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Allowance for Credit Losses and Unfunded Loan Commitments - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Private Education Loans allowance for losses | $ (1,435,920) | $ (1,339,772) | $ (1,357,075) | $ (1,165,335)
Private Education Loans, net | $ 20,902,158 | $ 20,306,357 |  |
Percentage of Private Education Loans in repayment, Year 1 | 56.50% | 55.60% | 56.90% |
Percentage of Private Education Loans in repayment, Year 2 | 63.50% | 65.30% | 67.10% |
Percentage of Private Education Loans in repayment, Year 3 | 73.70% | 75.20% | 76.10% |
Percentage of Private Education Loans in repayment, Year 4 | 80.30% | 81.10% | 80.70% |
Percentage of Private Education Loans in repayment, Year 5 | 84.30% | 85.00% | 84.50% |
Percentage of Private Education Loans in repayment, Year 6 | 89.70% | 89.30% | 89.30% |
Percentage of Private Education Loans in repayment | 72.40% | 73.30% | 74.50% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.70% | 1.00% | 0.90% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 2.00% | 2.10% | 1.90% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.80% | 3.60% | 3.10% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 4.30% | 4.10% | 4.00% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.40% | 4.50% | 4.70% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6 | 6.30% | 6.80% | 6.50% |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total | 3.70% | 3.90% | 3.80% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.70% | 0.80% | 0.70% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 3.60% | 2.70% | 2.10% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 3.60% | 2.60% | 1.90% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 3.10% | 2.30% | 2.10% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 2.70% | 2.20% | 2.20% |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6 | 1.80% | 1.90% | 2.00% |
Loans in forbearance as a percentage of loans in repayment and forbearance | 2.50% | 2.10% | 1.80% |
Student Loan | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | $ 2,840,129 | $ 2,494,814 | $ 2,432,838 |
Total Private Education Loans in forbearance, Year 2 | 3,379,077 | 3,323,327 | 3,049,477 |
Total Private Education Loans in forbearance, Year 3 | 2,348,504 | 2,209,734 | 1,967,756 |
Total Private Education Loans in forbearance, Year 4 | 1,661,502 | 1,475,858 | 1,754,219 |
Total Private Education Loans in forbearance, Year 5 | 1,158,044 | 1,386,889 | 1,365,979 |
Total Private Education Loans in forbearance, Year 6 | 4,719,495 | 4,519,192 | 4,559,281 |
Private Education Loans in forbearance, total | 16,106,751 | 15,409,814 | 15,129,550 |
Total Private Education Loans, gross, Year 1 | 5,024,592 | 4,490,119 | 4,276,533 |
Total Private Education Loans, gross, Year 2 | 5,321,510 | 5,088,739 | 4,547,159 |
Total Private Education Loans, gross, Year 3 | 3,185,247 | 2,937,422 | 2,584,622 |
Total Private Education Loans, gross, Year 4 | 2,069,436 | 1,819,089 | 2,172,660 |
Total Private Education Loans, gross, Year 5 | 1,373,996 | 1,632,300 | 1,615,842 |
Total Private Education Loans, gross, Year 6 | 5,260,227 | 5,058,175 | 5,106,872 |
Total Private Education Loans, gross | 22,235,008 | 21,025,844 | 20,303,688 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 47,659 | 35,616 | 26,714 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 25,599 | 18,556 | 15,933 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 10,788 | 9,465 | 9,062 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 6,142 | 5,809 | 5,496 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 4,057 | 3,556 | 3,575 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6 | 8,825 | 8,552 | 8,876 |
Private Education Loans deferred origination costs and unamortized premium/(discount), Total | 103,070 | 81,554 | 69,656 |
Total Private Education Loans, Year 1 | 5,072,251 | 4,525,735 | 4,303,247 |
Total Private Education Loans, Year 2 | 5,347,109 | 5,107,295 | 4,563,092 |
Total Private Education Loans, Year 3 | 3,196,035 | 2,946,887 | 2,593,684 |
Total Private Education Loans, Year 4 | 2,075,578 | 1,824,898 | 2,178,156 |
Total Private Education Loans, Year 5 | 1,378,053 | 1,635,856 | 1,619,417 |
Total Private Education Loans, Year 6 | 5,269,052 | 5,066,727 | 5,115,748 |
Total Private Education Loans | 22,338,078 | 21,107,398 | 20,373,344 |
Private Education Loans allowance for losses, Year 1 | (258,235) | (269,642) | (304,943) |
Private Education Loans allowance for losses, Year 2 | (326,207) | (335,090) | (323,506) |
Private Education Loans allowance for losses, Year 3 | (234,532) | (194,104) | (181,915) |
Private Education Loans allowance for losses, Year 4 | (150,324) | (118,755) | (141,424) |
Private Education Loans allowance for losses, Year 5 | (90,600) | (100,111) | (101,023) |
Private Education Loans allowance for losses, Year 6 | (376,022) | (317,403) | (300,820) |
Private Education Loans allowance for losses | (1,435,920) | (1,335,105) | (1,353,631) |
Private Education Loans, net, Year 1 | 4,814,016 | 4,256,093 | 3,998,304 |
Private Education Loans, net, Year 2 | 5,020,902 | 4,772,205 | 4,239,586 |
Private Education Loans, net, Year 3 | 2,961,503 | 2,752,783 | 2,411,769 |
Private Education Loans, net, Year 4 | 1,925,254 | 1,706,143 | 2,036,732 |
Private Education Loans, net, Year 5 | 1,287,453 | 1,535,745 | 1,518,394 |
Private Education Loans, net, Year 6 | 4,893,030 | 4,749,324 | 4,814,928 |
Private Education Loans, net | 20,902,158 | 19,772,293 | 19,019,713 |
Student Loan | Consumer Portfolio Segment | Loan delinquent, current |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 2,820,940 | 2,469,817 | 2,411,441 |
Year 2 | 3,312,916 | 3,254,534 | 2,991,839 |
Year 3 | 2,259,455 | 2,131,040 | 1,907,574 |
Year 4 | 1,590,812 | 1,416,069 | 1,683,986 |
Year 5 | 1,107,189 | 1,323,825 | 1,301,809 |
After Year 5 | 4,422,021 | 4,213,986 | 4,262,698 |
Loan type | 15,513,333 | 14,809,271 | 14,559,347 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 13,533 | 17,599 | 14,164 |
Year 2 | 36,441 | 34,627 | 30,740 |
Year 3 | 45,543 | 37,147 | 30,877 |
Year 4 | 35,245 | 28,020 | 35,213 |
Year 5 | 27,302 | 31,432 | 31,366 |
After Year 5 | 152,684 | 149,926 | 144,948 |
Loan type | 310,748 | 298,751 | 287,308 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 3,973 | 5,720 | 5,523 |
Year 2 | 15,239 | 17,227 | 15,056 |
Year 3 | 23,359 | 20,077 | 14,433 |
Year 4 | 18,365 | 16,614 | 18,201 |
Year 5 | 10,921 | 15,482 | 16,697 |
After Year 5 | 68,878 | 75,897 | 77,595 |
Loan type | 140,735 | 151,017 | 147,505 |
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 1,683 | 1,678 | 1,710 |
Year 2 | 14,481 | 16,939 | 11,842 |
Year 3 | 20,147 | 21,470 | 14,872 |
Year 4 | 17,080 | 15,155 | 16,819 |
Year 5 | 12,632 | 16,150 | 16,107 |
After Year 5 | 75,912 | 79,383 | 74,040 |
Loan type | 141,935 | 150,775 | 135,390 |
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Year 1 | 2,164,479 | 1,976,040 | 1,827,649 |
Year 2 | 1,817,705 | 1,672,333 | 1,433,322 |
Year 3 | 748,782 | 669,250 | 578,253 |
Year 4 | 355,248 | 307,781 | 380,639 |
Year 5 | 184,377 | 213,593 | 219,280 |
After Year 5 | 452,236 | 452,994 | 455,910 |
Loan type | 5,722,827 | 5,291,991 | 4,895,053 |
Student Loan | Loans In Forbearance | Consumer Portfolio Segment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 19,984 | 19,265 | 16,046 |
Total Private Education Loans in forbearance, Year 2 | 124,728 | 93,079 | 64,360 |
Total Private Education Loans in forbearance, Year 3 | 87,961 | 58,438 | 38,613 |
Total Private Education Loans in forbearance, Year 4 | 52,686 | 35,450 | 37,802 |
Total Private Education Loans in forbearance, Year 5 | 31,575 | 31,818 | 30,583 |
Total Private Education Loans in forbearance, Year 6 | 88,496 | 85,989 | 91,681 |
Private Education Loans in forbearance, total | $ 405,430 | $ 324,039 | $ 279,085 |

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