Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration and license revenue	$ 1,250		$ 2,500
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 13,699	$ 7,834	$ 25,352	$ 13,637
General and administrative	3,450	1,830	6,379	3,327
Total operating expenses	17,149	9,664	31,731	16,964
Loss from operations	(15,899)	(9,664)	(29,231)	(16,964)
Interest and other income, net	2,366	114	2,743	214
Net loss	(13,533)	(9,550)	(26,488)	(16,750)
Other comprehensive income (loss)	14	11	(41)	3
Comprehensive loss	$ (13,519)	$ (9,539)	$ (26,529)	$ (16,747)
Net loss per share, basic and diluted	$ (0.32)	$ (5.64)	$ (1.01)	$ (10.66)
Weighted-average number of shares used to compute basic and diluted net loss per share	42,533,641	1,693,150	26,236,007	1,571,905