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Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares outstanding, Beginning balance	797,931
Number of shares, Assumed as part of merger with Arsanis	271,230
Number of shares, Granted	136,913
Number of shares, Exercised	(16,483)
Number of shares, Forfeited	(22,378)
Number of shares Outstanding, Ending balance	1,167,213	797,931
Number of shares Options, Exercisable	556,774
Number of shares Options, Vested and expected to vest	979,466
Weighted average exercise price, Beginning balance	$ 8.29
Weighted average exercise price, Assumed as part of merger with Arsanis	62.60
Weighted average exercise price, Granted	17.22
Weighted average exercise price, Exercised	6.85
Weighted average exercise price, Forfeited	8.43
Weighted average exercise price, Ending balance	21.97	$ 8.29
Weighted average exercise price, Exercisable	30.13
Weighted average exercise price, Vested and expected to vest	$ 48.79
Weighted average contractual term outstanding, Beginning balance	7 years 7 months 24 days	8 years 5 months 1 day
Aggregate intrinsic value, Beginning balance	$ 6,486
Aggregate intrinsic value, Exercised	0
Aggregate intrinsic value, Ending balance	$ 7,168	$ 6,486

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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