Collaborations - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended	42 Months Ended	57 Months Ended
	Jul. 31, 2018 USD ($) shares	Apr. 30, 2016 USD ($)	Jan. 31, 2015 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) PerformanceObligation	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
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				10,616,000	$ 7,408,000	32,167,000	$ 22,554,000
Deferred revenue				35,144,000	46,264,000	35,144,000	46,264,000		35,144,000	35,144,000	$ 55,932,000	$ 59,868,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional payments received on each target						1,000,000.0
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				18,000,000.0		18,000,000.0			18,000,000.0	18,000,000.0
Research term			5 years
Proceeds from issuance of preferred stock						$ 9,000,000.0
One time collaboration payment											10,000,000.0
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] | Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Internalization fee								$ 50,000,000.0
Period allowed for editing limited number of platform targets								5 years
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] | 2014 Novartis Agreement [Member] | Maximum [Member] | Scenario, Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Internalization fee milestone payment								$ 20,000,000.0
Milestone payment for additional post-internalization targets								$ 4,000,000.0
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										56,400,000
Collaboration revenue				4,800,000	2,400,000	$ 14,300,000	7,200,000			53,200,000
Aggregate transaction price remaining to be recognized				4,100,000		$ 4,100,000			4,100,000	4,100,000
Aggregate transaction price remaining to be recognized, period						through December 2019
Accounts receivable				1,000,000.0		$ 1,000,000.0			1,000,000.0	1,000,000.0	6,000,000.0
Deferred revenue				3,100,000		$ 3,100,000			3,100,000	3,100,000	14,500,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions		$ 75,000,000.0
One time collaboration payment		$ 25,000,000.0
Termination period of agreement		180 days
Number of performance obligations | PerformanceObligation						3
Percentage of certain research and development costs to be funded		50.00%
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]
Development based milestone payments under agreement		$ 25,000,000.0
Sales based milestone payments under agreement		185,000,000.0
Regulatory based milestone payments under agreement		110,000,000.0
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	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	$ 5,000,000.0
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions						$ 75,000,000.0			75,000,000.0
Transaction price						125,000,000.0
Collaboration revenue				5,800,000	5,000,000.0	17,800,000	15,400,000		63,500,000
Aggregate transaction price remaining to be recognized				32,000,000.0		$ 32,000,000.0			32,000,000.0	32,000,000.0
Aggregate transaction price remaining to be recognized, period						through April 2022
Deferred revenue				32,000,000.0		$ 32,000,000.0			32,000,000.0	32,000,000.0	41,400,000
Purchase of common stock through private placement						50,000,000.0
Payments due				2,600,000	$ 1,800,000	8,400,000	$ 6,000,000.0
Accounts receivable				$ 2,600,000		$ 2,600,000			2,600,000	$ 2,600,000	$ 1,500,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									$ 20,500,000