CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 392	$ 1,395	$ 662	$ 2,524
Operating expenses:
Research and development	22,827	10,290	43,368	18,288
General and administrative	5,538	4,527	10,390	8,355
Total operating expenses	28,365	14,817	53,758	26,643
Loss from operations	(27,973)	(13,422)	(53,096)	(24,119)
Other income:
Interest income	1,703	996	2,974	1,476
Total other income	1,703	996	2,974	1,476
Net loss and net loss attributable to common stockholders-basic and diluted	$ (26,270)	$ (12,426)	$ (50,122)	$ (22,643)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.61)	$ (0.34)	$ (1.25)	$ (1.15)
Weighted-average common shares outstanding-basic and diluted	43,075,587	36,860,619	40,230,484	19,606,543