CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balances at Dec. 31, 2016	$ 1	$ 862,708	$ (253)	$ (512,225)	$ 350,231
Balances (in shares) at Dec. 31, 2016	52,833
Increase (Decrease) in Stockholders' Equity
Proceeds from sale of ordinary shares		1			1
Proceeds from ESPP purchases		3,980			3,980
Proceeds from ESPP purchases (in shares)	250
Employee share-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 gain (loss) on marketable securities			(480)		(480)
Net loss				(285,405)	(285,405)
Balances at Dec. 31, 2017	$ 1	913,650	(733)	(797,740)	115,178
Balances (in shares) at Dec. 31, 2017	54,381
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		4,173			4,173
Proceeds from ESPP purchases (in shares)	204
Employee share-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 ASC 606				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 gain (loss) on marketable securities			567		567
Net loss				(215,524)	(215,524)
Balances at Dec. 31, 2018	$ 1	960,721	(166)	(1,012,145)	$ (51,589)
Balances (in shares) at Dec. 31, 2018	55,681				55,681
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		3,474			$ 3,474
Proceeds from ESPP purchases (in shares)	203
Employee share-based compensation expense		60,450			60,450
Issuance of restricted shares (in shares)	1,105
Option exercises		3,142			3,142
Option exercises (in shares)	164
Repurchase of shares to satisfy tax withholding		(3,173)			(3,173)
Repurchase of shares to satisfy tax withholding (in shares)	(138)
Net unrealized gain (loss) on marketable securities			311		311
Net loss				(236,455)	(236,455)
Balances at Dec. 31, 2019	$ 1	$ 1,024,614	$ 145	$ (1,248,600)	$ (223,840)
Balances (in shares) at Dec. 31, 2019	57,015				57,015