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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,361,723) | $ (441,912) | $ (341,121) | $ (251,475)
Loans held for investment, net | $ 19,183,143 | $ 24,667,792 |  |
Percentage of Private Education Loans in repayment, 2013 and prior |  |  | 82.30% |
Percentage of Private Education Loans in repayment, 2020 | 59.60% |  |  |
Percentage of Private Education Loans in repayment, 2019 | 64.70% | 58.20% |  |
Percentage of Private Education Loans in repayment, 2018 | 71.30% | 65.40% | 58.20% |
Percentage of Private Education Loans in repayment, 2017 | 75.30% | 71.20% | 64.00% |
Percentage of Private Education Loans in repayment, 2016 | 80.00% | 77.20% | 72.00% |
Percentage of Private Education Loans in repayment, 2014 and prior | 84.70% | 85.40% | 78.30% |
Percentage of Private Education Loans in repayment | 72.50% | 72.40% | 71.50% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 0.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 1.20% | 0.60% |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 2.40% | 1.20% | 0.60% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 | 3.20% | 2.40% | 1.20% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 | 3.80% | 3.20% | 2.30% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2015 and prior | 4.80% | 4.00% | 3.10% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2014 and prior |  | 5.00% | 3.80% |
Delinquencies As Percentage Of Loans In Repayment, Originated Six Years Before Current Fiscal YearDelinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2013 and prior |  |  | 5.10% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total | 2.80% | 2.80% | 2.60% |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020 | 0.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 3.40% | 0.90% |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 5.00% | 3.30% | 0.60% |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 | 5.90% | 4.80% | 3.20% |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 | 5.90% | 5.20% | 4.30% |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015 |  | 5.20% | 4.80% |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2014 and prior | 4.90% | 4.70% | 5.00% |
Loans in forbearance as a percentage of loans in repayment and forbearance | 4.30% | 4.10% | 3.80% |
Student Loan | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2020 | $ 2,043,033 |  |  |
2019 | 2,573,228 | $ 2,340,221 |  |
2018 | 2,045,012 | 3,159,878 | $ 2,233,102 |
2017 | 1,850,539 | 2,781,132 | 2,800,142 |
2016 | 1,685,572 | 2,566,815 | 2,769,458 |
2015 and Prior | 3,701,564 | 2,225,721 | 2,533,550 |
2014 and Prior |  | 3,241,884 | 1,865,094 |
2013 and Prior |  |  | 2,088,359 |
Total | 13,898,948 | 16,315,651 | 14,289,705 |
Loans In Repayment, Originated Seven Or More Years Before Current Fiscal Year |  |  | 2,199,514 |
Total Private Education Loans in forbearance, 2019 | 2,603,996 | 2,355,395 |  |
Total Private Education Loans in forbearance, 2018 | 2,096,368 | 3,199,351 | 2,247,526 |
Total Private Education Loans in forbearance, 2017 | 1,912,370 | 2,850,380 | 2,832,910 |
Total Private Education Loans in forbearance, 2016 | 1,752,386 | 2,651,946 | 2,835,181 |
Total Private Education Loans in forbearance, 2015 and Prior | 3,887,180 | 2,317,788 | 2,613,796 |
Total Private Education Loans in repayment, 2014 and prior |  | 3,412,810 | 1,937,929 |
Private Education Loans in forbearance, total | 14,304,821 | 16,787,670 | 14,666,856 |
Total Private Education Loans, gross, 2020 | 3,442,765 |  |  |
Total Private Education Loans, gross, 2019 | 4,026,848 | 4,047,048 |  |
Total Private Education Loans, gross, 2018 | 2,940,511 | 4,888,373 | 3,864,545 |
Total Private Education Loans, gross, 2017 | 2,539,794 | 4,003,485 | 4,423,883 |
Total Private Education Loans, gross, 2016 | 2,189,222 | 3,433,858 | 3,938,366 |
Total Private Education Loans, gross, 2015 |  | 2,821,172 | 3,336,377 |
Total Private Education Loans, gross, 2014 |  |  | 2,353,607 |
Total Private Education Loans, gross, 2013 |  |  | 2,587,687 |
Total Private Education Loans, gross | 19,729,337 | 23,189,591 | 20,504,465 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 21,129 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 13,933 | 23,661 |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 8,671 | 17,699 | 20,433 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 | 6,708 | 13,843 | 16,362 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 | 5,721 | 12,304 | 14,747 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015 |  | 8,564 | 10,422 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2014 | 7,313 | 5,153 | 4,992 |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2013 |  |  | 1,365 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 63,475 | 81,224 | 68,321 |
Total Private Education Loans, 2020 | 3,463,894 |  |  |
Total Private Education Loans, 2019 | 4,040,781 | 4,070,709 |  |
Total Private Education Loans, 2018 | 2,949,182 | 4,906,072 | 3,884,978 |
Total Private Education Loans, 2017 | 2,546,502 | 4,017,328 | 4,440,245 |
Total Private Education Loans, 2016 | 2,194,943 | 3,446,162 | 3,953,113 |
Total Private Education Loans, 2015 |  | 2,829,736 | 3,346,799 |
Total Private Education Loans, 2014 and Prior | 4,597,510 | 4,000,808 | 2,358,599 |
Total Private Education Loans, 2013 and Prior |  |  | 2,589,052 |
Loans, Before Allowance For Credit Loss | 19,792,812 | 23,270,815 | 20,572,786 |
Financing Receivable, Allowance For Credit Loss, Originated Six Or More Years Before Current Fiscal Year |  |  | 90,041 |
Private Education Loans allowance for losses, 2020 | (210,875) |  |  |
Private Education Loans allowance for losses, 2019 | (298,776) | (3,013) |  |
Private Education Loans allowance for losses, 2018 | (218,136) | (19,105) | (2,670) |
Private Education Loans allowance for losses, 2017 | (184,265) | (44,858) | (15,469) |
Private Education Loans allowance for losses, 2016 | (150,150) | (71,598) | (43,469) |
Private Education Loans allowance for losses, 2015 |  | 80,974 | 63,442 |
Private Education Loans allowance for losses, 2015 and prior | (293,642) | (154,752) | (62,852) |
Private Education Loans allowance for losses | (1,355,844) | (374,300) | (277,943) |
Financing Receivable, after Allowance for Credit Loss, Originated Six Or More Years Before Current Fiscal Year |  |  | 2,499,011 |
Private Education Loans, net, 2020 | 3,253,019 |  |  |
Private Education Loans, net, 2019 | 3,742,005 | 4,067,696 |  |
Private Education Loans, net, 2018 | 2,731,046 | 4,886,967 | 3,882,308 |
Private Education Loans, net, 2017 | 2,362,237 | 3,972,470 | 4,424,776 |
Private Education Loans, net, 2016 | 2,044,793 | 3,374,564 | 3,909,644 |
Private Education Loans, net, 2015 |  | 2,748,762 | 3,283,357 |
Private Education Loans, net, 2014 and prior | 4,303,868 | 3,846,056 | 2,295,747 |
Loans held for investment, net | 18,436,968 | 22,896,515 | 20,294,843 |
Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2020 | 6,400 |  |  |
2019 | 16,983 | 11,152 |  |
2018 | 26,934 | 26,096 | 10,263 |
2017 | 30,771 | 44,382 | 22,219 |
2016 | 33,040 | 51,656 | 41,788 |
2015 and Prior | 91,400 | 54,559 | 49,762 |
2014 and Prior |  | 100,206 | 43,291 |
2013 and Prior |  |  | 63,893 |
Total | 205,528 | 288,051 | 231,216 |
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2020 | 2,628 |  |  |
2019 | 9,143 | 3,087 |  |
2018 | 15,026 | 9,527 | 3,521 |
2017 | 18,121 | 17,048 | 7,227 |
2016 | 19,064 | 21,161 | 15,893 |
2015 and Prior | 55,661 | 24,562 | 19,935 |
2014 and Prior |  | 45,917 | 18,561 |
2013 and Prior |  |  | 29,968 |
Total | 119,643 | 121,302 | 95,105 |
Student Loan | Consumer Portfolio Segment | Loans delinquent greater than 90 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2020 | 460 |  |  |
2019 | 4,642 | 935 |  |
2018 | 9,396 | 3,850 | 640 |
2017 | 12,939 | 7,818 | 3,322 |
2016 | 14,710 | 12,314 | 8,042 |
2015 and Prior | 38,555 | 12,946 | 10,549 |
2014 and Prior |  | 24,803 | 10,983 |
2013 and Prior |  |  | 17,294 |
Total | 80,702 | 62,666 | 50,830 |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
2020 | 1,374,085 |  |  |
2019 | 1,330,175 | 1,670,644 |  |
2018 | 733,824 | 1,580,513 | 1,602,476 |
2017 | 508,478 | 1,010,764 | 1,497,955 |
2016 | 327,763 | 635,798 | 975,796 |
2015 and Prior | 504,715 | 375,585 | 589,558 |
2014 and Prior |  | 414,101 | 314,393 |
2013 and Prior |  |  | 280,267 |
Total | 4,779,040 | 5,687,405 | 5,260,445 |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, 2020 | 16,159 |  |  |
Total Private Education Loans in forbearance, 2019 |  | 21,009 |  |
Total Private Education Loans in forbearance, 2018 |  | 108,509 |  |
Total Private Education Loans in forbearance, 2017 |  | 142,341 |  |
Total Private Education Loans in forbearance, 2016 |  | 146,114 |  |
Total Private Education Loans in forbearance, 2015 and Prior |  | 127,799 |  |
Total Private Education Loans in repayment, 2014 and prior |  | 168,744 |  |
Private Education Loans in forbearance, total |  | 714,516 |  |
Total Private Education Loans in forbearance, 2018 | 110,319 |  | 14,543 |
Total Private Education Loans in forbearance, 2017 | 118,946 |  | 93,018 |
Total Private Education Loans in forbearance, 2016 | 109,073 |  | 127,389 |
Private Education Loans in forbearance, total | 645,476 |  | 577,164 |
Total Private Education Loans in forbearance, 2015 and Prior | 198,302 |  | $ 133,023 |
Personal Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total | 0 | 1,049,007 |  |
Private Education Loans allowance for losses | 0 | (65,877) |  |
Loans held for investment, net | $ 0 | $ 983,643 |  |

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