Collaborations - Additional Information (Detail)			1 Months Ended			3 Months Ended								12 Months Ended			45 Months Ended	60 Months Ended
	Dec. 13, 2019	Dec. 12, 2019	Jul. 31, 2018 USD ($) shares	Apr. 30, 2016 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) PerformanceObligation	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred to obtain collaboration agreements and costs to fulfill contracts														$ 0
Costs to obtain or fulfill contract capitalized						$ 0								0			$ 0	$ 0
Collaboration revenue						10,936,000	$ 10,616,000	$ 11,118,000	$ 10,433,000	$ 7,880,000	$ 7,408,000	$ 7,677,000	$ 7,469,000	43,103,000	$ 30,434,000	$ 26,117,000
Deferred revenue						28,810,000				55,932,000				$ 28,810,000	55,932,000	59,868,000	28,810,000	28,810,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Strategic collaboration agreement amended date														2018-12
Regulatory based milestone payments under agreement														$ 5,000,000.0
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] | 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					50,000,000.0
Regulatory based milestone payments for second indication					50,000,000.0
Sales based milestone payments under agreement					100,000,000.0
Novartis [Member] | 2014 Novartis Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions					10,000,000.0									10,000,000.0
Technology access fees					20,000,000.0	20,000,000.0								20,000,000.0			20,000,000.0	20,000,000.0
Quarterly research payments					$ 1,000,000.0	20,000,000.0								20,000,000.0			20,000,000.0	20,000,000.0
Research Payments						19,000,000.0								19,000,000.0			19,000,000.0	19,000,000.0
Research term					5 years
Proceeds from issuance of preferred stock														$ 9,000,000.0
One time collaboration payment										10,000,000.0					10,000,000.0
Termination period of agreement														90 days
Number of performance obligations | PerformanceObligation														2
Transaction price														$ 59,000,000.0
Transaction price allocated to preferred units purchased at fair value														11,600,000
Novartis [Member] | 2014 Novartis Agreement [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
Novartis [Member] | 2014 Novartis Agreement [Member] | LSCs [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations | PerformanceObligation														1
Transaction price														$ 10,000,000.0
Combined performance obligation of licenses and associated research activities, revenue recognition period														1 year
Novartis [Member] | 2014 Novartis Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research funding					$ 20,000,000.0
Novartis [Member] | Unit Purchase Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from issuance of preferred stock					$ 9,000,000.0
Novartis [Member] | Novartis Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions																		57,400,000
Collaboration revenue														$ 18,500,000	10,300,000	9,300,000		57,400,000
Aggregate transaction price remaining to be recognized, period														Through December 31, 2019
Accounts receivable						1,000,000.0				6,000,000.0				$ 1,000,000.0	6,000,000.0		1,000,000.0	1,000,000.0
Deferred revenue						0				14,500,000				0	14,500,000		0	0
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions				$ 75,000,000.0										$ 75,000,000.0
One time collaboration payment				$ 25,000,000.0
Termination period of agreement				180 days
Number of performance obligations | PerformanceObligation														3
Transaction price														$ 125,000,000.0
Purchase of common stock through private placement														50,000,000.0
Collaboration term extension period				2 years
Royalty payment obligation expiration period				12 years
Transaction price allocated to common stock														50,000,000.0
Remaining transaction price allocated to combined performance obligation														75,000,000.0
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]
Regulatory based milestone payments under agreement				$ 110,000,000.0
Development based milestone payments under agreement				25,000,000.0
Sales based milestone payments under agreement				185,000,000.0
Regeneron Pharmaceuticals Inc. [Member] | Stock Purchase Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase of common stock through private placement				$ 50,000,000.0
Regeneron Pharmaceuticals Inc. [Member] | Co-Development and Co-Promotion Agreement [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	25.00%	50.00%	50.00%
Regeneron Pharmaceuticals Inc. [Member] | Co-Development and Co-Promotion Agreement [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 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions																	75,000,000.0
Collaboration revenue														$ 24,600,000	20,100,000	16,800,000	70,300,000
Aggregate transaction price remaining to be recognized, period														through April 2022
Deferred revenue						28,800,000				41,400,000				$ 28,800,000	41,400,000		28,800,000	28,800,000
Payments due														12,000,000.0	7,500,000	$ 4,100,000
Aggregate transaction price remaining to be recognized						28,800,000								28,800,000			28,800,000	28,800,000
Accounts receivable						$ 3,600,000				$ 1,500,000				$ 3,600,000	$ 1,500,000		3,600,000	$ 3,600,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue																	$ 24,100,000