Loans Held for Investment - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) payment	Dec. 31, 2016 USD ($)	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Percent of private loans indexed to LIBOR (as a percentage)	77.00%	81.00%
Tier 1 of government guarantee			97.00%
Tier 2 of government guarantee				98.00%
Tier 3 of government guarantee					100.00%
Estimated weighted average life of student loans	5 years 6 months	6 years
Number of payments to qualify for Private Education Loan program | payment	3
Period of interest rate reduction	24 months
Period of loans past due that have accrued interest	90 days
Loans in nonaccrual status	$ 0	$ 0
Percentage of loans concentrated in major states	40.40%	40.30%
Percentage of aggregate outstanding loans held for investment	5.00%
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	$ 66,900,000	$ 67,500,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	37,600,000	24,200,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to borrower in custody	$ 2,700,000,000	$ 2,700,000,000