Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Data (Unaudited)
Collaboration revenues	$ 5,281	$ 614	$ 1,522	$ 6,384	$ 1,057	$ 1,610	$ 1,661	$ 2,808	$ 13,801	$ 7,136	$ 2,060
Total operating expenses	10,045	10,867	9,510	9,678	8,599	7,967	6,619	5,697	40,100	28,882	18,273
Total other income (expense), net	220	440	(2,094)	(3,129)	901	(1,335)	(21)	(41)	(4,563)	(496)	(69)
Net loss before income tax provision	(4,544)	(9,813)	(10,082)	(6,423)	(6,641)	(7,692)	(4,979)	(2,930)	(30,862)	(22,242)	(16,282)
Benefit from income taxes	(138)	(331)	135	80				(396)	(254)	(396)	(23)
Net loss	$ (4,406)	$ (9,482)	$ (10,217)	$ (6,503)	$ (6,641)	$ (7,692)	$ (4,979)	$ (2,534)	$ (30,608)	$ (21,846)	$ (16,259)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.25)	$ (0.53)	$ (1.40)	$ (7.80)	$ (13.19)	$ (17.73)	$ (11.85)	$ (6.38)	$ (2.77)	$ (49.78)	$ (48.81)
Weighted average ordinary shares outstanding, basic and diluted	17,926,165	17,900,978	7,298,139	834,043	503,309	433,795	420,063	397,483	11,045,370	438,862	333,125