Allowance for Loan Losses - Education Loans - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Jun. 30, 2026
Accounts Notes And Loans Receivable [Line Items]
Write-offs	$ (79)	$ (121)	$ (271)	$ (310)
Change in balance during period	(6)	1	(13)	(6)
Loans held for sale	2,400		2,400
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Write-offs	(68)	(112)	(233)	(287)
Change in balance during period	(6)	1	(13)	(6)
Loans held for sale	2,400		2,400
Private Education Loans [Member] \| Loans Receivable For Previously Fully Charged Off Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Beginning of period expected future recoveries on previously fully charged-off loans	163	172	170	179
Expected future recoveries of current period defaults	9	16	31	39
Recoveries (cash collected)	(9)	(9)	(29)	(30)
Write-offs	(6)	(6)	(15)	(15)
End of period expected future recoveries on previously fully charged-off loans	157	173	157	173
Change in balance during period	(6)	$ 1	(13)	$ (6)
Legacy Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Write-offs	(121)		(121)
Classified held for sale	1,400		1,400
Loans held for sale	2,339		2,339		$ 5,700
In-School Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 235		$ 235

Expected future recoveries on previously fully charged-off loans.

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- Definition

Financing receivable decrease in expected future recoveries on previous fully charged-off loans.

+ References

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- Definition

Represents the difference between the loan balance and the company's estimate of the amount to be collected in the future.

+ References

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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.

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Amount of increase in allowance for credit loss on financing receivable from recovery.

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Amount of writeoff of financing receivable, charged against allowance for credit loss.

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Amount of financing receivable reclassified to held-for-sale.

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Amount, before valuation allowance, of financing receivable classified as held-for-sale.

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