Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Outstanding, beginning balance (in shares)	797,931
Assumed as part of Merger with Arsanis (in shares)	271,230
Granted, (in shares)	589,110
Exercised (in shares)	(51,617)
Forfeited (in shares)	(309,625)
Outstanding, ending balance (in shares)	1,297,029	797,931
Exercisable, ending balance (in shares)	495,740
Vested and expected to vest, ending balance (in shares)	1,122,542
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 8.29
Assumed as part of Merger with Arsanis (in dollars per share)	62.60
Granted (in dollars per share)	14.69
Exercised (in dollars per share)	7.05
Forfeited (in dollars per share)	31.55
Outstanding, ending balance (in dollars per share)	17.05	$ 8.29
Exercisable (in dollars per share)	21.66
Vested and expected to vest (in dollars per share)	$ 17.15
Weighted Average Contractual Term (Years)
Outstanding	8 years 4 months 28 days	8 years 5 months 1 day
Exercisable	7 years 2 months 15 days
Vested and expected to vest	8 years 3 months 25 days
Aggregate Intrinsic Value
Outstanding beginning balance	$ 6,486
Outstanding ending balance	1,286	$ 6,486
Exercisable	952
Vested and expected to vest	$ 1,245