Collaborations - Additional Information (Detail)	1 Months Ended			3 Months Ended								12 Months Ended				33 Months Ended	48 Months Ended
	Jul. 31, 2018 USD ($) shares	Apr. 30, 2016 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) PerformanceObligation	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue				$ 7,880,000	$ 7,408,000	$ 7,677,000	$ 7,469,000	$ 6,668,000	$ 7,317,000	$ 5,917,000	$ 6,215,000		$ 30,434,000	$ 26,117,000	$ 16,479,000
Deferred revenue				55,932,000				59,868,000					55,932,000	59,868,000		$ 55,932,000	$ 55,932,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional payments received on each target													1,000,000
Estimated fair value of units			$ 11,600,000
Difference between cash proceeds received and estimated fair value of preferred units			2,600,000
Novartis [Member] | Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period allowed for editing limited number of platform targets												5 years
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] | Maximum [Member] | Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Internalization fee milestone payment												$ 20,000,000
Milestone payment for additional post-internalization targets												4,000,000
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	15,000,000									15,000,000			15,000,000	15,000,000
Quarterly research payments			$ 1,000,000
Research Payments				15,000,000									15,000,000			15,000,000	15,000,000
Research term			5 years
One time collaboration payment				10,000,000									$ 10,000,000			10,000,000	10,000,000
Termination period of agreement													90 days
Novartis [Member] | Novartis Agreement [Member] | Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Internalization fee												$ 50,000,000
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]
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				53,400,000
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													10,300,000	9,300,000	7,800,000		38,900,000
Aggregate transaction price remaining to be recognized				18,500,000									$ 18,500,000			18,500,000	18,500,000
Aggregate transaction price remaining to be recognized, period													through December 2019
Accounts receivable				6,000,000				6,000,000					$ 6,000,000	6,000,000		6,000,000	6,000,000
Deferred revenue				14,500,000				11,200,000					14,500,000	11,200,000		14,500,000	14,500,000
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] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions		$ 75,000,000											$ 75,000,000
One time collaboration payment		$ 25,000,000
Termination period of agreement		180 days
Number of performance obligations | PerformanceObligation													3
Transaction price													$ 125,000,000
Purchase of common stock through private placement													50,000,000
Collaboration term extension period		2 years
Royalty payment obligation expiration period		12 years
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
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] | 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] | Co-Development and Co-Promotion Agreement Member [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Termination period of agreement	180 days
Compensation for prior work	$ 1,500,000
Payment of royalty percentage on net product sales	50.00%
Regeneron Pharmaceuticals Inc. [Member] | Co-Development and Co-Promotion Agreement Member [Member] | Minimum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Options exercisable, number of shares exercisable | shares	5
Collaborative arrangement obligation to be fund in development costs	$ 5,000,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions																75,000,000
Collaboration revenue													$ 20,100,000	16,800,000	8,700,000	45,600,000
Aggregate transaction price remaining to be recognized				41,400,000									$ 41,400,000			41,400,000	41,400,000
Aggregate transaction price remaining to be recognized, period													through April 2022
Deferred revenue				41,400,000				54,100,000					$ 41,400,000	54,100,000		41,400,000	41,400,000
Due from related party				7,500,000				4,100,000					7,500,000	4,100,000	$ 500,000	7,500,000	7,500,000
Accounts receivable				$ 1,500,000				$ 4,500,000					$ 1,500,000	$ 4,500,000		1,500,000	$ 1,500,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue																$ 12,100,000