Stockholders' Equity - Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
2003 Stock Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	713,211
Options exercised	(30,362)
Outstanding at end of period	682,849	713,211
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 1.25
Options exercised	1.24
Outstanding at end of period	$ 1.25	$ 1.25
VESTED AND EXPECTED TO VEST
Outstanding options	682,849
Weighted-Average Exercise Price	$ 1.25
Aggregate Intrinsic Value (in thousands)	$ 10,163
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	682,849
Weighted-average exercise price exercisable at end of period	$ 1.25
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 10,163
Weighted-average contractual term outstanding	1 year 8 months 12 days	2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 10,163
2013 Equity Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	2,664,163
Options granted	715,329
Options exercised	(54,624)
Options forfeited	(31,495)
Outstanding at end of period	3,293,373	2,664,163
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 7.34
Options granted	20.16
Options exercised	6.48
Options forfeited	13.10
Outstanding at end of period	$ 8.25	$ 7.34
VESTED AND EXPECTED TO VEST
Outstanding options	3,293,373
Weighted-Average Exercise Price	$ 8.25
Weighted-Average Remaining Contractual Term	7 years 4 months 24 days
Aggregate Intrinsic Value (in thousands)	$ 26,958
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	1,914,278
Weighted-average exercise price exercisable at end of period	$ 7.77
Weighted-average contractual term (years) exercisable at end of period	6 years 4 months 24 days
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 15,970
Weighted-average contractual term outstanding	7 years 4 months 24 days	7 years 4 months 24 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 26,958

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