Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Outstanding beginning balance	1,034,961
Number of options, Granted	1,277,528
Number of options, Exercised	(16,974)
Number of options, Cancelled or forfeited	(131,414)
Number of options, Outstanding ending balance	2,164,101	1,034,961
Number of options, Exercisable	538,214
Number of options, Unvested	1,625,886
Weighted average exercise price, Outstanding beginning balance	$ 0.78
Weighted average exercise price, Granted	12.78
Weighted average exercise price, Exercised	0.98
Weighted average exercise price, Cancelled or forfeited	2.54
Weighted average exercise price, Outstanding ending balance	7.75	$ 0.78
Weighted average exercise price, Exercisable	1.22
Weighted average exercise price, Unvested	$ 9.92
Weighted average remaining contractual term, Outstanding balance	8 years 9 months 7 days	8 years 11 months 8 days
Weighted average remaining contractual term, Exercisable	7 years 11 months 19 days
Weighted average remaining contractual term, Unvested	9 years 14 days
Aggregate intrinsic value, Outstanding balance	$ 23,083	$ 3,427
Aggregate intrinsic value, Exercisable	8,311
Aggregate intrinsic value, Unvested	$ 14,771