CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Ordinary shares	Additional paid-in capital	Accumulated other comprehensive (loss)/income	Accumulated deficit	Total
Beginning balance at Dec. 31, 2018	$ 2,299	$ 720,072	$ (7,259)	$ (535,506)	$ 179,606
Beginning balance (in shares) at Dec. 31, 2018	37,351,653
Increase (decrease) in shareholders' equity
Loss for the period				(59,171)	(59,171)
Other comprehensive income (loss)			(1,259)		(1,259)
Hercules warrants exercise	$ 2	1,271			1,273
Hercules warrants exercise (in shares)	37,175
Exercise of share options	$ 14	2,542			2,556
Exercise of share options (in shares)	236,654
Restricted and performance share units distributed during the period	$ 12	(12)
Restricted and performance share units distributed during the period (in shares)	209,481
Share-based compensation expense		8,886			8,886
Issuance of ordinary shares relating to employee stock purchase plan		165			165
Issuance of ordinary shares relating to employee stock purchase plan (in shares)	4,870
Ending balance at Jun. 30, 2019	$ 2,327	732,924	(8,518)	(594,677)	132,056
Ending balance (in shares) at Jun. 30, 2019	37,839,833
Beginning balance at Mar. 31, 2019	$ 2,322	727,795	(9,727)	(563,278)	157,112
Beginning balance (in shares) at Mar. 31, 2019	37,763,842
Increase (decrease) in shareholders' equity
Loss for the period				(31,399)	(31,399)
Other comprehensive income (loss)			1,209		1,209
Exercise of share options	$ 4	454			458
Exercise of share options (in shares)	52,847
Restricted and performance share units distributed during the period	$ 1	(1)
Restricted and performance share units distributed during the period (in shares)	21,400
Share-based compensation expense		4,592			4,592
Issuance of ordinary shares relating to employee stock purchase plan		84			84
Issuance of ordinary shares relating to employee stock purchase plan (in shares)	1,744
Ending balance at Jun. 30, 2019	$ 2,327	732,924	(8,518)	(594,677)	132,056
Ending balance (in shares) at Jun. 30, 2019	37,839,833
Beginning balance at Dec. 31, 2019	$ 2,651	986,803	(6,689)	(659,707)	$ 323,058
Beginning balance (in shares) at Dec. 31, 2019	43,711,954				43,711,954
Increase (decrease) in shareholders' equity
Loss for the period				(70,550)	$ (70,550)
Other comprehensive income (loss)			(630)		(630)
Exercise of share options	$ 12	3,390			3,402
Exercise of share options (in shares)	203,940
Restricted and performance share units distributed during the period	$ 29	(29)
Restricted and performance share units distributed during the period (in shares)	525,506
Share-based compensation expense		10,078			10,078
Issuance of ordinary shares relating to employee stock purchase plan		147			147
Issuance of ordinary shares relating to employee stock purchase plan (in shares)	3,005
Ending balance at Jun. 30, 2020	$ 2,692	1,000,389	(7,319)	(730,257)	$ 265,505
Ending balance (in shares) at Jun. 30, 2020	44,444,405				44,444,405
Beginning balance at Mar. 31, 2020	$ 2,683	992,136	(11,966)	(687,706)	$ 295,147
Beginning balance (in shares) at Mar. 31, 2020	44,299,596
Increase (decrease) in shareholders' equity
Loss for the period				(42,551)	(42,551)
Other comprehensive income (loss)			4,647		4,647
Exercise of share options	$ 9	2,461			2,470
Exercise of share options (in shares)	139,178
Restricted and performance share units distributed during the period (in shares)	4,427
Share-based compensation expense		5,723			5,723
Issuance of ordinary shares relating to employee stock purchase plan		69			69
Issuance of ordinary shares relating to employee stock purchase plan (in shares)	1,204
Ending balance at Jun. 30, 2020	$ 2,692	$ 1,000,389	$ (7,319)	$ (730,257)	$ 265,505
Ending balance (in shares) at Jun. 30, 2020	44,444,405				44,444,405