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Allowance for Loan Losses - Amount of Loans Modified for Borrowers Financial Difficulty and Payment Default (Parenthetical) (Details) - Private Education Loans [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 109	$ 128	$ 179	$ 163
Interest Rate Concession [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	71	101	119	121
More Than an Insignificant Payment Delay [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	33	22	52	35
Combination Rate Reduction and Term Extension [Member]
Accounts Notes And Loans Receivable [Line Items]
Modified Loans (Amortized Cost)	$ 5	$ 5	$ 8	$ 7

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
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-Name Accounting Standards Codification
-Section 40
-Paragraph 7
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-Topic 310
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