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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Parenthetical) (Detail) - With Cosigner [Member] | Jun. 30, 2019 | Dec. 31, 2018
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Accounts Notes And Loans Receivable [Line Items] |  |
Total Private Education Loan Credit Quality Indicators, Excluding Refinance Loans, Percent | 67.00% | 67.00%
Private Education Loans - TDRs [Member] |  |
Accounts Notes And Loans Receivable [Line Items] |  |
Total Private Education Loan Credit Quality Indicators, Excluding Refinance Loans, Percent | 62.00% | 62.00%

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