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Allowance for Loan Losses - Summary of Performance and Related Loan Status of Loans Modified (Details) - Private Education Loans [Member] - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 2,654	$ 2,695
Financing Asset, Not Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	2,111	2,037
Financial Asset, 31 to 60 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	181	172
Financial Asset, 61 to 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	100	117
Financial Asset, 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	164	186
School/Deferment [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	20	21
Forbearance [Member]
Accounts Notes And Loans Receivable [Line Items]
Payment status (Amortized Cost)	$ 78	$ 162

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

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

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-Name Accounting Standards Codification
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-Paragraph 2
-Publisher FASB
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- Definition

Accumulated amortized cost of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
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