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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Private Education Loans allowance for losses | $ (1,357,075) | $ (1,165,335) | $ (1,361,723) | $ (1,143,053) | $ (441,912)
Loans held for investment, net | $ 19,626,868 | $ 20,341,283 |  |  |
Percentage of Private Education Loans in repayment, Year 1 | 56.90% | 59.00% | 59.60% |  |
Percentage of Private Education Loans in repayment, Year 2 | 67.10% | 67.90% | 64.70% |  |
Percentage of Private Education Loans in repayment, Year 3 | 76.10% | 74.30% | 71.30% |  |
Percentage of Private Education Loans in repayment, Year 4 | 80.70% | 79.30% | 75.30% |  |
Percentage of Private Education Loans in repayment, Year 5 | 84.50% | 82.90% | 80.00% |  |
Percentage of Private Education Loans in repayment, Year 6 | 89.30% | 87.40% | 84.70% |  |
Percentage of Private Education Loans in repayment | 74.50% | 74.90% | 72.50% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.90% | 0.80% | 0.50% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 1.90% | 1.40% | 1.20% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.10% | 2.90% | 2.40% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 4.00% | 3.70% | 3.20% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.70% | 4.30% | 3.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6 | 6.50% | 5.20% | 4.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total | 3.80% | 3.30% | 2.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.70% | 0.50% | 0.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 2.10% | 1.90% | 3.40% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 1.90% | 2.10% | 5.00% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 2.10% | 2.30% | 5.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 2.20% | 2.50% | 5.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6 | 2.00% | 2.10% | 4.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 1.80% | 1.90% | 4.30% |  |
Student Loan | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 | $ 2,432,838 | $ 2,253,860 | $ 2,052,521 |  |
Total Private Education Loans in forbearance, Year 2 | 3,049,477 | 2,826,916 | 2,603,996 |  |
Total Private Education Loans in forbearance, Year 3 | 1,967,756 | 2,390,841 | 2,096,368 |  |
Total Private Education Loans in forbearance, Year 4 | 1,754,219 | 1,840,790 | 1,912,370 |  |
Total Private Education Loans in forbearance, Year 5 | 1,365,979 | 1,640,870 | 1,752,386 |  |
Total Private Education Loans in forbearance, Year 6 | 4,559,281 | 4,557,935 | 3,887,180 |  |
Private Education Loans in forbearance, total | 15,129,550 | 15,511,212 | 14,304,821 |  |
Total Private Education Loans, gross, Year 1 | 4,276,533 | 3,822,361 | 3,442,765 |  |
Total Private Education Loans, gross, Year 2 | 4,547,159 | 4,166,283 | 4,026,848 |  |
Total Private Education Loans, gross, Year 3 | 2,584,622 | 3,216,525 | 2,940,511 |  |
Total Private Education Loans, gross, Year 4 | 2,172,660 | 2,320,060 | 2,539,794 |  |
Total Private Education Loans, gross, Year 5 | 1,615,842 | 1,978,233 | 2,189,222 |  |
Total Private Education Loans, gross, Year 6 | 5,106,872 | 5,213,401 | 4,590,197 |  |
Total Private Education Loans, gross | 20,303,688 | 20,716,863 | 19,729,337 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 26,714 | 22,169 | 21,129 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 15,933 | 16,067 | 13,933 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 9,062 | 9,575 | 8,671 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 5,496 | 5,918 | 6,708 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 3,575 | 4,588 | 5,721 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6 | 8,876 | 9,171 | 7,313 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 69,656 | 67,488 | 63,475 |  |
Total Private Education Loans, Year 1 | 4,303,247 | 3,844,530 | 3,463,894 |  |
Total Private Education Loans, Year 2 | 4,563,092 | 4,182,350 | 4,040,781 |  |
Total Private Education Loans, Year 3 | 2,593,684 | 3,226,100 | 2,949,182 |  |
Total Private Education Loans, Year 4 | 2,178,156 | 2,325,978 | 2,546,502 |  |
Total Private Education Loans, Year 5 | 1,619,417 | 1,982,821 | 2,194,943 |  |
Total Private Education Loans, Year 6 | 5,115,748 | 5,222,572 | 4,597,510 |  |
Total Private Education Loans | 20,373,344 | 20,784,351 | 19,792,812 |  |
Private Education Loans allowance for losses, Year 1 | (304,943) | (248,102) | (210,875) |  |
Private Education Loans allowance for losses, Year 2 | (323,506) | (239,507) | (298,776) |  |
Private Education Loans allowance for losses, Year 3 | (181,915) | (195,223) | (218,136) |  |
Private Education Loans allowance for losses, Year 4 | (141,424) | (129,678) | (184,265) |  |
Private Education Loans allowance for losses, Year 5 | (101,023) | (99,982) | (150,150) |  |
Private Education Loans allowance for losses, Year 6 | (300,820) | (246,485) | (293,642) |  |
Private Education Loans allowance for losses | (1,353,631) | (1,158,977) | (1,355,844) |  |
Private Education Loans, net, Year 1 | 3,998,304 | 3,596,428 | 3,253,019 |  |
Private Education Loans, net, Year 2 | 4,239,586 | 3,942,843 | 3,742,005 |  |
Private Education Loans, net, Year 3 | 2,411,769 | 3,030,877 | 2,731,046 |  |
Private Education Loans, net, Year 4 | 2,036,732 | 2,196,300 | 2,362,237 |  |
Private Education Loans, net, Year 5 | 1,518,394 | 1,882,839 | 2,044,793 |  |
Private Education Loans, net, Year 6 | 4,814,928 | 4,976,087 | 4,303,868 |  |
Loans held for investment, net | 19,019,713 | 19,625,374 | 18,436,968 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 | 2,411,441 | 2,234,876 | 2,043,033 |  |
Year 2 | 2,991,839 | 2,786,646 | 2,573,228 |  |
Year 3 | 1,907,574 | 2,321,728 | 2,045,012 |  |
Year 4 | 1,683,986 | 1,772,651 | 1,850,539 |  |
Year 5 | 1,301,809 | 1,570,815 | 1,685,572 |  |
After Year 5 | 4,262,698 | 4,319,057 | 3,701,564 |  |
Loan type | 14,559,347 | 15,005,773 | 13,898,948 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 | 14,164 | 15,148 | 6,400 |  |
Year 2 | 30,740 | 29,146 | 16,983 |  |
Year 3 | 30,877 | 46,616 | 26,934 |  |
Year 4 | 35,213 | 43,197 | 30,771 |  |
Year 5 | 31,366 | 41,695 | 33,040 |  |
After Year 5 | 144,948 | 132,757 | 91,400 |  |
Loan type | 287,308 | 308,559 | 205,528 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 | 5,523 | 3,194 | 2,628 |  |
Year 2 | 15,056 | 7,441 | 9,143 |  |
Year 3 | 14,433 | 14,044 | 15,026 |  |
Year 4 | 18,201 | 14,310 | 18,121 |  |
Year 5 | 16,697 | 16,425 | 19,064 |  |
After Year 5 | 77,595 | 61,533 | 55,661 |  |
Loan type | 147,505 | 116,947 | 119,643 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 | 1,710 | 642 | 460 |  |
Year 2 | 11,842 | 3,683 | 4,642 |  |
Year 3 | 14,872 | 8,453 | 9,396 |  |
Year 4 | 16,819 | 10,632 | 12,939 |  |
Year 5 | 16,107 | 11,935 | 14,710 |  |
After Year 5 | 74,040 | 44,588 | 38,555 |  |
Loan type | 135,390 | 79,933 | 80,702 |  |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 | 1,827,649 | 1,556,550 | 1,374,085 |  |
Year 2 | 1,433,322 | 1,283,523 | 1,330,175 |  |
Year 3 | 578,253 | 773,320 | 733,824 |  |
Year 4 | 380,639 | 435,657 | 508,478 |  |
Year 5 | 219,280 | 296,008 | 327,763 |  |
After Year 5 | 455,910 | 559,356 | 504,715 |  |
Loan type | 4,895,053 | 4,904,414 | 4,779,040 |  |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 16,046 | 11,951 | 16,159 |  |
Total Private Education Loans in forbearance, Year 2 | 64,360 | 55,844 | 92,677 |  |
Total Private Education Loans in forbearance, Year 3 | 38,613 | 52,364 | 110,319 |  |
Total Private Education Loans in forbearance, Year 4 | 37,802 | 43,613 | 118,946 |  |
Total Private Education Loans in forbearance, Year 5 | 30,583 | 41,355 | 109,073 |  |
Total Private Education Loans in forbearance, Year 6 | 91,681 | 96,110 | 198,302 |  |
Private Education Loans in forbearance, total | $ 279,085 | $ 301,237 | 645,476 |  |
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 |  |  | 609,000 |  |
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Year 1 |  |  | 401,000 |  |
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 |  |  | $ 30,000 |  |

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