Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 6,615,000	$ 3,766,000
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Collaboration revenue - related party	6,615,000	3,766,000
Deferred revenue	130,644,000		$ 120,000,000
Payments under agreements with related party	0	$ 0
Due from related party for the reimbursement of development costs	$ 0

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

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The aggregate amount of receivables to be collected from related parties where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth, at the financial statement date. which are usually due within one year (or one business cycle).

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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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Amount of selling, general and administrative expenses resulting from transactions, excluding transactions that are eliminated in consolidated or combined financial statements, with related party.

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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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