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Summary of Quarterly Results of Operations (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Acquired in-process research and development |  |  |  | $ 154,272 | $ 4,750 | $ 30,000
Voyager Therapeutics | IPR&D | Collaboration and License Agreement |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Acquired in-process research and development |  | $ 5,000 | $ 113,100 |  |  |
Xenon | IPR&D | Collaboration and License Agreement |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Acquired in-process research and development | $ 36,200 |  |  |  |  |

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