Stock-Based Compensation - Summary of Activity under Stock Option Plan and Related Information (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of options outstanding, Outstanding, Beginning balance | shares	1,213,010
Number of options outstanding, Options granted | shares	1,111,901
Number of options outstanding, Options exercised | shares	(136,328)
Number of options outstanding, Options cancelled | shares	(10,917)
Number of options outstanding, Outstanding, Ending balance | shares	2,177,666	1,213,010
Number of options outstanding, options vested and exercisable | shares	747,477
Weighted average exercise price, Outstanding, Beginning balance | $ / shares	$ 1.54
Weighted average exercise price, Options granted | $ / shares	5.77
Weighted average exercise price, Options exercised | $ / shares	1.56
Weighted average exercise price, Options cancelled | $ / shares	0.90
Weighted average exercise price, Outstanding, Ending balance | $ / shares	3.70	$ 1.54
Weighted average exercise price, options vested and exercisable | $ / shares	$ 2.02
Weighted average remaining contractual term, Outstanding, Beginning balance	8 years 8 months 12 days	8 years 8 months 12 days
Weighted average remaining contractual term,options vested and exercisable	7 years 10 months 24 days
Aggregate intrinsic value, Outstanding, Beginning balance | $	$ 999
Aggregate intrinsic value, Options exercised | $	237
Aggregate intrinsic value, Outstanding, Ending balance | $	38,559	$ 999
Aggregate intrinsic value, options vested and exercisable | $	$ 14,493