Loans Held for Investment - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) payment	Dec. 31, 2014 USD ($)	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	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	6 years 2 months 6 days	6 years 2 months 12 days
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
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
90 or more days delinquent	122,900,000	201,700,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
90 or more days delinquent	20,900,000	10,700,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to Borrower in Custody	0	0
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to Borrower in Custody	$ 1,660,000,000	$ 1,410,000,000