Stock-Based Compensation - Summary of Company's Stock Option Activity and Related Information (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Shares Available for Grant
Shares Available for Grant, Beginning balance	1,908,626
Shares Available for Grant, Additional options authorized	3,853,280
Shares Available for Grant, Options granted	(2,624,505)
Shares Available for Grant, Options canceled	553,585
Shares Available fot Grant, Withheld shares to pay for taxes on RSU released shares	253,986
Shares Available for Grant, RSUs granted	(708,144)
Shares Available for Grant, RSUs canceled	70,732
Shares Available for Grant, Ending balance	3,307,560	1,908,626
Number of Stock Options Outstanding
Number of Stock Options Outstanding, Beginning balance	11,603,708
Number of Stock Options Outstanding, Options granted	2,624,505
Number of Stock Options Outstanding, Options exercised	(2,219,251)
Number of Stock Options Outstanding, Options canceled	(553,585)
Number of Stock Options Outstanding, Ending balance	11,455,377	11,603,708
Number of Stock Options Outstanding, Vested and exercisable	4,575,393
Number of Stock Options Outstanding, Vested and expected to vest	10,793,250
Weighted- Average Exercise Price
Weighted- Average Exercise Price, Beginning balance | $ / shares	$ 6.64
Weighted- Average Exercise Price, Options granted | $ / shares	8.19
Weighted- Average Exercise Price, Options exercised | $ / shares	4.34
Weighted- Average Exercise Price, Options canceled | $ / shares	6.73
Weighted- Average Exercise Price, Ending balance | $ / shares	7.58	$ 6.64
Weighted- Average Exercise Price, Vested and exercisable | $ / shares	6.25
Weighted- Average Exercise Price, Vested and expected to vest | $ / shares	$ 7.38
Weighted- Average Remaining Contractual Life (Years)
Options Outstanding, Weighted- Average Remaining Contractual Life (Years)	7 years 10 months 24 days	7 years 7 months 6 days
Weighted- Average Remaining Contractual Life (Years), Vested and exercisable	6 years 2 months 12 days
Weighted- Average Remaining Contractual Life (Years), Vested and expected to vest	7 years 8 months 12 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value | $	$ 1,011	$ 10,151
Aggregate Intrinsic Value, Vested and exercisable | $	1,010
Aggregate Intrinsic Value, Vested and expected to vest | $	$ 1,010