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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private Education Loans allowance for losses | $ (1,416,048) | $ (1,364,716) | $ (1,357,075) | $ (1,194,238) | $ (1,081,066) | $ (1,165,335)
Private Education Loans, net | $ 20,899,181 |  | $ 19,626,868 |  |  |
Percentage of Private Education Loans in repayment, Year 1 | 53.90% |  | 56.90% |  |  |
Percentage of Private Education Loans in repayment, Year 2 | 60.40% |  | 67.10% |  |  |
Percentage of Private Education Loans in repayment, Year 3 | 71.60% |  | 76.10% |  |  |
Percentage of Private Education Loans in repayment, Year 4 | 78.30% |  | 80.70% |  |  |
Percentage of Private Education Loans in repayment, Year 5 | 82.50% |  | 84.50% |  |  |
Percentage of Private Education Loans in repayment, After Year 5 | 88.60% |  | 89.30% |  |  |
Percentage of Private Education Loans in repayment | 71.50% |  | 74.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.30% |  | 0.90% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 1.70% |  | 1.90% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.40% |  | 3.10% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 3.70% |  | 4.00% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.20% |  | 4.70% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5 | 6.30% |  | 6.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.70% |  | 3.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.20% |  | 0.70% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 0.90% |  | 2.10% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 1.20% |  | 1.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 1.40% |  | 2.10% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 1.70% |  | 2.20% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5 | 2.10% |  | 2.00% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 1.40% |  | 1.80% |  |  |
Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in Repayment, Year 1 | $ 2,012,005 |  | $ 2,432,838 |  |  |
Total Private Education Loans in Repayment, Year 2 | 3,273,815 |  | 3,049,477 |  |  |
Total Private Education Loans in Repayment, Year 3 | 2,408,846 |  | 1,967,756 |  |  |
Total Private Education Loans in Repayment, Year 4 | 1,559,204 |  | 1,754,219 |  |  |
Total Private Education Loans in Repayment, Year 5 | 1,443,549 |  | 1,365,979 |  |  |
Total Private Education Loans in Repayment, After Year 5 | 4,807,726 |  | 4,559,281 |  |  |
Total Private Education Loans in repayment | 15,505,145 |  | 15,129,550 |  |  |
Total Private Education Loans, gross, Year 1 | 3,730,485 |  | 4,276,533 |  |  |
Total Private Education Loans, gross, Year 2 | 5,421,538 |  | 4,547,159 |  |  |
Total Private Education Loans, gross, Year 3 | 3,363,703 |  | 2,584,622 |  |  |
Total Private Education Loans, gross, Year 4 | 1,991,433 |  | 2,172,660 |  |  |
Total Private Education Loans, gross, Year 5 | 1,749,067 |  | 1,615,842 |  |  |
Total Private Education Loans, gross, After Year 5 | 5,424,641 |  | 5,106,872 |  |  |
Total Private Education Loans, gross | 21,680,867 |  | 20,303,688 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 26,268 |  | 26,714 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 21,367 |  | 15,933 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 11,090 |  | 9,062 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 6,555 |  | 5,496 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 3,993 |  | 3,575 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5 | 9,400 |  | 8,876 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount) | 78,673 |  | 69,656 |  |  |
Total Private Education Loans, Year 1 | 3,756,753 |  | 4,303,247 |  |  |
Total Private Education Loans, Year 2 | 5,442,905 |  | 4,563,092 |  |  |
Total Private Education Loans, Year 3 | 3,374,793 |  | 2,593,684 |  |  |
Total Private Education Loans, Year 4 | 1,997,988 |  | 2,178,156 |  |  |
Total Private Education Loans, Year 5 | 1,753,060 |  | 1,619,417 |  |  |
Total Private Education Loans, After Year 5 | 5,434,041 |  | 5,115,748 |  |  |
Total Private Education Loans | 21,759,540 |  | 20,373,344 |  |  |
Private Education Loans allowance for losses, Year 1 | (220,113) |  | (304,943) |  |  |
Private Education Loans allowance for losses, Year 2 | (361,902) |  | (323,506) |  |  |
Private Education Loans allowance for losses, Year 3 | (227,021) |  | (181,915) |  |  |
Private Education Loans allowance for losses, Year 4 | (134,487) |  | (141,424) |  |  |
Private Education Loans allowance for losses, Year 5 | (111,946) |  | (101,023) |  |  |
Private Education Loans allowance for losses, After Year 5 | (355,763) |  | (300,820) |  |  |
Private Education Loans allowance for losses | (1,411,232) |  | (1,353,631) |  |  |
Private Education Loans, net, Year 1 | 3,536,640 |  | 3,998,304 |  |  |
Private Education Loans, net, Year 2 | 5,081,003 |  | 4,239,586 |  |  |
Private Education Loans, net, Year 3 | 3,147,772 |  | 2,411,769 |  |  |
Private Education Loans, net, Year 4 | 1,863,501 |  | 2,036,732 |  |  |
Private Education Loans, net, Year 5 | 1,641,114 |  | 1,518,394 |  |  |
Private Education Loans, net, After Year 5 | 5,078,278 |  | 4,814,928 |  |  |
Private Education Loans, net | 20,348,308 |  | 19,019,713 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent, current |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 2,005,363 |  | 2,411,441 |  |  |
Year 2 | 3,218,301 |  | 2,991,839 |  |  |
Year 3 | 2,327,629 |  | 1,907,574 |  |  |
Year 4 | 1,500,801 |  | 1,683,986 |  |  |
Year 5 | 1,382,249 |  | 1,301,809 |  |  |
After Year 5 | 4,504,119 |  | 4,262,698 |  |  |
Loan type | 14,938,462 |  | 14,559,347 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 4,272 |  | 14,164 |  |  |
Year 2 | 32,282 |  | 30,740 |  |  |
Year 3 | 41,945 |  | 30,877 |  |  |
Year 4 | 28,901 |  | 35,213 |  |  |
Year 5 | 30,468 |  | 31,366 |  |  |
After Year 5 | 145,753 |  | 144,948 |  |  |
Loan type | 283,621 |  | 287,308 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 1,788 |  | 5,523 |  |  |
Year 2 | 15,013 |  | 15,056 |  |  |
Year 3 | 22,612 |  | 14,433 |  |  |
Year 4 | 16,566 |  | 18,201 |  |  |
Year 5 | 16,311 |  | 16,697 |  |  |
After Year 5 | 81,159 |  | 77,595 |  |  |
Loan type | 153,449 |  | 147,505 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 582 |  | 1,710 |  |  |
Year 2 | 8,219 |  | 11,842 |  |  |
Year 3 | 16,660 |  | 14,872 |  |  |
Year 4 | 12,936 |  | 16,819 |  |  |
Year 5 | 14,521 |  | 16,107 |  |  |
After Year 5 | 76,695 |  | 74,040 |  |  |
Loan type | 129,613 |  | 135,390 |  |  |
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 1,713,896 |  | 1,827,649 |  |  |
Year 2 | 2,118,976 |  | 1,433,322 |  |  |
Year 3 | 925,728 |  | 578,253 |  |  |
Year 4 | 410,375 |  | 380,639 |  |  |
Year 5 | 280,963 |  | 219,280 |  |  |
After Year 5 | 511,941 |  | 455,910 |  |  |
Loan type | 5,961,879 |  | 4,895,053 |  |  |
Loans In Forbearance | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 4,584 |  | 16,046 |  |  |
Total Private Education Loans in forbearance, Year 2 | 28,747 |  | 64,360 |  |  |
Total Private Education Loans in forbearance, Year 3 | 29,129 |  | 38,613 |  |  |
Total Private Education Loans in forbearance, Year 4 | 21,854 |  | 37,802 |  |  |
Total Private Education Loans in forbearance, Year 5 | 24,555 |  | 30,583 |  |  |
Total Private Education Loans in forbearance, After Year 5 | 104,974 |  | 91,681 |  |  |
Total Private Education Loans in forbearance | $ 213,843 |  | $ 279,085 |  |  |

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