Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share (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
Numerator
Net loss	$ (18,413)	$ (15,461)	$ (36,233)	$ (30,223)
Net loss attributable to common stockholders, basic and diluted	$ (18,413)	$ (15,461)	$ (36,233)	$ (30,223)
Denominator
Weighted average shares outstanding	37,490,960	31,446,235	36,694,820	31,437,448
Less: weighted average shares subject to repurchase	(50,936)	(245,462)	(74,073)	(286,232)
Weighted average shares used to compute basic and diluted net loss per share	37,440,024	31,200,773	36,620,747	31,151,216
Net loss per share attributable to common stockholders, basic and diluted	$ (0.49)	$ (0.50)	$ (0.99)	$ (0.97)

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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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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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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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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