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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,350,058) | $ (1,339,772) | $ (1,479,306) | $ (1,357,075)
Private Education Loans, net | $ 20,200,789 | $ 20,306,357 |  |
Percentage of Private Education Loans in repayment, Year 1 | 52.10% | 55.60% |  |
Percentage of Private Education Loans in repayment, Year 2 | 56.30% | 65.30% |  |
Percentage of Private Education Loans in repayment, Year 3 | 66.20% | 75.20% |  |
Percentage of Private Education Loans in repayment, Year 4 | 75.10% | 81.10% |  |
Percentage of Private Education Loans in repayment, Year 5 | 80.60% | 85.00% |  |
Percentage of Private Education Loans in repayment, After Year 5 | 88.00% | 89.30% |  |
Percentage of Private Education Loans in repayment | 71.40% | 73.30% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.50% | 1.00% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 1.00% | 2.10% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 2.80% | 3.60% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 3.60% | 4.10% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 3.70% | 4.50% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5 | 5.30% | 6.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.40% | 3.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.20% | 0.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 1.00% | 2.70% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 4.20% | 2.60% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 3.60% | 2.30% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 3.20% | 2.20% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5 | 2.20% | 1.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 2.50% | 2.10% |  |
Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in Repayment, Year 1 | $ 482,998 | $ 2,494,814 |  |
Total Private Education Loans in Repayment, Year 2 | 3,267,486 | 3,323,327 |  |
Total Private Education Loans in Repayment, Year 3 | 2,643,579 | 2,209,734 |  |
Total Private Education Loans in Repayment, Year 4 | 1,842,892 | 1,475,858 |  |
Total Private Education Loans in Repayment, Year 5 | 1,294,286 | 1,386,889 |  |
Total Private Education Loans in Repayment, After Year 5 | 5,430,451 | 4,519,192 |  |
Total Private Education Loans in repayment | 14,961,692 | 15,409,814 |  |
Total Private Education Loans, gross, Year 1 | 926,224 | 4,490,119 |  |
Total Private Education Loans, gross, Year 2 | 5,803,342 | 5,088,739 |  |
Total Private Education Loans, gross, Year 3 | 3,994,361 | 2,937,422 |  |
Total Private Education Loans, gross, Year 4 | 2,454,339 | 1,819,089 |  |
Total Private Education Loans, gross, Year 5 | 1,606,219 | 1,632,300 |  |
Total Private Education Loans, gross, After Year 5 | 6,167,861 | 5,058,175 |  |
Total Private Education Loans, gross | 20,952,346 | 21,025,844 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 10,902 | 35,616 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 31,667 | 18,556 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 14,460 | 9,465 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 7,834 | 5,809 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 5,078 | 3,556 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5 | 10,927 | 8,552 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount) | 80,868 | 81,554 |  |
Total Private Education Loans, Year 1 | 937,126 | 4,525,735 |  |
Total Private Education Loans, Year 2 | 5,835,009 | 5,107,295 |  |
Total Private Education Loans, Year 3 | 4,008,821 | 2,946,887 |  |
Total Private Education Loans, Year 4 | 2,462,173 | 1,824,898 |  |
Total Private Education Loans, Year 5 | 1,611,297 | 1,635,856 |  |
Total Private Education Loans, After Year 5 | 6,178,788 | 5,066,727 |  |
Total Private Education Loans | 21,033,214 | 21,107,398 |  |
Private Education Loans allowance for losses, Year 1 | (67,459) | (269,642) |  |
Private Education Loans allowance for losses, Year 2 | (354,779) | (335,090) |  |
Private Education Loans allowance for losses, Year 3 | (271,004) | (194,104) |  |
Private Education Loans allowance for losses, Year 4 | (164,549) | (118,755) |  |
Private Education Loans allowance for losses, Year 5 | (105,385) | (100,111) |  |
Private Education Loans allowance for losses, After Year 5 | (382,255) | (317,403) |  |
Private Education Loans allowance for losses | (1,345,431) | (1,335,105) |  |
Private Education Loans, net, Year 1 | 869,667 | 4,256,093 |  |
Private Education Loans, net, Year 2 | 5,480,230 | 4,772,205 |  |
Private Education Loans, net, Year 3 | 3,737,817 | 2,752,783 |  |
Private Education Loans, net, Year 4 | 2,297,624 | 1,706,143 |  |
Private Education Loans, net, Year 5 | 1,505,912 | 1,535,745 |  |
Private Education Loans, net, After Year 5 | 5,796,533 | 4,749,324 |  |
Private Education Loans, net | 19,687,783 | 19,772,293 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent, current |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 480,422 | 2,469,817 |  |
Year 2 | 3,236,356 | 3,254,534 |  |
Year 3 | 2,569,306 | 2,131,040 |  |
Year 4 | 1,776,854 | 1,416,069 |  |
Year 5 | 1,245,882 | 1,323,825 |  |
After Year 5 | 5,142,786 | 4,213,986 |  |
Loan type | 14,451,606 | 14,809,271 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 2,576 | 17,599 |  |
Year 2 | 18,739 | 34,627 |  |
Year 3 | 32,476 | 37,147 |  |
Year 4 | 30,460 | 28,020 |  |
Year 5 | 22,553 | 31,432 |  |
After Year 5 | 133,231 | 149,926 |  |
Loan type | 240,035 | 298,751 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 0 | 5,720 |  |
Year 2 | 7,600 | 17,227 |  |
Year 3 | 20,016 | 20,077 |  |
Year 4 | 17,424 | 16,614 |  |
Year 5 | 12,482 | 15,482 |  |
After Year 5 | 76,399 | 75,897 |  |
Loan type | 133,921 | 151,017 |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 0 | 1,678 |  |
Year 2 | 4,791 | 16,939 |  |
Year 3 | 21,781 | 21,470 |  |
Year 4 | 18,154 | 15,155 |  |
Year 5 | 13,369 | 16,150 |  |
After Year 5 | 78,035 | 79,383 |  |
Loan type | 136,130 | 150,775 |  |
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 442,021 | 1,976,040 |  |
Year 2 | 2,501,814 | 1,672,333 |  |
Year 3 | 1,235,053 | 669,250 |  |
Year 4 | 541,726 | 307,781 |  |
Year 5 | 268,959 | 213,593 |  |
After Year 5 | 613,124 | 452,994 |  |
Loan type | 5,602,697 | 5,291,991 |  |
Loans In Forbearance | Consumer Portfolio Segment | Student Loan |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 1,205 | 19,265 |  |
Total Private Education Loans in forbearance, Year 2 | 34,042 | 93,079 |  |
Total Private Education Loans in forbearance, Year 3 | 115,729 | 58,438 |  |
Total Private Education Loans in forbearance, Year 4 | 69,721 | 35,450 |  |
Total Private Education Loans in forbearance, Year 5 | 42,974 | 31,818 |  |
Total Private Education Loans in forbearance, After Year 5 | 124,286 | 85,989 |  |
Total Private Education Loans in forbearance | $ 387,957 | $ 324,039 |  |

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