Loans Held for Investment - Student Loan Portfolio by Program (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Private Education Loans	$ 17,432,167	$ 14,251,675
Deferred origination costs	56,378	44,206
Allowance for loan losses	(243,715)	(182,472)
Total Private Education Loans, net	17,244,830	14,113,409
FFELP Loans	927,660	1,010,908
Unamortized acquisition costs, net	2,631	2,941
Allowance for loan losses	(1,132)	(2,171)
Total FFELP Loans, net	929,159	1,011,678
Personal Loans	400,280	12,893
Allowance for loan losses	(6,628)	(58)
Total Personal Loans, net	393,652	12,835
Loans held for investment, net	$ 18,567,641	$ 15,137,922

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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