CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 270	$ 1,129
Operating expenses:
Research and development	20,536	7,997
General and administrative	4,857	3,828
Total operating expenses	25,393	11,825
Loss from operations	(25,123)	(10,696)
Other income (expense):
Interest income	1,271	479
Total other income (expense)	1,271	479
Net loss and net loss attributable to common stockholders-basic and diluted	$ (23,852)	$ (10,217)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.64)	$ (4.09)
Weighted-average common shares outstanding-basic and diluted	37,384,507	2,500,178