CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	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		2,742			2,742
Proceeds from ESPP purchases (in shares)	135
Employee share-based compensation expense		39,599			39,599
Issuance of restricted shares (in shares)	971
Option exercises		1,306			1,306
Option exercises (in shares)	69
Cumulative effect upon the adoption of ASC 606				1,119	1,119
Repurchase of shares to satisfy tax withholding		(8,453)			(8,453)
Repurchase of shares to satisfy tax withholding (in shares)	(146)
Net unrealized gain (loss) on marketable securities			402		402
Net loss				(165,338)	(165,338)
Balances at Sep. 30, 2018	$ 1	948,844	(331)	(961,959)	(13,445)
Balances (in shares) at Sep. 30, 2018	55,410
Balances at Jun. 30, 2018	$ 1	937,437	(525)	(902,526)	34,387
Balances (in shares) at Jun. 30, 2018	55,104
Increase (Decrease) in Stockholders' Equity
Employee share-based compensation expense		11,730			11,730
Issuance of restricted shares (in shares)	242
Option exercises		1,225			1,225
Option exercises (in shares)	64
Repurchase of shares to satisfy tax withholding		(1,548)			(1,548)
Net unrealized gain (loss) on marketable securities			194		194
Net loss				(59,433)	(59,433)
Balances at Sep. 30, 2018	$ 1	948,844	(331)	(961,959)	(13,445)
Balances (in shares) at Sep. 30, 2018	55,410
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		2,605			$ 2,605
Proceeds from ESPP purchases (in shares)	145
Employee share-based compensation expense		36,538			36,538
Issuance of restricted shares (in shares)	898
Option exercises		2,973			2,973
Option exercises (in shares)	151
Repurchase of shares to satisfy tax withholding		(2,743)			(2,743)
Repurchase of shares to satisfy tax withholding (in shares)	(113)
Net unrealized gain (loss) on marketable securities			260		260
Net loss				(170,849)	(170,849)
Balances at Sep. 30, 2019	$ 1	1,000,094	94	(1,182,994)	$ (182,805)
Balances (in shares) at Sep. 30, 2019	56,762				56,762
Balances at Jun. 30, 2019	$ 1	987,209	129	(1,124,563)	$ (137,224)
Balances (in shares) at Jun. 30, 2019	56,637
Increase (Decrease) in Stockholders' Equity
Employee share-based compensation expense		13,019			13,019
Issuance of restricted shares (in shares)	209
Option exercises		428			428
Option exercises (in shares)	29
Repurchase of shares to satisfy tax withholding		(562)			(562)
Repurchase of shares to satisfy tax withholding (in shares)	(113)
Net unrealized gain (loss) on marketable securities			(35)		(35)
Net loss				(58,431)	(58,431)
Balances at Sep. 30, 2019	$ 1	$ 1,000,094	$ 94	$ (1,182,994)	$ (182,805)
Balances (in shares) at Sep. 30, 2019	56,762				56,762