Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Parenthetical) (Detail)	Sep. 30, 2023	Sep. 30, 2022
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%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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