Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 1,464	$ 4,830
Operating Expenses [Abstract]
Research and development	14,072	8,732
General and administrative	4,914	3,565
Total operating expenses	18,986	12,297
Operating loss	(17,522)	(7,467)
Other expense, net
Interest income	253	465
Other expense, net	395	(186)
Total other income (expense)	648	279
Loss before income taxes	(16,874)	(7,188)
Income tax benefit	226
Net loss	(16,648)	(7,188)
Other comprehensive loss
Net unrealized gain on available-for-sale-securities, net of tax expense of $244 for the three months ended March, 31 2017	377	281
Total other comprehensive gain	377	281
Comprehensive loss	(16,271)	(6,907)
Reconciliation of net loss to net loss attributable to common stockholder:
Net loss attributable to common stockholders	$ (16,648)	$ (7,188)
Net loss per share attributable to common stock, basic and diluted	$ (0.65)	$ (0.29)
Weighted-average common shares outstanding, basic and diluted	25,791,591	25,076,769