Stockholders' Equity - Summary of the Company's Stock Options Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Options expected to vest, end of period (in shares)	1,100
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Options outstanding, beginning of year (in shares)	804
Number of options, Granted (in shares)	6,366
Number of options, Exercised (in shares)	(445)
Number of options, Expired / terminated (in shares)	(43)
Number of options, Options outstanding, end of year (in shares)	6,682
Number of options, Options exercisable, end of period (in shares)	311
Number of options, Options expected to vest, end of period (in shares)	1,061
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, Options outstanding, beginning of year (in dollars per share) | $ / shares	$ 4.99
Weighted average exercise price, Granted (in dollars per share) | $ / shares	28.58
Weighted average exercise price, Exercised (in dollars per share) | $ / shares	5.26
Weighted average exercise price, Expired / terminated (in dollars per share) | $ / shares	4.46
Weighted average exercise price, Options outstanding, end of period (in dollars per share) | $ / shares	27.45
Weighted average exercise price, Options exercisable, end of period (in dollars per share) | $ / shares	$ 4.66
Weighted average remaining contractual life, Options outstanding, end of period	9 years 2 months 23 days
Weighted average remaining contractual life, Options exercisable, end of period	8 months 9 days
Aggregate intrinsic value, Options outstanding, end of period | $	$ 238,748
Aggregate intrinsic value, Options exercisable, end of period | $	$ 19,871