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		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
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
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		47,425			47,425
Issuance of restricted shares (in shares)	1,590
Option exercises		960			960
Option exercises (in shares)	43
Repurchase of shares to satisfy tax withholding		(8,935)			(8,935)
Repurchase of shares to satisfy tax withholding (in shares)	(360)
Net unrealized gain/loss on marketable securities			(23)		(23)
Net loss				(219,583)	(219,583)
Balances at Sep. 30, 2020	$ 1	1,206,524	122	(1,468,183)	$ (261,536)
Balances (in shares) at Sep. 30, 2020	63,956				63,956
Balances at Jun. 30, 2020	$ 1	1,191,923	280	(1,394,540)	$ (202,336)
Balances (in shares) at Jun. 30, 2020	63,515
Increase (Decrease) in Stockholders' Equity
Employee share-based compensation expense		15,564			15,564
Issuance of restricted shares (in shares)	493
Option exercises		24			24
Option exercises (in shares)	2
Repurchase of shares to satisfy tax withholding		(987)			(987)
Repurchase of shares to satisfy tax withholding (in shares)	(54)
Net unrealized gain/loss on marketable securities			(158)		(158)
Net loss				(73,643)	(73,643)
Balances at Sep. 30, 2020	$ 1	$ 1,206,524	$ 122	$ (1,468,183)	$ (261,536)
Balances (in shares) at Sep. 30, 2020	63,956				63,956