CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' 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, 2018	$ 1	$ 960,721	$ (166)	$ (1,012,145)	$ (51,589)
Balances (in shares) at Dec. 31, 2018	55,681
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		2,605			2,605
Proceeds from ESPP purchases (in shares)	145
Employee share-based compensation expense		23,519			23,519
Issuance of restricted shares (in shares)	689
Option exercises		2,545			2,545
Option exercises (in shares)	122
Repurchase of shares to satisfy tax withholding		(2,181)			(2,181)
Net unrealized gain/loss on marketable securities			295		295
Net loss				(112,418)	(112,418)
Balances at Jun. 30, 2019	$ 1	987,209	129	(1,124,563)	(137,224)
Balances (in shares) at Jun. 30, 2019	56,637
Balances at Mar. 31, 2019	$ 1	971,508	(36)	(1,084,725)	(113,252)
Balances (in shares) at Mar. 31, 2019	56,122
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		2,605			2,605
Proceeds from ESPP purchases (in shares)	145
Employee share-based compensation expense		11,298			11,298
Issuance of restricted shares (in shares)	263
Option exercises		2,292			2,292
Option exercises (in shares)	107
Repurchase of shares to satisfy tax withholding		(494)			(494)
Net unrealized gain/loss on marketable securities			165		165
Net loss				(39,838)	(39,838)
Balances at Jun. 30, 2019	$ 1	987,209	129	(1,124,563)	(137,224)
Balances (in shares) at Jun. 30, 2019	56,637
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
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		2,545			2,545
Proceeds from ESPP purchases (in shares)	168
Employee share-based compensation expense		31,861			31,861
Issuance of restricted shares (in shares)	1,097
Option exercises		936			936
Option exercises (in shares)	41
Repurchase of shares to satisfy tax withholding		(7,948)			(7,948)
Repurchase of shares to satisfy tax withholding (in shares)	(306)
Net unrealized gain/loss on marketable securities			135		135
Net loss				(145,940)	(145,940)
Balances at Jun. 30, 2020	$ 1	1,191,923	280	(1,394,540)	$ (202,336)
Balances (in shares) at Jun. 30, 2020	63,515				63,515
Balances at Mar. 31, 2020	$ 1	1,173,204	512	(1,331,653)	$ (157,936)
Balances (in shares) at Mar. 31, 2020	63,004
Increase (Decrease) in Stockholders' Equity
Proceeds from ESPP purchases		2,545			2,545
Proceeds from ESPP purchases (in shares)	168
Employee share-based compensation expense		16,585			16,585
Issuance of restricted shares (in shares)	353
Option exercises		733			733
Option exercises (in shares)	33
Repurchase of shares to satisfy tax withholding		(1,144)			(1,144)
Repurchase of shares to satisfy tax withholding (in shares)	(43)
Net unrealized gain/loss on marketable securities			(232)		(232)
Net loss				(62,887)	(62,887)
Balances at Jun. 30, 2020	$ 1	$ 1,191,923	$ 280	$ (1,394,540)	$ (202,336)
Balances (in shares) at Jun. 30, 2020	63,515				63,515