CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2015		$ 564,691	$ (70)	$ (321,556)	$ 243,065
Balance (in shares) at Dec. 31, 2015	37,981
Increase (Decrease) in Stockholders' Equity
Net proceeds from sale of ordinary shares	$ 1	253,027			253,028
Net proceeds from sale of ordinary shares (in shares)	11,978
Proceeds from ESPP purchases		3,172			3,172
Proceeds from ESPP purchases (in shares)	245
Employee shared-based compensation expense		41,290			41,290
Issuance of restricted shares (in shares)	2,466
Option exercises		4,378			4,378
Option exercises (in shares)	197
Repurchase of shares to satisfy tax withholding		(3,871)			(3,871)
Repurchase of shares to satisfy tax withholding (in shares)	(34)
Excess tax benefit of share-based compensation		21			21
Net unrealized loss on marketable securities			(183)		(183)
Net loss				(190,669)	(190,669)
Balance at Dec. 31, 2016	$ 1	862,708	(253)	(512,225)	350,231
Balance (in shares) at Dec. 31, 2016	52,833
Increase (Decrease) in Stockholders' Equity
Net proceeds from sale of ordinary shares		1			1
Proceeds from ESPP purchases		3,980			3,980
Proceeds from ESPP purchases (in shares)	250
Employee shared-based compensation expense		49,175			49,175
Issuance of restricted shares (in shares)	1,025
Option exercises		6,236			6,236
Option exercises (in shares)	276
Cumulative effect upon the adoption of ASU 2016-09		110		(110)
Repurchase of shares to satisfy tax withholding		(8,560)			(8,560)
Repurchase of shares to satisfy tax withholding (in shares)	(3)
Net unrealized loss on marketable securities			(480)		(480)
Net loss				(285,405)	(285,405)
Balance at Dec. 31, 2017	$ 1	913,650	(733)	(797,740)	$ 115,178
Balance (in shares) at Dec. 31, 2017	54,381				54,381,000
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		4,173			$ 4,173
Proceeds from ESPP purchases (in shares)	204
Employee shared-based compensation expense		51,313			51,313
Issuance of restricted shares (in shares)	1,168
Option exercises		1,393			1,393
Option exercises (in shares)	75
Cumulative effect upon the adoption of ASU 2016-09				1,119	1,119
Repurchase of shares to satisfy tax withholding		(9,808)			(9,808)
Repurchase of shares to satisfy tax withholding (in shares)	(147)
Net unrealized loss on marketable securities			567		567
Net loss				(215,524)	(215,524)
Balance at Dec. 31, 2018	$ 1	$ 960,721	$ (166)	$ (1,012,145)	$ (51,589)
Balance (in shares) at Dec. 31, 2018	55,681				55,681,000