Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Collaboration revenue	$ 41,439	$ 5,890	$ 42,393	$ 11,730
Operating expenses:
Research and development	58,573	33,271	108,527	61,758
General and administrative	12,333	6,833	22,244	12,516
Total operating expenses	70,906	40,104	130,771	74,274
Other income (expense):
Other income, net	2,442	861	4,836	1,286
Interest expense	(23)	(59)	(55)	(131)
Total other income	2,419	802	4,781	1,155
Net loss	(27,048)	(33,412)	(83,597)	(61,389)
Other comprehensive loss:
Unrealized gain (loss) on investments	210	(200)	(112)	(296)
Comprehensive loss	$ (26,838)	$ (33,612)	$ (83,709)	$ (61,685)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (0.62)	$ (0.86)	$ (1.91)	$ (1.71)
Weighted-average number of common shares used in net loss per share applicable to common stockholders — basic and diluted	43,856	38,775	43,779	35,998