Stockholders' Equity - Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
2003 Stock Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	713,211
Options exercised	(43,131)
Outstanding at end of period	670,080	713,211
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 1.25
Options exercised	1.31
Outstanding at end of period	$ 1.24	$ 1.25
VESTED AND EXPECTED TO VEST
Aggregate Intrinsic Value (in thousands)	$ 8,816
ADDITIONAL DISCLOSURES
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 8,816
Weighted-average contractual term outstanding	1 year 4 months 24 days	2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 8,816
2013 Equity Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	2,664,163
Options granted	812,979
Options exercised	(123,658)
Options forfeited	(52,035)
Outstanding at end of period	3,301,449	2,664,163
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 7.34
Options granted	19.53
Options exercised	6.79
Options forfeited	13.02
Outstanding at end of period	$ 8.44	$ 7.34
VESTED AND EXPECTED TO VEST
Outstanding options	3,301,449
Weighted-Average Exercise Price	$ 8.44
Weighted-Average Remaining Contractual Term	7 years 3 months 18 days
Aggregate Intrinsic Value (in thousands)	$ 21,243
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	1,940,699
Weighted-average exercise price exercisable at end of period	$ 7.72
Weighted-average contractual term (years) exercisable at end of period	6 years 3 months 18 days
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 13,014
Weighted-average contractual term outstanding	7 years 3 months 18 days	7 years 4 months 24 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 21,243

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Number of options outstanding, including both vested and non-vested options.

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