Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Parenthetical) (Detail)	Jun. 30, 2024	Jun. 30, 2023
With Cosigner [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators, Excluding Refinance Loans, Percent	66.00%	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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

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