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%