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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private Education Loans allowance for losses | $ (1,215,407) | $ (1,160,244) | $ (1,361,723) | $ (1,734,559) | $ (1,929,323) | $ (441,912)
Loans held for investment, net | $ 21,281,527 |  | $ 19,183,143 |  |  |
Percentage of Private Education Loans in repayment, 2021 | 57.50% |  |  |  |  |
Percentage of Private Education Loans in repayment, 2020 | 62.00% |  | 59.60% |  |  |
Percentage of Private Education Loans in repayment, 2019 | 67.80% |  | 64.70% |  |  |
Percentage of Private Education Loans in repayment, 2018 | 73.80% |  | 71.30% |  |  |
Percentage of Private Education Loans in repayment, 2017 | 77.90% |  | 75.30% |  |  |
Percentage of Private Education Loans in repayment, 2016 and prior | 85.10% |  | 80.00% |  |  |
Percentage of Private Education Loans in repayment, 2015 and prior |  |  | 84.70% |  |  |
Percentage of Private Education Loans in repayment | 71.40% |  | 72.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2021 | 0.20% |  |  |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020 | 0.80% |  | 0.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019 | 1.90% |  | 1.20% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018 | 2.70% |  | 2.40% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017 | 3.20% |  | 3.20% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016 and prior | 4.00% |  | 3.80% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2015 and prior |  |  | 4.80% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 2.40% |  | 2.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2021 | 0.20% |  |  |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020 | 1.20% |  | 0.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019 | 2.20% |  | 3.40% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018 | 2.80% |  | 5.00% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017 | 3.30% |  | 5.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016 and Prior | 3.10% |  | 5.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015 and Prior |  |  | 4.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 2.30% |  | 4.30% |  |  |
Student Loan | Consumer Portfolio Segment |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in repayment, 2021 | $ 1,834,164 |  |  |  |  |
Total Private Education Loans in repayment, 2020 | 2,767,086 |  | $ 2,052,521 |  |  |
Total Private Education Loans in repayment, 2019 | 2,450,321 |  | 2,603,996 |  |  |
Total Private Education Loans in repayment, 2018 | 1,892,065 |  | 2,096,368 |  |  |
Total Private Education Loans in repayment, 2017 | 1,691,930 |  | 1,912,370 |  |  |
Total Private Education Loans in repayment, 2016 | 4,854,566 |  | 1,752,386 |  |  |
Total Private Education Loans in repayment, 2015 and prior |  |  | 3,887,180 |  |  |
Total Private Education Loans in repayment | 15,490,132 |  | 14,304,821 |  |  |
Total Private Education Loans, gross, 2021 | 3,189,827 |  |  |  |  |
Total Private Education Loans, gross, 2020 | 4,460,869 |  | 3,442,765 |  |  |
Total Private Education Loans, gross, 2019 | 3,612,813 |  | 4,026,848 |  |  |
Total Private Education Loans, gross, 2018 | 2,562,132 |  | 2,940,511 |  |  |
Total Private Education Loans, gross, 2017 | 2,172,125 |  | 2,539,794 |  |  |
Total Private Education Loans, gross, 2016 and prior | 5,705,071 |  | 2,189,222 |  |  |
Total Private Education Loans, gross, 2015 and prior |  |  | 4,590,197 |  |  |
Total Private Education Loans, gross | 21,702,837 |  | 19,729,337 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2021 | 16,763 |  |  |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020 | 17,970 |  | 21,129 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019 | 11,357 |  | 13,933 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018 | 6,901 |  | 8,671 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017 | 5,275 |  | 6,708 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016 and prior | 10,318 |  | 5,721 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015 and prior |  |  | 7,313 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 68,584 |  | 63,475 |  |  |
Total Private Education Loans, 2021 | 3,206,590 |  |  |  |  |
Total Private Education Loans, 2020 | 4,478,839 |  | 3,463,894 |  |  |
Total Private Education Loans, 2019 | 3,624,170 |  | 4,040,781 |  |  |
Total Private Education Loans, 2018 | 2,569,033 |  | 2,949,182 |  |  |
Total Private Education Loans, 2017 | 2,177,400 |  | 2,546,502 |  |  |
Total Private Education Loans, 2016 and Prior | 5,715,389 |  | 2,194,943 |  |  |
Total Private Education Loans, 2015 and Prior |  |  | 4,597,510 |  |  |
Total Private Education Loans | 21,771,421 |  | 19,792,812 |  |  |
Private Education Loans allowance for losses, 2021 | (173,374) |  |  |  |  |
Private Education Loans allowance for losses, 2020 | (252,460) |  | (210,875) |  |  |
Private Education Loans allowance for losses, 2019 | (220,891) |  | (298,776) |  |  |
Private Education Loans allowance for losses, 2018 | (148,040) |  | (218,136) |  |  |
Private Education Loans allowance for losses, 2017 | (118,966) |  | (184,265) |  |  |
Private Education Loans allowance for losses, 2016 and prior | (295,729) |  | (150,150) |  |  |
Private Education Loans allowance for losses, 2015 and prior |  |  | (293,642) |  |  |
Private Education Loans allowance for losses | (1,209,460) |  | (1,355,844) |  |  |
Private Education Loans, net, 2021 | 3,033,216 |  |  |  |  |
Private Education Loans, net, 2020 | 4,226,379 |  | 3,253,019 |  |  |
Private Education Loans, net, 2019 | 3,403,279 |  | 3,742,005 |  |  |
Private Education Loans, net, 2018 | 2,420,993 |  | 2,731,046 |  |  |
Private Education Loans, net, 2017 | 2,058,434 |  | 2,362,237 |  |  |
Private Education Loans, net, 2016 and prior | 5,419,660 |  | 2,044,793 |  |  |
Private Education Loans, net, 2015 and prior |  |  | 4,303,868 |  |  |
Loans held for investment, net | 20,561,961 |  | 18,436,968 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2021 | 1,830,182 |  |  |  |  |
2020 | 2,743,889 |  | 2,043,033 |  |  |
2019 | 2,404,326 |  | 2,573,228 |  |  |
2018 | 1,840,620 |  | 2,045,012 |  |  |
2017 | 1,636,992 |  | 1,850,539 |  |  |
2016 and prior | 4,659,532 |  | 1,685,572 |  |  |
2015 and Prior |  |  | 3,701,564 |  |  |
Total | 15,115,541 |  | 13,898,948 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2021 | 3,051 |  |  |  |  |
2020 | 15,107 |  | 6,400 |  |  |
2019 | 25,793 |  | 16,983 |  |  |
2018 | 27,398 |  | 26,934 |  |  |
2017 | 28,340 |  | 30,771 |  |  |
2016 and prior | 100,253 |  | 33,040 |  |  |
2015 and Prior |  |  | 91,400 |  |  |
Total | 199,942 |  | 205,528 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2021 | 650 |  |  |  |  |
2020 | 5,787 |  | 2,628 |  |  |
2019 | 12,784 |  | 9,143 |  |  |
2018 | 14,313 |  | 15,026 |  |  |
2017 | 15,251 |  | 18,121 |  |  |
2016 and prior | 52,727 |  | 19,064 |  |  |
2015 and Prior |  |  | 55,661 |  |  |
Total | 101,512 |  | 119,643 |  |  |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2021 | 281 |  |  |  |  |
2020 | 2,303 |  | 460 |  |  |
2019 | 7,418 |  | 4,642 |  |  |
2018 | 9,734 |  | 9,396 |  |  |
2017 | 11,347 |  | 12,939 |  |  |
2016 and prior | 42,054 |  | 14,710 |  |  |
2015 and Prior |  |  | 38,555 |  |  |
Total | 73,137 |  | 80,702 |  |  |
Student Loan | Loans in-school/grace/deferment | Consumer Portfolio Segment |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
2021 | 1,351,990 |  |  |  |  |
2020 | 1,661,199 |  | 1,374,085 |  |  |
2019 | 1,108,155 |  | 1,330,175 |  |  |
2018 | 616,392 |  | 733,824 |  |  |
2017 | 423,102 |  | 508,478 |  |  |
2016 and prior | 694,442 |  | 327,763 |  |  |
2015 and Prior |  |  | 504,715 |  |  |
Total | 5,855,280 |  | 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 | 3,673 |  |  |  |  |
Total Private Education Loans in forbearance, 2020 | 32,584 |  | 16,159 |  |  |
Total Private Education Loans in forbearance, 2019 | 54,337 |  | 92,677 |  |  |
Total Private Education Loans in forbearance, 2018 | 53,675 |  | 110,319 |  |  |
Total Private Education Loans in forbearance, 2017 | 57,093 |  | 118,946 |  |  |
Total Private Education Loans in forbearance, 2016 and Prior | 156,063 |  | 109,073 |  |  |
Total Private Education Loans in forbearance, 2015 and Prior |  |  | 198,302 |  |  |
Total Private Education Loans in forbearance | $ 357,425 |  | 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 |  |  | 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 |  |  | 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 |  |  | $ 30,000 |  |  |

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