Convertible Notes - Summary of Carrying Value of Convertible Notes (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Gross proceeds	$ 125,000,000	$ 125,000,000
Unamortized debt discount and issuance costs	(1,078,000)	(1,155,000)
Carrying value	$ 123,922,000	$ 123,845,000

Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_ConvertibleLongTermNotesPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant