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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue				$ 5,411
Operating expenses:
Research and development	$ 17,218	$ 13,689	$ 33,836	25,606
General and administrative	8,912	5,082	16,225	10,558
Total operating expenses	26,130	18,771	50,061	36,164
Loss from operations	(19,491)	(15,770)	(38,091)	(30,753)
Interest and other income, net	1,078	309	1,858	530
Net loss	(18,413)	(15,461)	(36,233)	(30,223)
Other comprehensive income (loss)	70	(3)	(67)	(58)
Comprehensive loss	(18,343)	(15,464)	(36,300)	(30,281)
Net loss attributable to common stockholders	$ (18,413)	$ (15,461)	$ (36,233)	$ (30,223)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.49)	$ (0.50)	$ (0.99)	$ (0.97)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted	37,440,024	31,200,773	36,620,747	31,151,216
As Originally Reported | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue				$ 11,250
Operating expenses:
Research and development				25,606
General and administrative				10,558
Total operating expenses				36,164
Loss from operations				(24,914)
Interest and other income, net				530
Net loss				(24,384)
Other comprehensive income (loss)				(58)
Comprehensive loss				(24,442)
Net loss attributable to common stockholders				$ (24,384)
Net loss per share attributable to common stockholders, basic and diluted				$ (0.78)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted				31,151,216
Effect of Change | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue				$ (5,839)
Operating expenses:
Loss from operations				(5,839)
Net loss				(5,839)
Comprehensive loss				(5,839)
Net loss attributable to common stockholders				$ (5,839)
Net loss per share attributable to common stockholders, basic and diluted				$ (0.19)