Convertible Senior Notes and Capped Calls - Carrying Amount of Equity Component (Details) - Convertible Debt [Member] - The Notes [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Aug. 31, 2018
Debt Instrument [Line Items]
Debt discount for conversion option	$ 53,820	$ 53,820
Issuance costs	(1,626)	(1,626)
Net carrying amount	$ 52,194	$ 52,194	$ 53,800

The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

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