | 4.40% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5 | 5.70% | 6.30% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.60% | 3.70% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.20% | 0.70% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 1.00% | 3.60% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 5.00% | 3.60% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 4.50% | 3.10% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 4.00% | 2.70% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5 | 2.30% | 1.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 2.80% | 2.50% |  |
Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in Repayment, Year 1 | $ 505,279 | $ 2,840,129 |  |
Total Private Education Loans in Repayment, Year 2 | 3,696,844 | 3,379,077 |  |
Total Private Education Loans in Repayment, Year 3 | 2,659,437 | 2,348,504 |  |
Total Private Education Loans in Repayment, Year 4 | 2,052,134 | 1,661,502 |  |
Total Private Education Loans in Repayment, Year 5 | 1,507,821 | 1,158,044 |  |
Total Private Education Loans in Repayment, After Year 5 | 5,482,282 | 4,719,495 |  |
Total Private Education Loans in repayment | 15,903,797 | 16,106,751 |  |
Total Private Education Loans, gross, Year 1 | 942,378 | 5,024,592 |  |
Total Private Education Loans, gross, Year 2 | 6,461,366 | 5,321,510 |  |
Total Private Education Loans, gross, Year 3 | 4,141,150 | 3,185,247 |  |
Total Private Education Loans, gross, Year 4 | 2,784,645 | 2,069,436 |  |
Total Private Education Loans, gross, Year 5 | 1,890,758 | 1,373,996 |  |
Total Private Education Loans, gross, After Year 5 | 6,211,828 | 5,260,227 |  |
Total Private Education Loans, gross | 22,432,125 | 22,235,008 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 13,142 | 47,659 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 42,843 | 25,599 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 20,119 | 10,788 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 9,329 | 6,142 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 5,521 | 4,057 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5 | 11,840 | 8,825 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount) | 102,794 | 103,070 |  |
Total Private Education Loans, Year 1 | 955,520 | 5,072,251 |  |
Total Private Education Loans, Year 2 | 6,504,209 | 5,347,109 |  |
Total Private Education Loans, Year 3 | 4,161,269 | 3,196,035 |  |
Total Private Education Loans, Year 4 | 2,793,974 | 2,075,578 |  |
Total Private Education Loans, Year 5 | 1,896,279 | 1,378,053 |  |
Total Private Education Loans, After Year 5 | 6,223,668 | 5,269,052 |  |
Total Private Education Loans | 22,534,919 | 22,338,078 |  |
Private Education Loans allowance for losses, Year 1 | (36,944) | (258,235) |  |
Private Education Loans allowance for losses, Year 2 | (332,859) | (326,207) |  |
Private Education Loans allowance for losses, Year 3 | (283,874) | (234,532) |  |
Private Education Loans allowance for losses, Year 4 | (217,640) | (150,324) |  |
Private Education Loans allowance for losses, Year 5 | (141,990) | (90,600) |  |
Private Education Loans allowance for losses, After Year 5 | (430,408) | (376,022) |  |
Private Education Loans allowance for losses | (1,443,715) | (1,435,920) |  |
Private Education Loans, net, Year 1 | 918,576 | 4,814,016 |  |
Private Education Loans, net, Year 2 | 6,171,350 | 5,020,902 |  |
Private Education Loans, net, Year 3 | 3,877,395 | 2,961,503 |  |
Private Education Loans, net, Year 4 | 2,576,334 | 1,925,254 |  |
Private Education Loans, net, Year 5 | 1,754,289 | 1,287,453 |  |
Private Education Loans, net, After Year 5 | 5,793,260 | 4,893,030 |  |
Private Education Loans, net | 21,091,204 | 20,902,158 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent, current |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 504,171 | 2,820,940 |  |
Year 2 | 3,673,471 | 3,312,916 |  |
Year 3 | 2,578,666 | 2,259,455 |  |
Year 4 | 1,965,257 | 1,590,812 |  |
Year 5 | 1,439,732 | 1,107,189 |  |
After Year 5 | 5,172,375 | 4,422,021 |  |
Loan type | 15,333,672 | 15,513,333 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 1,108 | 13,533 |  |
Year 2 | 13,654 | 36,441 |  |
Year 3 | 35,819 | 45,543 |  |
Year 4 | 39,736 | 35,245 |  |
Year 5 | 31,663 | 27,302 |  |
After Year 5 | 154,299 | 152,684 |  |
Loan type | 276,279 | 310,748 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 0 | 3,973 |  |
Year 2 | 5,932 | 15,239 |  |
Year 3 | 23,863 | 23,359 |  |
Year 4 | 24,895 | 18,365 |  |
Year 5 | 19,818 | 10,921 |  |
After Year 5 | 78,104 | 68,878 |  |
Loan type | 152,612 | 140,735 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 0 | 1,683 |  |
Year 2 | 3,787 | 14,481 |  |
Year 3 | 21,089 | 20,147 |  |
Year 4 | 22,246 | 17,080 |  |
Year 5 | 16,608 | 12,632 |  |
After Year 5 | 77,504 | 75,912 |  |
Loan type | 141,234 | 141,935 |  |
Loans In-school/grace/deferment | Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 436,059 | 2,164,479 |  |
Year 2 | 2,726,541 | 1,817,705 |  |
Year 3 | 1,341,863 | 748,782 |  |
Year 4 | 635,952 | 355,248 |  |
Year 5 | 320,015 | 184,377 |  |
After Year 5 | 603,297 | 452,236 |  |
Loan type | 6,063,727 | 5,722,827 |  |
Loans In forbearance | Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 1,040 | 19,984 |  |
Total Private Education Loans in forbearance, Year 2 | 37,981 | 124,728 |  |
Total Private Education Loans in forbearance, Year 3 | 139,850 | 87,961 |  |
Total Private Education Loans in forbearance, Year 4 | 96,559 | 52,686 |  |
Total Private Education Loans in forbearance, Year 5 | 62,922 | 31,575 |  |
Total Private Education Loans in forbearance, After Year 5 | 126,249 | 88,496 |  |
Total Private Education Loans in forbearance | $ 464,601 | $ 405,430 |  |

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