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Allowance for Loan Losses - Additional Information (Detail) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Jul. 01, 2006
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items] |  |  |
Period after grace period for forbearance allowance for loans | 9 months |  |
Forbearance period after grace period for loans | 6 months |  |
Period of forbearance period to be classified as TDR | 3 months |  |
Period of forbearance | 24 months |  |
Percentage of loans granted forbearance (as a percentage) | 66.00% | 69.00% |
Criteria for loans to be considered as nonperforming | 90 days |  |
Tier 1 of government guarantee (at least) |  |  | 97.00%
TDR payment default period | 60 days |  |
Period of loans past due that have accrued interest | 90 days |  |
FFELP Loans |  |  |
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items] |  |  |
Tier 1 of government guarantee (at least) | 97.00% |  |

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- Details
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