Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration and license revenue	$ 6,639	$ 3,001	$ 11,970	$ 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

Interest and other income (expense), net.

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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