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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Mar. 31, 2022 | Dec. 31, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,227,362) | $ (1,165,335) | $ (1,179,021) | $ (1,361,723)
Loans held for investment, net | $ 21,291,675 | $ 20,341,283 |  |
Percentage of Private Education Loans in repayment, 2022 | 53.40% |  |  |
Percentage of Private Education Loans in repayment, 2021 | 59.50% | 59.00% |  |
Percentage of Private Education Loans in repayment, 2020 | 69.10% | 67.90% |  |
Percentage of Private Education Loans in repayment, 2019 | 75.20% | 74.30% |  |
Percentage of Private Education Loans in repayment, 2018 | 80.40% | 79.30% |  |
Percentage of Private Education Loans in repayment, 2017 and prior | 87.70% | 82.90% |  |
Percentage of Private Education Loans in repayment, 2016 and prior |  | 87.40% |  |
Percentage of Private Education Loans in repayment | 74.10% | 74.90% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2022 | 0.40% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021 | 0.90% | 0.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 2.20% | 1.40% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 3.50% | 2.90% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 4.30% | 3.70% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 and prior | 5.40% | 4.30% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 and prior |  | 5.20% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.50% | 3.30% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2022 | 0.10% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2021 | 0.50% | 0.50% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020 | 1.30% | 1.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 1.50% | 2.10% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 1.70% | 2.30% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 and Prior | 1.90% | 2.50% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior |  | 2.10% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 1.40% | 1.90% |  |
Student Loan | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in repayment, 2022 | $ 417,417 |  |  |
Total Private Education Loans in repayment, 2021 | 3,000,284 | $ 2,253,860 |  |
Total Private Education Loans in repayment, 2020 | 2,751,187 | 2,826,916 |  |
Total Private Education Loans in repayment, 2019 | 2,285,817 | 2,390,841 |  |
Total Private Education Loans in repayment, 2018 | 1,749,113 | 1,840,790 |  |
Total Private Education Loans in repayment, 2017 | 5,891,339 | 1,640,870 |  |
Total Private Education Loans in repayment, 2016 and prior |  | 4,557,935 |  |
Total Private Education Loans in repayment | 16,095,157 | 15,511,212 |  |
Total Private Education Loans, gross, 2022 | 781,612 |  |  |
Total Private Education Loans, gross, 2021 | 5,040,792 | 3,822,361 |  |
Total Private Education Loans, gross, 2020 | 3,983,606 | 4,166,283 |  |
Total Private Education Loans, gross, 2019 | 3,038,337 | 3,216,525 |  |
Total Private Education Loans, gross, 2018 | 2,174,288 | 2,320,060 |  |
Total Private Education Loans, gross, 2017 and prior | 6,716,734 | 1,978,233 |  |
Total Private Education Loans, gross, 2016 and prior |  | 5,213,401 |  |
Total Private Education Loans, gross | 21,735,369 | 20,716,863 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2022 | 9,615 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021 | 20,732 | 22,169 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 14,883 | 16,067 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 8,753 | 9,575 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 5,380 | 5,918 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 and prior | 12,544 | 4,588 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 and prior |  | 9,171 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 71,907 | 67,488 |  |
Total Private Education Loans, 2022 | 791,227 |  |  |
Total Private Education Loans, 2021 | 5,061,524 | 3,844,530 |  |
Total Private Education Loans, 2020 | 3,998,489 | 4,182,350 |  |
Total Private Education Loans, 2019 | 3,047,090 | 3,226,100 |  |
Total Private Education Loans, 2018 | 2,179,668 | 2,325,978 |  |
Total Private Education Loans, 2017 and Prior | 6,729,278 | 1,982,821 |  |
Total Private Education Loans, 2016 and Prior |  | 5,222,572 |  |
Total Private Education Loans | 21,807,276 | 20,784,351 |  |
Private Education Loans allowance for losses, 2022 | (54,769) |  |  |
Private Education Loans allowance for losses, 2021 | (308,664) | (248,102) |  |
Private Education Loans allowance for losses, 2020 | (230,000) | (239,507) |  |
Private Education Loans allowance for losses, 2019 | (187,181) | (195,223) |  |
Private Education Loans allowance for losses, 2018 | (122,741) | (129,678) |  |
Private Education Loans allowance for losses, 2017 and prior | (317,698) | (99,982) |  |
Private Education Loans allowance for losses, 2016 and prior |  | (246,485) |  |
Private Education Loans allowance for losses | (1,221,053) | (1,158,977) |  |
Private Education Loans, net, 2022 | 736,458 |  |  |
Private Education Loans, net, 2021 | 4,752,860 | 3,596,428 |  |
Private Education Loans, net, 2020 | 3,768,489 | 3,942,843 |  |
Private Education Loans, net, 2019 | 2,859,909 | 3,030,877 |  |
Private Education Loans, net, 2018 | 2,056,927 | 2,196,300 |  |
Private Education Loans, net, 2017 and prior | 6,411,580 | 1,882,839 |  |
Private Education Loans, net, 2016 and prior |  | 4,976,087 |  |
Loans held for investment, net | 20,586,223 | 19,625,374 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2022 | 415,657 |  |  |
2021 | 2,972,685 | 2,234,876 |  |
2020 | 2,690,157 | 2,786,646 |  |
2019 | 2,205,417 | 2,321,728 |  |
2018 | 1,674,303 | 1,772,651 |  |
2017 and Prior | 5,571,861 | 1,570,815 |  |
2016 and Prior |  | 4,319,057 |  |
Total | 15,530,080 | 15,005,773 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2022 | 1,760 |  |  |
2021 | 17,435 | 15,148 |  |
2020 | 28,523 | 29,146 |  |
2019 | 34,956 | 46,616 |  |
2018 | 33,362 | 43,197 |  |
2017 and Prior | 144,499 | 41,695 |  |
2016 and Prior |  | 132,757 |  |
Total | 260,535 | 308,559 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2022 | 0 |  |  |
2021 | 6,557 | 3,194 |  |
2020 | 18,618 | 7,441 |  |
2019 | 25,590 | 14,044 |  |
2018 | 23,023 | 14,310 |  |
2017 and Prior | 95,272 | 16,425 |  |
2016 and Prior |  | 61,533 |  |
Total | 169,060 | 116,947 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2022 | 0 |  |  |
2021 | 3,607 | 642 |  |
2020 | 13,889 | 3,683 |  |
2019 | 19,854 | 8,453 |  |
2018 | 18,425 | 10,632 |  |
2017 and Prior | 79,707 | 11,935 |  |
2016 and Prior |  | 44,588 |  |
Total | 135,482 | 79,933 |  |
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2022 | 363,754 |  |  |
2021 | 2,025,385 | 1,556,550 |  |
2020 | 1,195,182 | 1,283,523 |  |
2019 | 717,261 | 773,320 |  |
2018 | 394,590 | 435,657 |  |
2017 and Prior | 709,780 | 296,008 |  |
2016 and Prior |  | 559,356 |  |
Total | 5,405,952 | 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, 2022 | 441 |  |  |
Total Private Education Loans in forbearance, 2021 | 15,123 | 11,951 |  |
Total Private Education Loans in forbearance, 2020 | 37,237 | 55,844 |  |
Total Private Education Loans in forbearance, 2019 | 35,259 | 52,364 |  |
Total Private Education Loans in forbearance, 2018 | 30,585 | 43,613 |  |
Total Private Education Loans in forbearance, 2017 and Prior | 115,615 | 41,355 |  |
Total Private Education Loans in forbearance, 2016 and Prior |  | 96,110 |  |
Total Private Education Loans in forbearance | $ 234,260 | $ 301,237 |  |

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