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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Parenthetical) (Detail) | Sep. 30, 2022 | Sep. 30, 2021
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With Cosigner [Member] |  |
Accounts Notes And Loans Receivable [Line Items] |  |
Total Private Education Loan Credit Quality Indicators, Excluding Refinance Loans, Percent | 65.00% | 65.00%

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- Details
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