Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue
Collaborations, licenses and milestones		$ 12,500		$ 16,000
Operating expenses
Research and development	$ 23,332	16,919	$ 49,418	31,967
General and administrative	4,958	4,091	9,520	8,903
Total operating expenses	28,290	21,010	58,938	40,870
Loss from operations	(28,290)	(8,510)	(58,938)	(24,870)
Other income (expenses)
Interest income	2,486	1,038	3,643	2,095
Interest expense	(3)	(3)	(6)	(5)
Other income (expense)	(62)	30	(60)	30
Total other income, net	2,421	1,065	3,577	2,120
Loss before income taxes	(25,869)	(7,445)	(55,361)	(22,750)
Income tax expense		280		450
Net loss	(25,869)	(7,725)	(55,361)	(23,200)
Other comprehensive loss
Net unrealized gain (loss) on marketable securities	193	(44)	(200)	201
Comprehensive loss	$ (25,676)	$ (7,769)	$ (55,561)	$ (22,999)
Net loss per share attributable to common shareholders:
Basic and diluted net loss (in dollars per share)	$ (0.46)	$ (0.17)	$ (1.07)	$ (0.50)
Weighted average shares used to compute net loss per share attributable to common stockholders:
Basic and diluted (in shares)	55,678,990	46,767,759	51,738,348	46,683,744