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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021 | Jun. 30, 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 | $ (1,081,066) | $ (1,227,362) | $ (1,165,335) | $ (1,160,244) | $ (1,179,021) | $ (1,361,723)
Loans held for investment, net | $ 19,201,328 |  | $ 20,341,283 |  |  |
Percentage of Private Education Loans in repayment, 2022 | 53.40% |  |  |  |  |
Percentage of Private Education Loans in repayment, 2021 | 60.80% |  | 59.00% |  |  |
Percentage of Private Education Loans in repayment, 2020 | 71.20% |  | 67.90% |  |  |
Percentage of Private Education Loans in repayment, 2019 | 76.90% |  | 74.30% |  |  |
Percentage of Private Education Loans in repayment, 2018 | 81.60% |  | 79.30% |  |  |
Percentage of Private Education Loans in repayment, 2017 and prior | 88.60% |  | 82.90% |  |  |
Percentage of Private Education Loans in repayment, 2016 and prior |  |  | 87.40% |  |  |
Percentage of Private Education Loans in repayment | 74.40% |  | 74.90% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2022 | 0.70% |  |  |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021 | 1.20% |  | 0.80% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 2.40% |  | 1.40% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 3.60% |  | 2.90% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 4.50% |  | 3.70% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 and prior | 6.00% |  | 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 | 3.70% |  | 3.30% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2022 | 0.20% |  |  |  |  |
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 | 0.90% |  | 1.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 1.10% |  | 2.10% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 1.50% |  | 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 | 1.30% |  | 1.90% |  |  |
Student Loan | Consumer Portfolio Segment |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in repayment, 2022 | $ 683,933 |  |  |  |  |
Total Private Education Loans in repayment, 2021 | 2,951,456 |  | $ 2,253,860 |  |  |
Total Private Education Loans in repayment, 2020 | 2,221,088 |  | 2,826,916 |  |  |
Total Private Education Loans in repayment, 2019 | 1,933,334 |  | 2,390,841 |  |  |
Total Private Education Loans in repayment, 2018 | 1,509,043 |  | 1,840,790 |  |  |
Total Private Education Loans in repayment, 2017 | 5,226,405 |  | 1,640,870 |  |  |
Total Private Education Loans in repayment, 2016 and prior |  |  | 4,557,935 |  |  |
Total Private Education Loans in repayment | 14,525,259 |  | 15,511,212 |  |  |
Total Private Education Loans, gross, 2022 | 1,279,752 |  |  |  |  |
Total Private Education Loans, gross, 2021 | 4,854,861 |  | 3,822,361 |  |  |
Total Private Education Loans, gross, 2020 | 3,120,690 |  | 4,166,283 |  |  |
Total Private Education Loans, gross, 2019 | 2,513,490 |  | 3,216,525 |  |  |
Total Private Education Loans, gross, 2018 | 1,848,355 |  | 2,320,060 |  |  |
Total Private Education Loans, gross, 2017 and prior | 5,902,037 |  | 1,978,233 |  |  |
Total Private Education Loans, gross, 2016 and prior |  |  | 5,213,401 |  |  |
Total Private Education Loans, gross | 19,519,185 |  | 20,716,863 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2022 | 14,647 |  |  |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021 | 18,565 |  | 22,169 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 11,502 |  | 16,067 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 6,923 |  | 9,575 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 4,421 |  | 5,918 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 and prior | 10,751 |  | 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 | 66,809 |  | 67,488 |  |  |
Total Private Education Loans, 2022 | 1,294,399 |  |  |  |  |
Total Private Education Loans, 2021 | 4,873,426 |  | 3,844,530 |  |  |
Total Private Education Loans, 2020 | 3,132,192 |  | 4,182,350 |  |  |
Total Private Education Loans, 2019 | 2,520,413 |  | 3,226,100 |  |  |
Total Private Education Loans, 2018 | 1,852,776 |  | 2,325,978 |  |  |
Total Private Education Loans, 2017 and Prior | 5,912,788 |  | 1,982,821 |  |  |
Total Private Education Loans, 2016 and Prior |  |  | 5,222,572 |  |  |
Total Private Education Loans | 19,585,994 |  | 20,784,351 |  |  |
Private Education Loans allowance for losses, 2022 | (118,056) |  |  |  |  |
Private Education Loans allowance for losses, 2021 | (287,653) |  | (248,102) |  |  |
Private Education Loans allowance for losses, 2020 | (168,698) |  | (239,507) |  |  |
Private Education Loans allowance for losses, 2019 | (144,986) |  | (195,223) |  |  |
Private Education Loans allowance for losses, 2018 | (95,567) |  | (129,678) |  |  |
Private Education Loans allowance for losses, 2017 and prior | (259,784) |  | (99,982) |  |  |
Private Education Loans allowance for losses, 2016 and prior |  |  | (246,485) |  |  |
Private Education Loans allowance for losses | (1,074,744) |  | (1,158,977) |  |  |
Private Education Loans, net, 2022 | 1,176,343 |  |  |  |  |
Private Education Loans, net, 2021 | 4,585,773 |  | 3,596,428 |  |  |
Private Education Loans, net, 2020 | 2,963,494 |  | 3,942,843 |  |  |
Private Education Loans, net, 2019 | 2,375,427 |  | 3,030,877 |  |  |
Private Education Loans, net, 2018 | 1,757,209 |  | 2,196,300 |  |  |
Private Education Loans, net, 2017 and prior | 5,653,004 |  | 1,882,839 |  |  |
Private Education Loans, net, 2016 and prior |  |  | 4,976,087 |  |  |
Loans held for investment, net | 18,511,250 |  | 19,625,374 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2022 | 679,111 |  |  |  |  |
2021 | 2,917,375 |  | 2,234,876 |  |  |
2020 | 2,167,862 |  | 2,786,646 |  |  |
2019 | 1,864,069 |  | 2,321,728 |  |  |
2018 | 1,440,757 |  | 1,772,651 |  |  |
2017 and Prior | 4,911,636 |  | 1,570,815 |  |  |
2016 and Prior |  |  | 4,319,057 |  |  |
Total | 13,980,810 |  | 15,005,773 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2022 | 3,513 |  |  |  |  |
2021 | 21,115 |  | 15,148 |  |  |
2020 | 25,780 |  | 29,146 |  |  |
2019 | 31,433 |  | 46,616 |  |  |
2018 | 30,911 |  | 43,197 |  |  |
2017 and Prior | 142,525 |  | 41,695 |  |  |
2016 and Prior |  |  | 132,757 |  |  |
Total | 255,277 |  | 308,559 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2022 | 761 |  |  |  |  |
2021 | 8,512 |  | 3,194 |  |  |
2020 | 15,626 |  | 7,441 |  |  |
2019 | 21,292 |  | 14,044 |  |  |
2018 | 22,816 |  | 14,310 |  |  |
2017 and Prior | 100,649 |  | 16,425 |  |  |
2016 and Prior |  |  | 61,533 |  |  |
Total | 169,656 |  | 116,947 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2022 | 548 |  |  |  |  |
2021 | 4,454 |  | 642 |  |  |
2020 | 11,820 |  | 3,683 |  |  |
2019 | 16,540 |  | 8,453 |  |  |
2018 | 14,559 |  | 10,632 |  |  |
2017 and Prior | 71,595 |  | 11,935 |  |  |
2016 and Prior |  |  | 44,588 |  |  |
Total | 119,516 |  | 79,933 |  |  |
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2022 | 594,307 |  |  |  |  |
2021 | 1,887,781 |  | 1,556,550 |  |  |
2020 | 878,657 |  | 1,283,523 |  |  |
2019 | 557,968 |  | 773,320 |  |  |
2018 | 315,998 |  | 435,657 |  |  |
2017 and Prior | 575,312 |  | 296,008 |  |  |
2016 and Prior |  |  | 559,356 |  |  |
Total | 4,810,023 |  | 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 | 1,512 |  |  |  |  |
Total Private Education Loans in forbearance, 2021 | 15,624 |  | 11,951 |  |  |
Total Private Education Loans in forbearance, 2020 | 20,945 |  | 55,844 |  |  |
Total Private Education Loans in forbearance, 2019 | 22,188 |  | 52,364 |  |  |
Total Private Education Loans in forbearance, 2018 | 23,314 |  | 43,613 |  |  |
Total Private Education Loans in forbearance, 2017 and Prior | 100,320 |  | 41,355 |  |  |
Total Private Education Loans in forbearance, 2016 and Prior |  |  | 96,110 |  |  |
Total Private Education Loans in forbearance | $ 183,903 |  | $ 301,237 |  |  |

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