Loans Held for Investment - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) payment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Private loans indexed to LIBOR (as a percentage)		67.00%	77.00%
Gains on sales of loans, net		$ 2,060,000	$ 0	$ 230,000
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 5 months 5 days	5 years 6 months
Number of payments to qualify for Private Education Loan program | payment		3
Interest Rate Reduction, Percentage		$ 0.02
Period of loans past due that have accrued interest		90 days
Loans in nonaccrual status		$ 0	$ 0
Percentage of loans concentrated in major states		39.90%	40.40%
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		$ 42,900,000	$ 66,900,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		50,800,000	37,600,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to borrower in custody		$ 3,400,000,000	$ 2,700,000,000
Discontinued Operations, Disposed of by Sale
Finance Receivable Transferred To Held For Sale [Line Items]
Split Loans portfolio sales price	$ 43,000,000
Gains on sales of loans, net	$ 2,000,000