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Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | $ 441,912 | $ 441,912 | $ 341,121 | $ 251,475
Transfer from unfunded commitment liability |  |  | 320,808 |  |
Provision for current period |  |  | 190,898 |  |
Loan sale reduction to provision |  |  | (204,709) |  |
Loan transfer to held-for-sale |  |  | (205,669) |  |
Total provision |  |  | (219,480) | 353,934 | 244,584
Net charge-offs: |  |  |  |  |
Charge-offs |  |  | (245,043) | (284,114) | (175,526)
Recoveries |  |  | 29,007 | 30,971 | 21,804
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery |  |  | (216,036) | (253,143) | (153,722)
Loan sales |  |  | (108,534) |  | (1,216)
Ending Balance |  |  | 1,361,723 | 441,912 | 341,121
Allowance: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 104,265 | 186,697 | 120,110
Ending balance: collectively evaluated for impairment |  |  | 1,257,458 | 255,215 | 221,011
Loans: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 1,274,590 | 1,581,966 | 1,257,856
Ending balance: collectively evaluated for impairment |  |  | 19,204,578 | 23,443,822 | 21,283,187
Provision for loan losses |  |  | (218,789) |  |
Provision for unfunded commitments |  |  | 312,613 |  |
Total Private Education Loan provision for credit losses |  |  | 93,824 |  |
Additional provisions related to other impacts |  |  | (691) |  |
Less: provisions for credit losses |  | 121,000 | 93,133 |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 1,584,965 | 1,584,965 |  |
Day 1 adjustment for the adoption of CECL |  |  |  | 1,143,053 |
Net charge-offs: |  |  |  |  |
Ending Balance |  |  |  | 1,584,965 |
FFELP Loans |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 1,633 | 1,633 | 977 | 1,132
Transfer from unfunded commitment liability |  |  | 0 |  |
Provision for current period |  |  | 412 |  |
Loan sale reduction to provision |  |  | 0 |  |
Loan transfer to held-for-sale |  |  | 0 |  |
Total provision |  |  | 412 | 1,478 | 980
Net charge-offs: |  |  |  |  |
Charge-offs |  |  | (519) | (822) | (1,135)
Recoveries |  |  | 0 | 0 | 0
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery |  |  | (519) | (822) | (1,135)
Loan sales |  |  | 0 |  | 0
Ending Balance |  |  | 4,378 | 1,633 | 977
Allowance: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 | 0
Ending balance: collectively evaluated for impairment |  |  | 4,378 | 1,633 | 977
Loans: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 | 0
Ending balance: collectively evaluated for impairment |  |  | $ 737,593 | $ 783,306 | $ 846,487
Net charge-offs as a percentage of average loans in repayment (annualized) |  |  | 0.09% | 0.13% | 0.16%
Allowance as a percentage of the ending total loan balance |  |  | 0.59% | 0.21% | 0.12%
Allowance as a percentage of the ending loans in repayment |  |  | 0.76% | 0.26% | 0.15%
Allowance coverage of net charge-offs (annualized) |  |  | 8.44 | 1.99 | 0.86
Ending total loans, gross |  |  | $ 737,593 | $ 783,306 | $ 846,487
Average loans in repayment |  |  | 549,584 | 631,029 | 691,406
Ending loans in repayment |  |  | 573,361 | 617,646 | 665,807
Additional provisions related to other impacts |  |  | 412 |  |
FFELP Loans | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 4,485 | 4,485 |  |
Day 1 adjustment for the adoption of CECL |  |  |  | 2,852 |
Net charge-offs: |  |  |  |  |
Ending Balance |  |  |  | 4,485 |
Private Education Loans |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 374,300 | 374,300 | 277,943 | 243,715
Transfer from unfunded commitment liability |  |  | 320,808 |  |
Provision for current period |  |  | 148,673 |  |
Loan sale reduction to provision |  |  | (161,793) |  |
Loan transfer to held-for-sale |  |  | (205,669) |  |
Total provision |  |  | (218,789) | 279,570 | 169,287
Net charge-offs: |  |  |  |  |
Charge-offs |  |  | (205,326) | (208,978) | (154,701)
Recoveries |  |  | 24,021 | 25,765 | 20,858
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery |  |  | (181,305) | (183,213) | (133,843)
Loan sales |  |  | 0 |  | (1,216)
Ending Balance |  |  | 1,355,844 | 374,300 | 277,943
Allowance: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 104,265 | 186,697 | 120,110
Ending balance: collectively evaluated for impairment |  |  | 1,251,579 | 187,603 | 157,833
Loans: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 1,274,590 | 1,581,966 | 1,257,856
Ending balance: collectively evaluated for impairment |  |  | $ 18,454,747 | $ 21,607,625 | $ 19,246,609
Net charge-offs as a percentage of average loans in repayment (annualized) |  |  | 1.17% | 1.17% | 1.01%
Allowance as a percentage of the ending total loan balance |  |  | 6.87% | 1.61% | 1.36%
Allowance as a percentage of the ending loans in repayment |  |  | 9.48% | 2.23% | 1.90%
Allowance coverage of net charge-offs (annualized) |  |  | 7.48 | 2.04 | 2.08
Ending total loans, gross |  |  | $ 19,729,337 | $ 23,189,591 | $ 20,504,465
Average loans in repayment |  |  | 15,518,851 | 15,605,927 | 13,303,801
Ending loans in repayment |  |  | 14,304,821 | 16,787,670 | 14,666,856
Private Education Loans | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 1,435,130 | 1,435,130 |  |
Day 1 adjustment for the adoption of CECL |  |  |  | 1,060,830 |
Net charge-offs: |  |  |  |  |
Ending Balance |  |  |  | 1,435,130 |
Personal Loans |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 65,877 | 65,877 | 62,201 | 6,628
Transfer from unfunded commitment liability |  |  | 0 |  |
Provision for current period |  |  | 40,485 |  |
Loan sale reduction to provision | $ 43,000 |  | (42,916) |  |
Loan transfer to held-for-sale |  |  | 0 |  |
Total provision |  |  | (2,431) | 72,783 | 74,317
Net charge-offs: |  |  |  |  |
Charge-offs |  |  | (39,079) | (74,313) | (19,690)
Recoveries |  |  | 4,984 | 5,206 | 946
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery |  |  | (34,095) | (69,107) | (18,744)
Loan sales |  |  | (108,534) |  | 0
Ending Balance |  |  | 0 | 65,877 | 62,201
Allowance: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 | 0
Ending balance: collectively evaluated for impairment |  |  | 0 | 65,877 | 62,201
Loans: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 | 0
Ending balance: collectively evaluated for impairment |  |  | $ 0 | $ 1,049,007 | $ 1,190,091
Net charge-offs as a percentage of average loans in repayment (annualized) |  |  | 0.00% | 6.07% | 2.11%
Allowance as a percentage of the ending total loan balance |  |  | 0.00% | 6.28% | 5.23%
Allowance as a percentage of the ending loans in repayment |  |  | 0.00% | 6.28% | 5.23%
Allowance coverage of net charge-offs (annualized) |  |  | 0 | 0.95 | 3.32
Ending total loans, gross |  |  | $ 0 | $ 1,049,007 | $ 1,190,091
Average loans in repayment |  |  | 0 | 1,138,887 | 889,348
Ending loans in repayment |  |  | 0 | 1,049,007 | 1,190,091
Additional provisions related to other impacts |  |  | (2,431) |  |
Personal Loans | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 145,060 | 145,060 |  |
Day 1 adjustment for the adoption of CECL |  |  |  | 79,183 |
Net charge-offs: |  |  |  |  |
Ending Balance |  |  |  | 145,060 |
Credit Cards |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | 102 | 102 | 0 |
Transfer from unfunded commitment liability |  |  | 0 |  |
Provision for current period |  |  | 1,328 |  |
Loan sale reduction to provision |  |  | 0 |  |
Loan transfer to held-for-sale |  |  | 0 |  |
Total provision |  |  | 1,328 | 103 |
Net charge-offs: |  |  |  |  |
Charge-offs |  |  | (119) | (1) |
Recoveries |  |  | 2 | 0 |
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery |  |  | (117) | (1) |
Loan sales |  |  | 0 |  |
Ending Balance |  |  | 1,501 | 102 | $ 0
Allowance: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 |
Ending balance: collectively evaluated for impairment |  |  | 1,501 | 102 |
Loans: |  |  |  |  |
Ending balance: individually evaluated for impairment |  |  | 0 | 0 |
Ending balance: collectively evaluated for impairment |  |  | $ 12,238 | $ 3,884 |
Net charge-offs as a percentage of average loans in repayment (annualized) |  |  | 1.26% | 0.13% |
Allowance as a percentage of the ending total loan balance |  |  | 12.27% | 2.63% |
Allowance as a percentage of the ending loans in repayment |  |  | 12.27% | 2.63% |
Allowance coverage of net charge-offs (annualized) |  |  | 12.83 | 102.00 |
Ending total loans, gross |  |  | $ 12,238 | $ 3,884 |
Average loans in repayment |  |  | 9,286 | 786 |
Ending loans in repayment |  |  | 12,238 | 3,884 |
Additional provisions related to other impacts |  |  | 1,328 |  |
Credit Cards | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |
Beginning Balance |  | $ 290 | $ 290 |  |
Day 1 adjustment for the adoption of CECL |  |  |  | 188 |
Net charge-offs: |  |  |  |  |
Ending Balance |  |  |  | $ 290 |

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