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 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(c)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: 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 4: 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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Period Type:	duration

Allowance for Loan Losses - Education Loans - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Sep. 30, 2025
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 2,400
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	2,400
In-School Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 235
With Cosigner [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Private Education Loan Credit Quality Indicators, Excluding Refinance Loans, Percent	65.00%	67.00%