Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance | shares	2,164,100
Number of options, Granted | shares	1,094,879
Number of options, Exercised | shares	(227,149)
Number of options, Cancelled or forfeited | shares	(74,373)
Number of options, Outstanding ending balance | shares	2,957,457	2,164,100
Number of options, Exercisable | shares	669,887
Number of options, Vested and expected to vest | shares	2,957,457
Weighted average exercise price, Outstanding beginning balance | $ / shares	$ 7.75
Weighted average exercise price, Granted | $ / shares	16.22
Weighted average exercise price, Exercised | $ / shares	2.16
Weighted average exercise price, Cancelled or forfeited | $ / shares	4.15
Weighted average exercise price, Outstanding ending balance | $ / shares	11.41	$ 7.75
Weighted average exercise price, Exercisable | $ / shares	2.59
Weighted average exercise price, Vested and expected to vest | $ / shares	$ 11.41
Weighted average remaining contractual term, Outstanding balance	8 years 9 months 18 days	8 years 9 months 7 days
Weighted average remaining contractual term, Exercisable	7 years 8 months 4 days
Weighted average remaining contractual term, vested and expected to vest	8 years 9 months 18 days
Aggregate intrinsic value, Outstanding balance | $	$ 18,956	$ 23,083
Aggregate intrinsic value, Exercisable | $	9,224
Aggregate intrinsic value, vested and expected to vest | $	$ 18,956