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Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2015 | Dec. 31, 2014 | Jul. 01, 2006
Receivables [Abstract] |  |  |
Troubled Debt Restructuring | $ 187.1 |  |
Criteria for loans to be considered as nonperforming | 90 days |  |
Tier 1 of government guarantee | 97.00% |  | 97.00%
TDR payment default period | 60 days |  |
Ffelp loans | $ 722.0 |  |
Percentage of FFELP rehabilitation loans | 61.20% | 62.10% |
Period of loans past due that have accrued interest | 90 days |  |

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