CONSOLIDATED STATEMENTS OF SHAREHOLDERS' DEFICIT - USD ($) shares in Thousands, $ in Thousands	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment [Member]	Accumulated Deficit	Cumulative Effect, Period of Adoption, Adjustment [Member]	Total
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
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 available-for-sale investments			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
Increase (Decrease) in Stockholders' Equity
Cumulative effect upon the adoption of ASC 606				$ 1,119		$ 1,119
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 available-for-sale investments			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
Increase (Decrease) in Stockholders' Equity
Cumulative effect upon the adoption of ASC 606							$ (1,248,600)
Net proceeds from sale of ordinary shares		139,915					139,915
Net proceeds from sale of ordinary shares (in shares)	5,500
Proceeds from ESPP purchases		3,701					3,701
Proceeds from ESPP purchases (in shares)	245
Employee share-based compensation expense		62,976					62,976
Issuance of restricted shares (in shares)	1,907
Option exercises		1,361					1,361
Option exercises (in shares)	68
Repurchase of shares to satisfy tax withholding		(9,749)					(9,749)
Repurchase of shares to satisfy tax withholding (in shares)	(407)
Net unrealized gain (loss) on available-for-sale investments			(98)				(98)
Net loss					(278,017)		(278,017)
Balances at Dec. 31, 2020	$ 1	$ 1,222,818	$ 47		$ (1,526,617)		$ (303,751)
Balances (in shares) at Dec. 31, 2020	64,328						64,328
Increase (Decrease) in Stockholders' Equity
Cumulative effect upon the adoption of ASC 606							$ (1,526,617)