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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions | Jun. 30, 2025 | Mar. 31, 2025 | Dec. 31, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Dec. 31, 2023
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Loan allowance for losses | $ (530) | $ (579) | $ (621) | $ (687) | $ (744) | $ (832)
Private Education Loans [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Loans, gross | 15,878 |  |  | 16,731 |  |
Loan allowance for losses | (348) | $ (397) | (441) | (493) | $ (538) | $ (617)
Loans, net | 15,530 |  | $ 15,716 | 16,238 |  |
Loan, Net Charge-Offs in Year One | 0 |  |  | 0 |  |
Loan, Net Charge-Offs in Year Two | (2) |  |  | (1) |  |
Loan, Net Charge-Offs in Year Three | (3) |  |  | (5) |  |
Loan, Net Charge-Offs in Year Four | (6) |  |  | (9) |  |
Loan, Net Charge-Offs in Year Five | (12) |  |  | (2) |  |
Loan, Net Charge-Offs in Year Prior | (129) |  |  | (149) |  |
Total Net Charge-Offs | (152) |  |  | (166) |  |
FICO Scores [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | 954 |  |  | 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
FICO Scores [Member] | 640 and Above [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 940 |  |  | $ 490 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,146 |  |  | 841 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 635 |  |  | 1,453 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,167 |  |  | 3,591 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 2,991 |  |  | 1,119 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 7,020 |  |  | 7,513 |  |
Total Loans, gross | $ 13,899 |  |  | $ 15,007 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 88.00% |  |  | 90.00% |  |
FICO Scores [Member] | Below 640 [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 14 |  |  | $ 8 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 48 |  |  | 20 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 46 |  |  | 75 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 102 |  |  | 137 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 166 |  |  | 31 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 1,603 |  |  | 1,453 |  |
Total Loans, gross | $ 1,979 |  |  | $ 1,724 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 12.00% |  |  | 10.00% |  |
Loan Status [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Loan Status [Member] | In-school/Grace/Deferment/Forbearance [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 29 |  |  | $ 22 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 91 |  |  | 69 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 55 |  |  | 64 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 51 |  |  | 80 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 73 |  |  | 19 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 312 |  |  | 390 |  |
Total Loans, gross | $ 611 |  |  | $ 644 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 4.00% |  |  | 4.00% |  |
Loan Status [Member] | Current/90 Days or Less Delinquent [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 924 |  |  | $ 476 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,097 |  |  | 789 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 618 |  |  | 1,454 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,201 |  |  | 3,634 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,063 |  |  | 1,127 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 7,905 |  |  | 8,256 |  |
Total Loans, gross | $ 14,808 |  |  | $ 15,736 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 93.00% |  |  | 94.00% |  |
Loan Status [Member] | Greater than 90 days Delinquent [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 1 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 6 |  |  | 3 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 8 |  |  | 10 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 17 |  |  | 14 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 21 |  |  | 4 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 406 |  |  | 320 |  |
Total Loans, gross | $ 459 |  |  | $ 351 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 3.00% |  |  | 2.00% |  |
Certain Loan Modifications [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Certain Loan Modifications [Member] | Modified [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 0 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 6 |  |  | 3 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 27 |  |  | 64 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 98 |  |  | 161 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 203 |  |  | 53 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 5,077 |  |  | 5,538 |  |
Total Loans, gross | $ 5,411 |  |  | $ 5,819 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 34.00% |  |  | 35.00% |  |
Certain Loan Modifications [Member] | Non-Modified [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,188 |  |  | 858 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 654 |  |  | 1,464 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,171 |  |  | 3,567 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 2,954 |  |  | 1,097 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 3,546 |  |  | 3,428 |  |
Total Loans, gross | $ 10,467 |  |  | $ 10,912 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 66.00% |  |  | 65.00% |  |
Cosigners [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Cosigners [Member] | With Cosigner [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 152 |  |  | $ 76 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 342 |  |  | 264 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 225 |  |  | 168 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 136 |  |  | 87 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 70 |  |  | 21 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 4,179 |  |  | 4,808 |  |
Total Loans, gross | $ 5,104 |  |  | $ 5,424 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 32.00% |  |  | 32.00% |  |
Cosigners [Member] | Without Cosigner [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 802 |  |  | $ 422 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 852 |  |  | 597 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 456 |  |  | 1,360 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,133 |  |  | 3,641 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,087 |  |  | 1,129 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 4,444 |  |  | 4,158 |  |
Total Loans, gross | $ 10,774 |  |  | $ 11,307 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 68.00% |  |  | 68.00% |  |
School Type [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
School Type [Member] | Not For Profit [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 893 |  |  | $ 335 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,127 |  |  | 814 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 643 |  |  | 1,447 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,201 |  |  | 3,511 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 2,971 |  |  | 1,099 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 7,489 |  |  | 7,674 |  |
Total Loans, gross | $ 14,324 |  |  | $ 14,880 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 90.00% |  |  | 89.00% |  |
School Type [Member] | For Profit [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 61 |  |  | $ 163 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 67 |  |  | 47 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 38 |  |  | 81 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 68 |  |  | 217 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 186 |  |  | 51 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 1,134 |  |  | 1,292 |  |
Total Loans, gross | $ 1,554 |  |  | $ 1,851 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 10.00% |  |  | 11.00% |  |
Repayments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 954 |  |  | $ 498 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 1,194 |  |  | 861 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 681 |  |  | 1,528 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 1,269 |  |  | 3,728 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 3,157 |  |  | 1,150 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,623 |  |  | 8,966 |  |
Total Loans, gross | $ 15,878 |  |  | $ 16,731 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Repayments [Member] | 1-12 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 932 |  |  | $ 479 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 879 |  |  | 632 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 28 |  |  | 36 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 22 |  |  | 24 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 11 |  |  | 3 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 30 |  |  | 45 |  |
Total Loans, gross | $ 1,902 |  |  | $ 1,219 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 12.00% |  |  | 7.00% |  |
Repayments [Member] | 13-24 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 0 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 238 |  |  | 172 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 484 |  |  | 656 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 48 |  |  | 82 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 37 |  |  | 11 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 41 |  |  | 61 |  |
Total Loans, gross | $ 848 |  |  | $ 982 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 6.00% |  |  | 6.00% |  |
Repayments [Member] | 25-36 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 0 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 125 |  |  | 791 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 524 |  |  | 2,602 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 107 |  |  | 28 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 85 |  |  | 111 |  |
Total Loans, gross | $ 841 |  |  | $ 3,532 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 5.00% |  |  | 21.00% |  |
Repayments [Member] | 37-48 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 0 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 645 |  |  | 976 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 2,182 |  |  | 683 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 160 |  |  | 206 |  |
Total Loans, gross | $ 2,987 |  |  | $ 1,865 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 19.00% |  |  | 11.00% |  |
Repayments [Member] | More than 48 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 0 |  |  | $ 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 0 |  |  | 0 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 784 |  |  | 416 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 8,155 |  |  | 8,367 |  |
Total Loans, gross | $ 8,939 |  |  | $ 8,783 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 56.00% |  |  | 53.00% |  |
Repayments [Member] | School/Grace/Deferment [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year One | $ 22 |  |  | $ 19 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Two | 77 |  |  | 57 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Three | 44 |  |  | 45 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Four | 30 |  |  | 44 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Five | 36 |  |  | 9 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 152 |  |  | 176 |  |
Total Loans, gross | $ 361 |  |  | $ 350 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 2.00% |  |  | 2.00% |  |

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