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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
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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

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