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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,339,772) | $ (1,357,075) | $ (1,165,335) | $ (1,361,723)
Private Education Loans, net | $ 20,306,357 | $ 19,626,868 |  |
Percentage of Private Education Loans in repayment, Year 1 | 55.60% | 56.90% | 59.00% |
Percentage of Private Education Loans in repayment, Year 2 | 65.30% | 67.10% | 67.90% |
Percentage of Private Education Loans in repayment, Year 3 | 75.20% | 76.10% | 74.30% |
Percentage of Private Education Loans in repayment, Year 4 | 81.10% | 80.70% | 79.30% |
Percentage of Private Education Loans in repayment, Year 5 | 85.00% | 84.50% | 82.90% |
Percentage of Private Education Loans in repayment, Year 6 | 89.30% | 89.30% | 87.40% |
Percentage of Private Education Loans in repayment | 73.30% | 74.50% | 74.90% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 1.00% | 0.90% | 0.80% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 2.10% | 1.90% | 1.40% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.60% | 3.10% | 2.90% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 4.10% | 4.00% | 3.70% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.50% | 4.70% | 4.30% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6 | 6.80% | 6.50% | 5.20% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total | 3.90% | 3.80% | 3.30% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.80% | 0.70% | 0.50% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 2.70% | 2.10% | 1.90% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 2.60% | 1.90% | 2.10% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 2.30% | 2.10% | 2.30% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 2.20% | 2.20% | 2.50% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6 | 1.90% | 2.00% | 2.10% |
Loans in forbearance as a percentage of loans in repayment and forbearance | 2.10% | 1.80% | 1.90% |
Student Loan | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | $ 2,494,814 | $ 2,432,838 | $ 2,253,860 |
Total Private Education Loans in forbearance, Year 2 | 3,323,327 | 3,049,477 | 2,826,916 |
Total Private Education Loans in forbearance, Year 3 | 2,209,734 | 1,967,756 | 2,390,841 |
Total Private Education Loans in forbearance, Year 4 | 1,475,858 | 1,754,219 | 1,840,790 |
Total Private Education Loans in forbearance, Year 5 | 1,386,889 | 1,365,979 | 1,640,870 |
Total Private Education Loans in forbearance, Year 6 | 4,519,192 | 4,559,281 | 4,557,935 |
Private Education Loans in forbearance, total | 15,409,814 | 15,129,550 | 15,511,212 |
Total Private Education Loans, gross, Year 1 | 4,490,119 | 4,276,533 | 3,822,361 |
Total Private Education Loans, gross, Year 2 | 5,088,739 | 4,547,159 | 4,166,283 |
Total Private Education Loans, gross, Year 3 | 2,937,422 | 2,584,622 | 3,216,525 |
Total Private Education Loans, gross, Year 4 | 1,819,089 | 2,172,660 | 2,320,060 |
Total Private Education Loans, gross, Year 5 | 1,632,300 | 1,615,842 | 1,978,233 |
Total Private Education Loans, gross, Year 6 | 5,058,175 | 5,106,872 | 5,213,401 |
Total Private Education Loans, gross | 21,025,844 | 20,303,688 | 20,716,863 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 35,616 | 26,714 | 22,169 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 18,556 | 15,933 | 16,067 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 9,465 | 9,062 | 9,575 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 5,809 | 5,496 | 5,918 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 3,556 | 3,575 | 4,588 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6 | 8,552 | 8,876 | 9,171 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 81,554 | 69,656 | 67,488 |
Total Private Education Loans, Year 1 | 4,525,735 | 4,303,247 | 3,844,530 |
Total Private Education Loans, Year 2 | 5,107,295 | 4,563,092 | 4,182,350 |
Total Private Education Loans, Year 3 | 2,946,887 | 2,593,684 | 3,226,100 |
Total Private Education Loans, Year 4 | 1,824,898 | 2,178,156 | 2,325,978 |
Total Private Education Loans, Year 5 | 1,635,856 | 1,619,417 | 1,982,821 |
Total Private Education Loans, Year 6 | 5,066,727 | 5,115,748 | 5,222,572 |
Total Private Education Loans | 21,107,398 | 20,373,344 | 20,784,351 |
Private Education Loans allowance for losses, Year 1 | (269,642) | (304,943) | (248,102) |
Private Education Loans allowance for losses, Year 2 | (335,090) | (323,506) | (239,507) |
Private Education Loans allowance for losses, Year 3 | (194,104) | (181,915) | (195,223) |
Private Education Loans allowance for losses, Year 4 | (118,755) | (141,424) | (129,678) |
Private Education Loans allowance for losses, Year 5 | (100,111) | (101,023) | (99,982) |
Private Education Loans allowance for losses, Year 6 | (317,403) | (300,820) | (246,485) |
Private Education Loans allowance for losses | (1,335,105) | (1,353,631) | (1,158,977) |
Private Education Loans, net, Year 1 | 4,256,093 | 3,998,304 | 3,596,428 |
Private Education Loans, net, Year 2 | 4,772,205 | 4,239,586 | 3,942,843 |
Private Education Loans, net, Year 3 | 2,752,783 | 2,411,769 | 3,030,877 |
Private Education Loans, net, Year 4 | 1,706,143 | 2,036,732 | 2,196,300 |
Private Education Loans, net, Year 5 | 1,535,745 | 1,518,394 | 1,882,839 |
Private Education Loans, net, Year 6 | 4,749,324 | 4,814,928 | 4,976,087 |
Private Education Loans, net | 19,772,293 | 19,019,713 | 19,625,374 |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 2,469,817 | 2,411,441 | 2,234,876 |
Year 2 | 3,254,534 | 2,991,839 | 2,786,646 |
Year 3 | 2,131,040 | 1,907,574 | 2,321,728 |
Year 4 | 1,416,069 | 1,683,986 | 1,772,651 |
Year 5 | 1,323,825 | 1,301,809 | 1,570,815 |
After Year 5 | 4,213,986 | 4,262,698 | 4,319,057 |
Loan type | 14,809,271 | 14,559,347 | 15,005,773 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 17,599 | 14,164 | 15,148 |
Year 2 | 34,627 | 30,740 | 29,146 |
Year 3 | 37,147 | 30,877 | 46,616 |
Year 4 | 28,020 | 35,213 | 43,197 |
Year 5 | 31,432 | 31,366 | 41,695 |
After Year 5 | 149,926 | 144,948 | 132,757 |
Loan type | 298,751 | 287,308 | 308,559 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 5,720 | 5,523 | 3,194 |
Year 2 | 17,227 | 15,056 | 7,441 |
Year 3 | 20,077 | 14,433 | 14,044 |
Year 4 | 16,614 | 18,201 | 14,310 |
Year 5 | 15,482 | 16,697 | 16,425 |
After Year 5 | 75,897 | 77,595 | 61,533 |
Loan type | 151,017 | 147,505 | 116,947 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 1,678 | 1,710 | 642 |
Year 2 | 16,939 | 11,842 | 3,683 |
Year 3 | 21,470 | 14,872 | 8,453 |
Year 4 | 15,155 | 16,819 | 10,632 |
Year 5 | 16,150 | 16,107 | 11,935 |
After Year 5 | 79,383 | 74,040 | 44,588 |
Loan type | 150,775 | 135,390 | 79,933 |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 1,976,040 | 1,827,649 | 1,556,550 |
Year 2 | 1,672,333 | 1,433,322 | 1,283,523 |
Year 3 | 669,250 | 578,253 | 773,320 |
Year 4 | 307,781 | 380,639 | 435,657 |
Year 5 | 213,593 | 219,280 | 296,008 |
After Year 5 | 452,994 | 455,910 | 559,356 |
Loan type | 5,291,991 | 4,895,053 | 4,904,414 |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 19,265 | 16,046 | 11,951 |
Total Private Education Loans in forbearance, Year 2 | 93,079 | 64,360 | 55,844 |
Total Private Education Loans in forbearance, Year 3 | 58,438 | 38,613 | 52,364 |
Total Private Education Loans in forbearance, Year 4 | 35,450 | 37,802 | 43,613 |
Total Private Education Loans in forbearance, Year 5 | 31,818 | 30,583 | 41,355 |
Total Private Education Loans in forbearance, Year 6 | 85,989 | 91,681 | 96,110 |
Private Education Loans in forbearance, total | $ 324,039 | $ 279,085 | $ 301,237 |

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