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Merger Accounting - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Acquisition Date [Line Items]
Goodwill	$ 27,407
Arsanis [Member]
Acquisition Date [Line Items]
Cash and cash equivalents and restricted cash	26,406
Other current assets	2,147
Property and equipment, net	68
IPR&D indefinite-lived intangible assets	4,900
Other assets, non-current	879
Current liabilities	(5,221)
Loans payable	(8,713)
Other liabilities, non-current	(1,515)
Goodwill	27,407
Purchase price	$ 46,358

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