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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

X

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of the write-off for research and development assets that were acquired in a transaction other than a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

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