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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
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%

X

- Definition

Loans and leases receivable gross carrying amount percent.

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: