The entity's obligation to fund intellectual property costs of the related party.

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The related party's percentage of fully diluted equity participation in the entity.

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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 total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

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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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No definition available.

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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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Number of new stock issued during the period.

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Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	May 31, 2016	Jul. 31, 2014	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
General and administrative expenses			$ 13,118,000	$ 7,805,000	$ 23,651,000	$ 15,211,000
Collaboration revenue			11,118,000	7,677,000	21,551,000	15,146,000
Deferred revenue			$ 42,160,000	$ 50,859,000	$ 42,160,000	$ 50,859,000	$ 55,932,000	$ 59,868,000
Caribou [Member]
Related Party Transaction [Line Items]
Percentage of related party obligation		30.00%	30.00%	30.00%	30.00%	30.00%
General and administrative expenses			$ 700,000	$ 300,000	$ 900,000	$ 500,000
Caribou [Member] \| Minimum [Member]
Related Party Transaction [Line Items]
Percentage of shares owned			5.00%		5.00%
Novartis [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Issuance of common stock through at-the-market offering, net of issuance costs, shares	277,777
Collaboration revenue			$ 4,800,000	$ 2,400,000	$ 9,500,000	$ 4,700,000
Accounts receivable			1,000,000.0		1,000,000.0		6,000,000.0
Deferred revenue			$ 7,000,000.0		$ 7,000,000.0		$ 14,500,000
Novartis [Member] \| Minimum [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Percentage of shares owned			10.00%		10.00%