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, 2019	$ 1	$ 1,024,614	$ 145	$ (1,248,600)	$ (223,840)
Balances (in shares) at Dec. 31, 2019	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 available-for-sale investments			(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
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 available-for-sale investments			(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
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
Net proceeds from sale of ordinary shares		108,180			$ 108,180
Net proceeds from sale of ordinary shares (in shares)	7,705
Proceeds from ESPP purchases		2,862			2,862
Proceeds from ESPP purchases (in shares)	188
Employee share-based compensation expense		45,143			45,143
Issuance of restricted shares (in shares)	1,978
Option exercises		5			5
Repurchase of shares to satisfy tax withholding		(8,606)			(8,606)
Repurchase of shares to satisfy tax withholding (in shares)	(501)
Net unrealized gain (loss) on available-for-sale investments			(33)		(33)
Net loss				(167,392)	(167,392)
Balances at Sep. 30, 2021	$ 1	1,370,402	14	(1,694,009)	$ (323,592)
Balances (in shares) at Sep. 30, 2021	73,698				73,698
Balances at Jun. 30, 2021	$ 1	1,358,318	8	(1,658,701)	$ (300,374)
Balances (in shares) at Jun. 30, 2021	73,470
Increase (Decrease) in Stockholders' Equity
Employee share-based compensation expense		14,370			14,370
Issuance of restricted shares (in shares)	391
Repurchase of shares to satisfy tax withholding		(2,286)			(2,286)
Repurchase of shares to satisfy tax withholding (in shares)	(163)
Net unrealized gain (loss) on available-for-sale investments			6		6
Net loss				(35,308)	(35,308)
Balances at Sep. 30, 2021	$ 1	$ 1,370,402	$ 14	$ (1,694,009)	$ (323,592)
Balances (in shares) at Sep. 30, 2021	73,698				73,698