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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

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