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Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loan Modifications, Amortized Cost |  | $ 250 | $ 149
Interest Rate Reduction [Member] | Private Education Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loan Modifications, Amortized Cost | $ 1,668 |  |
Loan Modifications, % of Loan Type | 9.50% |  |
Loans receivable, weighted average rate of interest | 5.30% | 13.20% |
More Than an Insignificant Payment Delay [Member] | Private Education Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loan Modifications, Amortized Cost | $ 923 |  |
Loan Modifications, % of Loan Type | 5.30% |  |
Loans receivable, weighted average period added to remaining life of loan | 7 months |  |
Combination Rate Reduction and Term Extension [Member] | Private Education Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loan Modifications, Amortized Cost | $ 138 |  |
Loan Modifications, % of Loan Type | 0.80% |  |
Loans receivable, weighted average rate of interest | 5.20% | 12.70% |
Loans receivable, weighted average period added to remaining life of loan | 8 years |  |

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- Definition Amount, before troubled debt restructuring, of financing receivable to be modified. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 33 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-33 + Details
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