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