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Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Apr. 29, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Jul. 01, 2006
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items] |  |  |  |
Period of loans delinquent sold to subsidiary | 90 days |  |  |
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 |  |
Percentage of loans granted forbearance |  | 23.00% | 10.00% |
Criteria for loans to be considered as nonperforming |  | 90 days |  |
Tier 1 of government guarantee |  |  |  | 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 |  | 97.00% |  |
Private Education Loans |  |  |  |
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items] |  |  |  |
Troubled debt restructuring |  | $ 307.2 |  |

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