Related Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 26,917,000	$ 32,100,000	$ 21,766,000
Flagship Pioneering [Member] \| Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0	0	17,000
Due to related party	0	0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0
Collaboration revenue - related party	26,917,000	32,100,000	$ 21,766,000
Deferred revenue	137,259,000	$ 96,926,000		$ 120,000,000
Due from related party	$ 0

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

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Carrying amount as of the balance sheet date of obligations due all related parties.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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