Net Loss Per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (9,607)	$ (11,265)	$ (10,620)	$ (17,713)	$ (4,311)	$ (6,230)	$ (11,610)	$ (14,507)	$ (49,205)	$ (36,658)	$ (31,778)
Weighted average shares outstanding									30,051,961	29,424,360	28,492,091
Less: weighted average shares subject to repurchase										(306)	(7,088)
Weighted average shares used to compute basic and diluted net loss per share	30,214,454	30,123,188	30,028,985	29,836,277	29,664,926	29,513,842	29,351,414	29,159,509	30,051,961	29,424,054	28,485,003
Net loss per share, basic and diluted	$ (0.32)	$ (0.37)	$ (0.35)	$ (0.59)	$ (0.15)	$ (0.21)	$ (0.40)	$ (0.50)	$ (1.64)	$ (1.25)	$ (1.12)

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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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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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