Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	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
Percentage of loans granted forbearance (as a percentage)	26.00%	23.00%
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
Private Education Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Troubled debt restructuring	$ 705.5
FFELP Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Tier 1 of government guarantee	97.00%