Net Loss per Share - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net loss - basic and diluted	$ (10,611)	$ (12,935)	$ (12,978)	$ (11,643)	$ (23,546)	$ (24,621)
Denominator:
Weighted average common shares outstanding	29,428,249		29,329,229		29,419,741	29,328,075
Less: weighted average common shares subject to repurchase			(20,079)			(26,570)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	29,428,249		29,309,150		29,419,741	29,301,505
Net loss per share, basic and diluted (in dollars per share)	$ (0.36)		$ (0.44)		$ (0.80)	$ (0.84)
Shares of common stock issuable on the conversion of pre-funded warrants	1,458,000				1,458,000

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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Number of shares of common stock subject to repurchase or cancellation determined by relating the portion of time within a reporting period that these shares have been outstanding to the total time in that period. Common stock subject to repurchase are outstanding common shares that are contingently returnable (that is, subject to recall).

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