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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 13, 2019	Jan. 01, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - non-current	$ 947			$ 364
Fair value of derivative liability	0	$ 5,235
Comprehensive loss included of foreign currency translation gain adjustments	23
Operating lease liabilities	4,116
Right-of-use assets	2,705
Collateralized Credit Card Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - non-current	150
Accounting Standards Update 2016-02 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Operating lease liabilities	2,538		$ 2,538
Right-of-use assets	2,026		$ 2,026
Company's Office Leases
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - non-current	$ 250
Lease expiration date	Dec. 31, 2023
Company's Office Leases \| Waltham Lease [Member] \| Arsanis [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - non-current	$ 283
Letter of Credit [Member] \| Cambridge Lease Agreement [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash - non-current	$ 264			$ 264