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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Transfers between fair value levels	$ 0
Measurement Input, Expected Dividend Rate [Member]
Expected dividend yield	0.00%
Genzyme Agreement [Member]
Fair value of derivative liability	$ 0	$ 183,000
Arsanis [Member] | Genzyme Agreement [Member]
Fair value of derivative liability	$ 0
Series A Convertible Preferred Stock [Member] | Arsanis And Merger Sub [Member]
Preferred stock Fair value per share		$ 1.70
Series B Convertible Preferred Stock [Member] | Arsanis And Merger Sub [Member]
Preferred stock Fair value per share		$ 1.86
Convertible Preferred Stock [Member]
Number of warrants for purchase of convertible preferred shares	0

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- Definition

Number of securities into which each warrant or right may be converted. For example, but not limited to, each warrant may be converted into two shares.

+ References

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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- Definition

The percentage rate used to calculate dividend payments on preferred stock.

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- Definition

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- Definition

Preferred stock fair value per share.

+ References

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