Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	797,931
Assumed as part of Merger with Arsanis (in shares) | shares	271,230
Granted (in shares) | shares	481,660
Exercised (in shares) | shares	(25,627)
Forfeited (in shares) | shares	(254,053)
Ending balance (in shares) | shares	1,271,141	797,931
Number of shares Options, Exercisable (in shares) | shares	480,025
Number of shares Options, Vested and expected to vest (in shares) | shares	1,092,790
Weighted Average Exercise Price
Beginning balance (in usd per share) | $ / shares	$ 8.29
Assumed as part of merger with Arsanis (in usd per share) | $ / shares	62.60
Granted (in usd per share) | $ / shares	15.48
Exercised (in usd per share) | $ / shares	6.90
Forfeited (in usd per share) | $ / shares	35.92
Ending balance (in usd per share) | $ / shares	17.11	$ 8.29
Weighted average exercise price, Exercisable (in usd per share) | $ / shares	21.49
Weighted average exercise price, Vested and expected to vest (in usd per share) | $ / shares	$ 17.18
Weighted average contractual term outstanding (in years)	8 years 5 months 12 days	8 years 5 months 1 day
Weighted average contractual term outstanding, Exercisable (in years)	7 years 1 month 24 days
Weighted average contractual term outstanding, Vested and expected to vest (in years)	8 years 4 months 6 days
Aggregate intrinsic value, Beginning balance | $	$ 6,486
Aggregate intrinsic value, Ending balance | $	2,760	$ 6,486
Aggregate intrinsic value, exercisable | $	1,709
Aggregate intrinsic value, vested and expected to vest | $	$ 2,595