Basis of Presentation and Summary of Significant Accounting Policies - Condensed Consolidated Statement of Operations and Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue	$ 5,331	$ 2,410
Operating expenses:
Research and development	16,618	11,917
General and administrative	7,313	5,476
Total operating expenses	23,931	17,393
Loss from operations	(18,600)	(14,983)
Interest and other income, net	780	221
Net loss	(17,820)	(14,762)
Other comprehensive loss	(137)	(55)
Comprehensive loss	(17,957)	(14,817)
Net loss attributable to common stockholders	$ (17,820)	$ (14,762)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.47)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted	35,827,235	31,089,310
As Originally Reported | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue		$ 5,625
Operating expenses:
Research and development		11,917
General and administrative		5,476
Total operating expenses		17,393
Loss from operations		(11,768)
Interest and other income, net		221
Net loss		(11,547)
Other comprehensive loss		(55)
Comprehensive loss		(11,602)
Net loss attributable to common stockholders		$ (11,547)
Net loss per share attributable to common stockholders, basic and diluted		$ (0.37)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted		31,089,310
Effect of Change | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue		$ (3,215)
Operating expenses:
Loss from operations		(3,215)
Net loss		(3,215)
Comprehensive loss		(3,215)
Net loss attributable to common stockholders		$ (3,215)
Net loss per share attributable to common stockholders, basic and diluted		$ (0.10)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted		31,089,310