Collaborations - Additional Information (Detail)	1 Months Ended		3 Months Ended		9 Months Ended		30 Months Ended	46 Months Ended
	Apr. 30, 2016 USD ($)	Jan. 31, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) PerformanceObligation	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue			$ 7,408,000	$ 7,317,000	$ 22,554,000	$ 19,449,000
Deferred revenue			46,264,000		46,264,000		$ 46,264,000	$ 46,264,000	$ 59,868,000
Novartis [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development based milestone payments under agreement		$ 30,300,000
Regulatory based milestone payments for first indication		50,000,000
Regulatory based milestone payments for second indication		50,000,000
Sales based milestone payments under agreement		100,000,000
Novartis [Member] | Novartis Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions		10,000,000
Technology access fees		20,000,000
Quarterly research payments		$ 1,000,000
Research term		5 years
Novartis [Member] | Novartis Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research funding		$ 20,000,000
Novartis [Member] | Unit Purchase Agreement [Member] | Class A Preferred Units [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from issuance of preferred stock		$ 9,000,000
Novartis [Member] | Novartis Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions					10,000,000			37,400,000
Technology access fees			20,000,000		20,000,000		20,000,000	20,000,000
Quarterly research payments			20,000,000		20,000,000		20,000,000	20,000,000
Proceeds from issuance of preferred stock					$ 9,000,000
Number of performance obligations | PerformanceObligation					2
Transaction price					$ 59,000,000
Transaction price allocated to preferred units purchased at fair value					11,600,000
Collaboration revenue			2,400,000	2,400,000	7,200,000	6,800,000		35,700,000
Aggregate transaction price remaining to be recognized			11,600,000		$ 11,600,000		11,600,000	11,600,000
Aggregate transaction price remaining to be recognized, period					through December 2019
Accounts receivable			1,000,000		$ 1,000,000		1,000,000	1,000,000	6,000,000
Deferred revenue			1,600,000		1,600,000		1,600,000	1,600,000	11,200,000
Novartis [Member] | Novartis Arrangement [Member] | HSC and CAR-T Cell Products [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Remaining transaction price allocated to combined performance obligation of licenses and associated research activities					$ 47,400,000
Combined performance obligation of licenses and associated research activities, revenue recognition period					5 years
Regeneron Pharmaceuticals Inc. [Member] | Stock Purchase Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase of common stock through private placement	$ 50,000,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions	$ 75,000,000
Percentage of certain research and development costs to be funded	50.00%
One time collaboration payment	$ 25,000,000
Collaboration term extension period	2 years
Royalty payment obligation expiration period	12 years
Termination period of agreement	180 days
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development based milestone payments under agreement	$ 25,000,000
Sales based milestone payments under agreement	185,000,000
Regulatory based milestone payments under agreement	$ 110,000,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions					$ 75,000,000		75,000,000
Number of performance obligations | PerformanceObligation					3
Transaction price					$ 125,000,000
Collaboration revenue			5,000,000	$ 4,900,000	15,400,000	$ 12,600,000	40,900,000
Aggregate transaction price remaining to be recognized			44,600,000		$ 44,600,000		44,600,000	44,600,000
Aggregate transaction price remaining to be recognized, period					through April 2022
Deferred revenue			44,600,000		$ 44,600,000		44,600,000	44,600,000	54,100,000
Purchase of common stock through private placement					50,000,000
Transaction price allocated to common stock					50,000,000
Remaining transaction price allocated to combined performance obligation					75,000,000
Amount allocated to licenses to targets and associated research activities and evaluation plans					63,800,000
Amount allocated to technology collaboration and associated research activities					$ 11,200,000
Licenses to targets and associated research activities and evaluation plans performance period					6 years
Accounts receivable			$ 1,800,000		$ 1,800,000		1,800,000	$ 1,800,000	$ 4,500,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue							$ 10,500,000