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Allowance for Loan Losses - Additional Information (Detail) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Jul. 01, 2006
Receivables [Abstract] |  |  |
Percentage of loans granted forbearance qualified as TDR | 62.00% | 66.00% |
Criteria for loans to be considered as nonperforming (greater than) | 90 days |  |
Tier 1 of government guarantee (at least) (in percentage) | 97.00% |  | 97.00%
TDR payment default period (more than) | 60 days |  |
Percentage of FFELP loans insured and guaranteed (at least) | 97.00% |  |
Period of loans past due that have accrued interest (greater than) | 90 days |  |

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- Definition Tier One of Government Guarantee + References No definition available. + Details
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- References No definition available. + Details
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