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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Jun. 30, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Dec. 31, 2018
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Private Education Loans allowance for losses | $ (1,734,559) | $ (1,929,323) | $ (441,912) | $ (441,912) | $ (414,406) | $ (383,997) | $ (341,121)
Loans held for investment, net | $ 21,709,771 |  |  | $ 24,667,792 |  |  |
Percentage of Private Education Loans in repayment, 2020 | 58.20% |  |  | 58.20% |  |  |
Percentage of Private Education Loans in repayment, 2019 | 60.20% |  |  | 65.40% |  |  |
Percentage of Private Education Loans in repayment, 2018 | 67.60% |  |  | 71.20% |  |  |
Percentage of Private Education Loans in repayment, 2017 | 71.80% |  |  | 77.20% |  |  |
Percentage of Private Education Loans in repayment, 2016 | 77.40% |  |  | 82.20% |  |  |
Percentage of Private Education Loans in repayment, 2015 and prior | 83.60% |  |  | 85.40% |  |  |
Percentage of Private Education Loans in repayment | 70.10% |  |  | 72.40% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 0.30% |  |  | 0.60% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 1.20% |  |  | 1.20% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 2.60% |  |  | 2.40% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 | 3.50% |  |  | 3.20% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 | 4.00% |  |  | 4.00% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2015 and prior | 4.90% |  |  | 5.00% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.00% |  |  | 2.80% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2020 | 0.30% |  |  | 0.60% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2019 | 1.20% |  |  | 1.20% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2018 | 2.60% |  |  | 2.40% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2017 | 3.50% |  |  | 3.20% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2016 | 4.10% |  |  | 4.00% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2015 and Prior | 4.90% |  |  | 5.00% |  |  |
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans | 3.00% |  |  | 2.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020 | 0.50% |  |  | 0.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 2.00% |  |  | 3.30% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 4.40% |  |  | 4.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 | 5.60% |  |  | 5.20% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 | 5.90% |  |  | 5.20% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015 and Prior | 5.70% |  |  | 4.70% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 4.30% |  |  | 4.10% |  |  |
Consumer Portfolio Segment |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Private Education Loans allowance for losses | $ (114,778) | $ (85,958) |  |  | $ (1,967) | $ (755) |
Personal Loans |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Total | 0 |  |  | $ 1,049,007 |  |  |
Private Education Loans allowance for losses | 0 |  | $ (65,877) | (65,877) |  |  |
Loans held for investment, net | 0 |  |  | 983,643 |  |  |
Student Loan | Consumer Portfolio Segment |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 1,758,337 |  |  |  |  |  |
2019 | 3,031,984 |  |  | 2,340,221 |  |  |
2018 | 2,374,526 |  |  | 3,159,878 |  |  |
2017 | 2,092,242 |  |  | 2,781,132 |  |  |
2016 | 1,920,310 |  |  | 2,566,815 |  |  |
2015 and Prior | 4,197,607 |  |  | 2,225,721 |  |  |
2014 and Prior |  |  |  | 3,241,884 |  |  |
Total | 15,375,006 |  |  | 16,315,651 |  |  |
Total Private Education Loans in repayment, 2019 | 3,068,154 |  |  | 2,355,395 |  |  |
Total Private Education Loans in repayment, 2018 | 2,437,078 |  |  | 3,199,351 |  |  |
Total Private Education Loans in repayment, 2017 | 2,169,127 |  |  | 2,850,380 |  |  |
Total Private Education Loans in repayment, 2016 | 2,001,314 |  |  | 2,651,946 |  |  |
Total Private Education Loans in repayment, 2015 | 4,413,952 |  |  | 2,317,788 |  |  |
Total Private Education Loans in repayment, 2014 and prior |  |  |  | 3,412,810 |  |  |
Total Private Education Loans in repayment | 15,853,309 |  |  | 16,787,670 |  |  |
Total Private Education Loans, gross, 2020 | 3,028,657 |  |  | 4,047,048 |  |  |
Total Private Education Loans, gross, 2019 | 5,095,700 |  |  | 4,888,373 |  |  |
Total Private Education Loans, gross, 2018 | 3,605,096 |  |  | 4,003,485 |  |  |
Total Private Education Loans, gross, 2017 | 3,019,169 |  |  | 3,433,858 |  |  |
Total Private Education Loans, gross, 2016 | 2,585,078 |  |  | 2,821,172 |  |  |
Total Private Education Loans, gross, 2014 and prior | 5,280,627 |  |  | 3,995,655 |  |  |
Total Private Education Loans, gross | 22,614,327 |  |  | 23,189,591 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 16,654 |  |  | 23,661 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 18,633 |  |  | 17,699 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 11,151 |  |  | 13,843 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 | 8,354 |  |  | 12,304 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 | 6,971 |  |  | 8,564 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015 | 8,643 |  |  | 5,153 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 70,406 |  |  | 81,224 |  |  |
Total Private Education Loans, 2020 | 3,045,311 |  |  | 4,070,709 |  |  |
Total Private Education Loans, 2019 | 5,114,333 |  |  | 4,906,072 |  |  |
Total Private Education Loans, 2018 | 3,616,247 |  |  | 4,017,328 |  |  |
Total Private Education Loans, 2017 | 3,027,523 |  |  | 3,446,162 |  |  |
Total Private Education Loans, 2016 | 2,592,049 |  |  | 2,829,736 |  |  |
Total Private Education Loans, 2015 and Prior | 5,289,270 |  |  | 4,000,808 |  |  |
Total Private Education Loans | 22,684,733 |  |  | 23,270,815 |  |  |
Private Education Loans allowance for losses, 2020 | (208,507) |  |  | (3,013) |  |  |
Private Education Loans allowance for losses, 2019 | (407,435) |  |  | (19,105) |  |  |
Private Education Loans allowance for losses, 2018 | (293,033) |  |  | (44,858) |  |  |
Private Education Loans allowance for losses, 2017 | (243,715) |  |  | (71,598) |  |  |
Private Education Loans allowance for losses, 2016 | (199,119) |  |  | (80,974) |  |  |
Private Education Loans allowance for losses, 2015 and prior | (377,002) |  |  | (154,752) |  |  |
Private Education Loans allowance for losses | (1,728,811) |  |  | (374,300) |  |  |
Private Education Loans, net, 2020 | 2,836,804 |  |  | 4,067,696 |  |  |
Private Education Loans, net, 2019 | 4,706,898 |  |  | 4,886,967 |  |  |
Private Education Loans, net, 2018 | 3,323,214 |  |  | 3,972,470 |  |  |
Private Education Loans, net, 2017 | 2,783,808 |  |  | 3,374,564 |  |  |
Private Education Loans, net, 2016 | 2,392,930 |  |  | 2,748,762 |  |  |
Private Education Loans, net, 2015 and prior | 4,912,268 |  |  | 3,846,056 |  |  |
Loans held for investment, net | 20,955,922 |  |  | 22,896,515 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency 31-60 days |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 4,209 |  |  |  |  |  |
2019 | 24,834 |  |  | 11,152 |  |  |
2018 | 37,444 |  |  | 26,096 |  |  |
2017 | 43,055 |  |  | 44,382 |  |  |
2016 | 43,560 |  |  | 51,656 |  |  |
2015 and Prior | 112,149 |  |  | 54,559 |  |  |
2014 and Prior |  |  |  | 100,206 |  |  |
Total | 265,251 |  |  | 288,051 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency 61-90 |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 828 |  |  |  |  |  |
2019 | 8,421 |  |  | 3,087 |  |  |
2018 | 17,289 |  |  | 9,527 |  |  |
2017 | 22,429 |  |  | 17,048 |  |  |
2016 | 24,515 |  |  | 21,161 |  |  |
2015 and Prior | 66,341 |  |  | 24,562 |  |  |
2014 and Prior |  |  |  | 45,917 |  |  |
Total | 139,823 |  |  | 121,302 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquency greater than 90 days |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 310 |  |  |  |  |  |
2019 | 2,915 |  |  | 935 |  |  |
2018 | 7,819 |  |  | 3,850 |  |  |
2017 | 11,401 |  |  | 7,818 |  |  |
2016 | 12,929 |  |  | 12,314 |  |  |
2015 and Prior | 37,855 |  |  | 12,946 |  |  |
2014 and Prior |  |  |  | 24,803 |  |  |
Total | 73,229 |  |  | 62,666 |  |  |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 1,256,630 |  |  |  |  |  |
2019 | 1,963,374 |  |  | 1,670,644 |  |  |
2018 | 1,057,069 |  |  | 1,580,513 |  |  |
2017 | 722,081 |  |  | 1,010,764 |  |  |
2016 | 457,475 |  |  | 635,798 |  |  |
2015 and Prior | 598,932 |  |  | 375,585 |  |  |
2014 and Prior |  |  |  | 414,101 |  |  |
Total | 6,055,561 |  |  | 5,687,405 |  |  |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 8,343 |  |  |  |  |  |
2019 | 64,167 |  |  | 21,009 |  |  |
2018 | 110,935 |  |  | 108,509 |  |  |
2017 | 127,915 |  |  | 142,341 |  |  |
2016 | 126,218 |  |  | 146,114 |  |  |
2015 and Prior | 267,584 |  |  | 127,799 |  |  |
2014 and Prior |  |  |  | 168,744 |  |  |
Total | 705,162 |  |  | 714,516 |  |  |
Total Private Education Loans in forbearance, 2020 | 8,343 |  |  |  |  |  |
Total Private Education Loans in forbearance, 2019 | 64,172 |  |  |  |  |  |
Total Private Education Loans in forbearance, 2018 | 110,949 |  |  |  |  |  |
Total Private Education Loans in forbearance, 2017 | 127,961 |  |  |  |  |  |
Total Private Education Loans in forbearance, 2016 | 126,289 |  |  |  |  |  |
Total Private Education Loans in forbearance, 2015 and Prior | 705,457 |  |  |  |  |  |
2015 and Prior | 267,743 |  |  |  |  |  |
Total Private Education Loans in repayment, 2019 |  |  |  | 21,009 |  |  |
Total Private Education Loans in repayment, 2018 |  |  |  | 108,509 |  |  |
Total Private Education Loans in repayment, 2017 |  |  |  | 142,341 |  |  |
Total Private Education Loans in repayment, 2016 |  |  |  | 146,114 |  |  |
Total Private Education Loans in repayment, 2015 |  |  |  | 127,799 |  |  |
Total Private Education Loans in repayment, 2014 and prior |  |  |  | 168,744 |  |  |
Total Private Education Loans in repayment |  |  |  | 714,516 |  |  |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency 31-60 days |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 0 |  |  |  |  |  |
2019 | 5 |  |  | 0 |  |  |
2018 | 14 |  |  | 0 |  |  |
2017 | 25 |  |  | 0 |  |  |
2016 | 71 |  |  | 0 |  |  |
2015 and Prior | 159 |  |  | 0 |  |  |
2014 and Prior |  |  |  | 0 |  |  |
Total | 274 |  |  | 0 |  |  |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency 61-90 |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 0 |  |  |  |  |  |
2019 | 0 |  |  | 0 |  |  |
2018 | 0 |  |  | 0 |  |  |
2017 | 21 |  |  | 0 |  |  |
2016 | 0 |  |  | 0 |  |  |
2015 and Prior | 0 |  |  | 0 |  |  |
2014 and Prior |  |  |  | 0 |  |  |
Total | 21 |  |  | 0 |  |  |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency greater than 90 days |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
2020 | 0 |  |  |  |  |  |
2019 | 0 |  |  | 0 |  |  |
2018 | 0 |  |  | 0 |  |  |
2017 | 0 |  |  | 0 |  |  |
2016 | 0 |  |  | 0 |  |  |
2015 and Prior | 0 |  |  | 0 |  |  |
2014 and Prior |  |  |  | 0 |  |  |
Total | $ 0 |  |  | $ 0 |  |  |

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