Convertible Senior Notes - Summary of Fair Value and Carrying Value of Convertible Senior Notes (Details) - Convertible Senior Notes - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Debt Instrument [Line Items]
Convertible senior notes	$ 204,344	$ 126,931
Carrying Value
Debt Instrument [Line Items]
Convertible senior notes	$ 92,916	$ 89,481

Fair value portion of notes payable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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