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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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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Aug. 31, 2015	Jul. 31, 2014
Related Party Transaction [Line Items]
General and administrative expenses		$ 8,270,000	$ 5,711,000	$ 23,481,000	$ 17,812,000
Collaboration revenue		7,408,000	7,317,000	22,554,000	19,449,000
Deferred revenue		46,264,000		46,264,000			$ 59,868,000
Caribou [Member]
Related Party Transaction [Line Items]
Junior preferred units issued									8,110,599
Percentage of shares owned						9.20%
General and administrative expenses		$ 100,000	$ 100,000	$ 700,000	$ 600,000
Percentage of related party obligation		30.00%	30.00%	30.00%	30.00%
Novartis [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Percentage of shares owned						9.90%
Common stock, shares acquired	277,777
Collaboration revenue		$ 2,400,000	$ 2,400,000	$ 7,200,000	$ 6,800,000
Accounts receivable		1,000,000		1,000,000			6,000,000
Deferred revenue		$ 1,600,000		$ 1,600,000			$ 11,200,000
Novartis [Member] \| Class A-1 Preferred Units [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Junior preferred units issued									4,761,905
Novartis [Member] \| Class A-2 Preferred Units [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Junior preferred units issued									2,666,666
Novartis [Member] \| Series B Preferred Stock [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Preferred stock, shares acquired								761,905