Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Collaboration and license revenue		$ 909	$ 10,640	$ 1,807
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 10,667	$ 8,008	$ 37,913	$ 18,461
General and administrative	3,035	1,236	8,822	3,462
Total operating expenses	13,702	9,244	46,735	21,923
Loss from operations	(13,702)	(8,335)	(36,095)	(20,116)
Interest income and other expense, net	855	163	2,243	316
Net loss attributable to common stockholders	(12,847)	(8,172)	(33,852)	(19,800)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	48	(19)	(50)	11
Comprehensive loss	$ (12,799)	$ (8,191)	$ (33,902)	$ (19,789)
Net loss attributable to common stockholders per share, basic and diluted	$ (0.56)	$ (1.60)	$ (1.60)	$ (3.89)
Weighted-average number of common shares used to compute basic and diluted net loss per share	22,977,793	5,116,165	21,164,069	5,093,418