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Allowance for Loan Losses - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2006
Receivables [Abstract]
Interest rate offered to borrowers facing financial difficulty	2.00%
Term of interest rate offered to borrowers facing financial difficulty	2 years
Percentage of loans in full and interest repayment status that are subject to interest rate reductions	7.40%		5.20%
Percentage of loans granted forbearance qualified as TDR	58.00%	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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Criteria for loans to be considered as nonperforming.

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Loans And Leases Receivable, Percentage Of Loans In Full And Interest Repayment Status That Are Subject To Interest Rate Reductions

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Loans And Leases Receivable, Temporary Interest Rate Reduction, Financial Difficulty Status

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Loans and Leases Receivable, Impaired, Loans Granted Forbearance Qualified as Trouble Debt Restructuring

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Loans and Leases Receivable, Percentage of FFELP Loans Insured and Guaranteed

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Period of Loans Past Due That Have Accrued Interest

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Tier One of Government Guarantee

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