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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Mar. 31, 2024 | Dec. 31, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 31, 2022
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private Education Loans allowance for losses | $ (1,269,652) | $ (1,350,058) | $ (1,339,772) | $ (1,364,716) | $ (1,479,306) | $ (1,357,075)
Private Education Loans, net | $ 18,915,333 |  | $ 20,306,357 |  |  |
Percentage of Private Education Loans in repayment, Year 1 | 52.00% |  | 55.60% |  |  |
Percentage of Private Education Loans in repayment, Year 2 | 57.80% |  | 65.30% |  |  |
Percentage of Private Education Loans in repayment, Year 3 | 69.20% |  | 75.20% |  |  |
Percentage of Private Education Loans in repayment, Year 4 | 77.60% |  | 81.10% |  |  |
Percentage of Private Education Loans in repayment, Year 5 | 82.60% |  | 85.00% |  |  |
Percentage of Private Education Loans in repayment, After Year 5 | 89.10% |  | 89.30% |  |  |
Percentage of Private Education Loans in repayment | 72.50% |  | 73.30% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.50% |  | 1.00% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 1.10% |  | 2.10% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 2.90% |  | 3.60% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 3.50% |  | 4.10% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 3.50% |  | 4.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5 | 5.20% |  | 6.80% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.30% |  | 3.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.30% |  | 0.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 1.10% |  | 2.70% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 2.60% |  | 2.60% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 2.10% |  | 2.30% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 2.00% |  | 2.20% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5 | 1.90% |  | 1.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 1.80% |  | 2.10% |  |  |
Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in Repayment, Year 1 | $ 769,857 |  | $ 2,494,814 |  |  |
Total Private Education Loans in Repayment, Year 2 | 3,134,870 |  | 3,323,327 |  |  |
Total Private Education Loans in Repayment, Year 3 | 2,281,790 |  | 2,209,734 |  |  |
Total Private Education Loans in Repayment, Year 4 | 1,688,961 |  | 1,475,858 |  |  |
Total Private Education Loans in Repayment, Year 5 | 1,215,583 |  | 1,386,889 |  |  |
Total Private Education Loans in Repayment, After Year 5 | 5,140,520 |  | 4,519,192 |  |  |
Total Private Education Loans in repayment | 14,231,581 |  | 15,409,814 |  |  |
Total Private Education Loans, gross, Year 1 | 1,480,348 |  | 4,490,119 |  |  |
Total Private Education Loans, gross, Year 2 | 5,423,019 |  | 5,088,739 |  |  |
Total Private Education Loans, gross, Year 3 | 3,297,769 |  | 2,937,422 |  |  |
Total Private Education Loans, gross, Year 4 | 2,177,720 |  | 1,819,089 |  |  |
Total Private Education Loans, gross, Year 5 | 1,472,239 |  | 1,632,300 |  |  |
Total Private Education Loans, gross, After Year 5 | 5,768,436 |  | 5,058,175 |  |  |
Total Private Education Loans, gross | 19,619,531 |  | 21,025,844 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 17,774 |  | 35,616 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 27,568 |  | 18,556 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 11,897 |  | 9,465 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 6,845 |  | 5,809 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 4,575 |  | 3,556 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5 | 10,002 |  | 8,552 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount) | 78,661 |  | 81,554 |  |  |
Total Private Education Loans, Year 1 | 1,498,122 |  | 4,525,735 |  |  |
Total Private Education Loans, Year 2 | 5,450,587 |  | 5,107,295 |  |  |
Total Private Education Loans, Year 3 | 3,309,666 |  | 2,946,887 |  |  |
Total Private Education Loans, Year 4 | 2,184,565 |  | 1,824,898 |  |  |
Total Private Education Loans, Year 5 | 1,476,814 |  | 1,635,856 |  |  |
Total Private Education Loans, After Year 5 | 5,778,438 |  | 5,066,727 |  |  |
Total Private Education Loans | 19,698,192 |  | 21,107,398 |  |  |
Private Education Loans allowance for losses, Year 1 | (80,912) |  | (269,642) |  |  |
Private Education Loans allowance for losses, Year 2 | (304,066) |  | (335,090) |  |  |
Private Education Loans allowance for losses, Year 3 | (228,421) |  | (194,104) |  |  |
Private Education Loans allowance for losses, Year 4 | (152,123) |  | (118,755) |  |  |
Private Education Loans allowance for losses, Year 5 | (94,330) |  | (100,111) |  |  |
Private Education Loans allowance for losses, After Year 5 | (405,740) |  | (317,403) |  |  |
Private Education Loans allowance for losses | (1,265,592) |  | (1,335,105) |  |  |
Private Education Loans, net, Year 1 | 1,417,210 |  | 4,256,093 |  |  |
Private Education Loans, net, Year 2 | 5,146,521 |  | 4,772,205 |  |  |
Private Education Loans, net, Year 3 | 3,081,245 |  | 2,752,783 |  |  |
Private Education Loans, net, Year 4 | 2,032,442 |  | 1,706,143 |  |  |
Private Education Loans, net, Year 5 | 1,382,484 |  | 1,535,745 |  |  |
Private Education Loans, net, After Year 5 | 5,372,698 |  | 4,749,324 |  |  |
Private Education Loans, net | 18,432,600 |  | 19,772,293 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent, current |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 765,729 |  | 2,469,817 |  |  |
Year 2 | 3,100,292 |  | 3,254,534 |  |  |
Year 3 | 2,215,133 |  | 2,131,040 |  |  |
Year 4 | 1,630,195 |  | 1,416,069 |  |  |
Year 5 | 1,172,591 |  | 1,323,825 |  |  |
After Year 5 | 4,872,598 |  | 4,213,986 |  |  |
Loan type | 13,756,538 |  | 14,809,271 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 2,542 |  | 17,599 |  |  |
Year 2 | 18,637 |  | 34,627 |  |  |
Year 3 | 29,618 |  | 37,147 |  |  |
Year 4 | 27,766 |  | 28,020 |  |  |
Year 5 | 19,455 |  | 31,432 |  |  |
After Year 5 | 126,427 |  | 149,926 |  |  |
Loan type | 224,445 |  | 298,751 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 759 |  | 5,720 |  |  |
Year 2 | 9,069 |  | 17,227 |  |  |
Year 3 | 19,011 |  | 20,077 |  |  |
Year 4 | 15,343 |  | 16,614 |  |  |
Year 5 | 11,809 |  | 15,482 |  |  |
After Year 5 | 69,393 |  | 75,897 |  |  |
Loan type | 125,384 |  | 151,017 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 827 |  | 1,678 |  |  |
Year 2 | 6,872 |  | 16,939 |  |  |
Year 3 | 18,028 |  | 21,470 |  |  |
Year 4 | 15,657 |  | 15,155 |  |  |
Year 5 | 11,728 |  | 16,150 |  |  |
After Year 5 | 72,102 |  | 79,383 |  |  |
Loan type | 125,214 |  | 150,775 |  |  |
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 708,196 |  | 1,976,040 |  |  |
Year 2 | 2,254,427 |  | 1,672,333 |  |  |
Year 3 | 955,723 |  | 669,250 |  |  |
Year 4 | 452,970 |  | 307,781 |  |  |
Year 5 | 231,555 |  | 213,593 |  |  |
After Year 5 | 525,887 |  | 452,994 |  |  |
Loan type | 5,128,758 |  | 5,291,991 |  |  |
Loans In Forbearance | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 2,295 |  | 19,265 |  |  |
Total Private Education Loans in forbearance, Year 2 | 33,722 |  | 93,079 |  |  |
Total Private Education Loans in forbearance, Year 3 | 60,256 |  | 58,438 |  |  |
Total Private Education Loans in forbearance, Year 4 | 35,789 |  | 35,450 |  |  |
Total Private Education Loans in forbearance, Year 5 | 25,101 |  | 31,818 |  |  |
Total Private Education Loans in forbearance, After Year 5 | 102,029 |  | 85,989 |  |  |
Total Private Education Loans in forbearance | $ 259,192 |  | $ 324,039 |  |  |

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