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Nature of the Business and Basis of Presentation - Additional Information (Detail) $ in Millions |  | 3 Months Ended |
--- | --- | --- | ---
May 14, 2019 USD ($) | Mar. 31, 2019 | Mar. 13, 2019
Schedule Of Accounting Policies [Line Items] |  |  |
Percentage of common stock owned |  |  | 63.70%
Net proceeds from sale of common stock and pre-funded warrants | $ 79 |  |
Arsanis [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Business acquisition share exchange ratio |  | 0.5702 |

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