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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,179,021) | $ (1,361,723) | $ (1,673,324) | $ (441,912)
Loans held for investment, net | $ 20,368,287 | $ 19,183,143 |  |
Percentage of Private Education Loans in repayment, 2021 | 55.80% | 59.60% |  |
Percentage of Private Education Loans in repayment, 2020 | 60.20% | 64.70% |  |
Percentage of Private Education Loans in repayment, 2019 | 65.20% | 71.30% |  |
Percentage of Private Education Loans in repayment, 2018 | 71.40% | 75.30% |  |
Percentage of Private Education Loans in repayment, 2017 | 75.20% | 80.00% |  |
Percentage of Private Education Loans in repayment, 2016 and prior | 83.10% | 84.70% |  |
Percentage of Private Education Loans in repayment | 71.20% | 72.50% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021 | 0.10% | 0.50% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 0.30% | 1.20% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 1.00% | 2.40% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 1.90% | 3.20% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 | 2.70% | 3.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 and prior | 3.50% | 4.80% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 2.10% | 2.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2021 | 0.30% | 0.80% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020 | 0.80% | 3.40% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 3.40% | 5.00% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 4.50% | 5.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 | 5.40% | 5.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior | 4.70% | 4.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 3.70% | 4.30% |  |
Student Loan | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2021 | $ 418,103 |  |  |
2020 | 2,747,747 | $ 2,043,033 |  |
2019 | 2,491,380 | 2,573,228 |  |
2018 | 1,955,468 | 2,045,012 |  |
2017 | 1,755,863 | 1,850,539 |  |
2016 and prior | 5,102,866 | 1,685,572 |  |
2015 and Prior |  | 3,701,564 |  |
Total | 14,471,427 | 13,898,948 |  |
Total Private Education Loans in repayment, 2021 | 418,469 |  |  |
Total Private Education Loans in repayment, 2020 | 2,757,024 | 2,052,521 |  |
Total Private Education Loans in repayment, 2019 | 2,517,627 | 2,603,996 |  |
Total Private Education Loans in repayment, 2018 | 1,993,924 | 2,096,368 |  |
Total Private Education Loans in repayment, 2017 | 1,803,993 | 1,912,370 |  |
Total Private Education Loans in repayment, 2016 | 5,286,902 | 1,752,386 |  |
Total Private Education Loans in repayment, 2015 and prior |  | 3,887,180 |  |
Total Private Education Loans in repayment | 14,777,939 | 14,304,821 |  |
Total Private Education Loans, gross, 2021 | 750,072 | 3,442,765 |  |
Total Private Education Loans, gross, 2020 | 4,577,417 | 4,026,848 |  |
Total Private Education Loans, gross, 2019 | 3,862,277 | 2,940,511 |  |
Total Private Education Loans, gross, 2018 | 2,791,354 | 2,539,794 |  |
Total Private Education Loans, gross, 2017 | 2,397,399 | 2,189,222 |  |
Total Private Education Loans, gross, 2016 and prior | 6,363,321 | 4,590,197 |  |
Total Private Education Loans, gross | 20,741,840 | 19,729,337 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021 | 5,487 | 21,129 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 19,922 | 13,933 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 12,925 | 8,671 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 7,977 | 6,708 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 | 6,153 | 5,721 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 | 12,004 | 7,313 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 64,468 | 63,475 |  |
Total Private Education Loans, 2021 | 755,559 | 3,463,894 |  |
Total Private Education Loans, 2020 | 4,597,339 | 4,040,781 |  |
Total Private Education Loans, 2019 | 3,875,202 | 2,949,182 |  |
Total Private Education Loans, 2018 | 2,799,331 | 2,546,502 |  |
Total Private Education Loans, 2017 | 2,403,552 | 2,194,943 |  |
Total Private Education Loans, 2016 and Prior | 6,375,325 | 4,597,510 |  |
Total Private Education Loans | 20,806,308 | 19,792,812 |  |
Private Education Loans allowance for losses, 2021 | (35,515) | (210,875) |  |
Private Education Loans allowance for losses, 2020 | (252,247) | (298,776) |  |
Private Education Loans allowance for losses, 2019 | (239,917) | (218,136) |  |
Private Education Loans allowance for losses, 2018 | (169,296) | (184,265) |  |
Private Education Loans allowance for losses, 2017 | (140,894) | (150,150) |  |
Private Education Loans allowance for losses, 2016 and prior | (335,506) | (293,642) |  |
Private Education Loans allowance for losses | (1,173,375) | (1,355,844) |  |
Private Education Loans, net, 2021 | 720,044 | 3,253,019 |  |
Private Education Loans, net, 2020 | 4,345,092 | 3,742,005 |  |
Private Education Loans, net, 2019 | 3,635,285 | 2,731,046 |  |
Private Education Loans, net, 2018 | 2,630,035 | 2,362,237 |  |
Private Education Loans, net, 2017 | 2,262,658 | 2,044,793 |  |
Private Education Loans, net, 2016 and prior | 6,039,819 | 4,303,868 |  |
Loans held for investment, net | 19,632,933 | 18,436,968 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2021 | 366 |  |  |
2020 | 6,587 | 6,400 |  |
2019 | 14,015 | 16,983 |  |
2018 | 19,145 | 26,934 |  |
2017 | 24,043 | 30,771 |  |
2016 and prior | 88,587 | 33,040 |  |
2015 and Prior |  | 91,400 |  |
Total | 152,743 | 205,528 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2021 | 0 |  |  |
2020 | 1,747 | 2,628 |  |
2019 | 7,654 | 9,143 |  |
2018 | 11,064 | 15,026 |  |
2017 | 13,166 | 18,121 |  |
2016 and prior | 52,759 | 19,064 |  |
2015 and Prior |  | 55,661 |  |
Total | 86,390 | 119,643 |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2021 | 0 |  |  |
2020 | 943 | 460 |  |
2019 | 4,578 | 4,642 |  |
2018 | 8,247 | 9,396 |  |
2017 | 10,921 | 12,939 |  |
2016 and prior | 42,690 | 14,710 |  |
2015 and Prior |  | 38,555 |  |
Total | 67,379 | 80,702 |  |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2021 | 330,417 |  |  |
2020 | 1,798,922 | 1,374,085 |  |
2019 | 1,255,241 | 1,330,175 |  |
2018 | 703,917 | 733,824 |  |
2017 | 489,599 | 508,478 |  |
2016 and prior | 815,578 | 327,763 |  |
2015 and Prior |  | 504,715 |  |
Total | 5,393,674 | 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, 2021 | 1,186 |  |  |
Total Private Education Loans in forbearance, 2020 | 21,471 | 16,159 |  |
Total Private Education Loans in forbearance, 2019 | 89,409 | 92,677 |  |
Total Private Education Loans in forbearance, 2018 | 93,513 | 110,319 |  |
Total Private Education Loans in forbearance, 2017 | 103,807 | 118,946 |  |
Total Private Education Loans in forbearance, 2016 and Prior | 260,841 | 109,073 |  |
Total Private Education Loans in forbearance, 2015 and Prior |  | 198,302 |  |
Total Private Education Loans in forbearance | 570,227 | 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 repayment, 2021 | 482,000 | 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] |  |  |  |
2021 | 273,000 | 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, 2021 | $ 29,000 | $ 30,000 |  |

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