Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Quarterly Financial Data (Unaudited)
Collaboration revenues	$ 3,848	$ 3,842	$ 1,571	$ 1,129	$ 5,281	$ 614	$ 1,522	$ 6,384	$ 10,390	$ 13,801	$ 7,136
Total operating expenses	20,907	14,517	14,154	12,772	10,045	10,867	9,510	9,678	62,350	40,100	28,882
Total other income (expense), net	(429)	72	371	212	220	440	(2,094)	(3,129)	226	(4,563)	(496)
Net loss before income tax provision	(17,488)	(10,603)	(12,212)	(11,431)	(4,544)	(9,813)	(10,082)	(6,423)	(51,734)	(30,862)	(22,242)
(Benefit from) provision for income taxes	(55)	(465)	(97)	(107)	(138)	(331)	135	80	(724)	(254)	(396)
Net loss	$ (17,433)	$ (10,138)	$ (12,115)	$ (11,324)	$ (4,406)	$ (9,482)	$ (10,217)	$ (6,503)	$ (51,010)	$ (30,608)	$ (21,846)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.83)	$ (0.52)	$ (0.67)	$ (0.63)	$ (0.25)	$ (0.53)	$ (1.40)	$ (7.80)	$ (2.66)	$ (2.77)	$ (49.78)
Weighted average ordinary shares outstanding, basic and diluted	21,057,855	19,426,833	18,077,770	17,997,929	17,926,165	17,900,978	7,298,139	834,043	19,145,938	11,045,370	438,862
Revenue recognized by non exercising of option									$ 1,702	$ 9,984
Sanofi
Selected Quarterly Financial Data (Unaudited)
Collaboration revenues									0	$ 10,724	$ 4,007
Sanofi | Sickle Cell License Option Material Right
Selected Quarterly Financial Data (Unaudited)
Revenue recognized by non exercising of option								$ 5,300
Sanofi | Hemophilia License Option Material Right
Selected Quarterly Financial Data (Unaudited)
Revenue recognized by non exercising of option					$ 4,700
Settlement And License Agreement With Pepscan
Selected Quarterly Financial Data (Unaudited)
Litigation settlement amount	$ 4,700								$ 4,700