Collaborations - Additional Information (Detail)				1 Months Ended		3 Months Ended		6 Months Ended		51 Months Ended	66 Months Ended
	May 30, 2020 USD ($) Targetcap Product Agreement $ / shares shares	Dec. 13, 2019	Dec. 12, 2019	Jul. 31, 2018 USD ($) shares	Apr. 30, 2016 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) PerformanceObligation $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock, par value | $ / shares						$ 0.0001		$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Collaboration revenue						$ 16,263,000	$ 11,118,000	$ 29,179,000	$ 21,551,000
Deferred revenue						100,679,000	42,160,000	100,679,000	42,160,000	$ 100,679,000	$ 100,679,000	$ 28,810,000	$ 55,932,000
2020 Regeneron Amendment [Member] | Hemophilia Co Co Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment of royalty percentage on net product sales	35.00%
Regeneron Pharmaceuticals Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Stock purchase agreement date	May 30, 2020
Regeneron Pharmaceuticals Inc. [Member] | License Agreement Terms [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate transaction price remaining to be recognized						15,300,000		15,300,000		15,300,000	15,300,000
Regeneron Pharmaceuticals Inc. [Member] | 2016 Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions					$ 75,000,000.0			75,000,000.0
Purchase of common stock through private placement								$ 50,000,000.0
Number of performance obligations | PerformanceObligation								3
Transaction price								$ 125,000,000.0
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] | 2016 Regeneron Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development based milestone payments under agreement					25,000,000.0
Regulatory based milestone payments under agreement					110,000,000.0
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
Premium on sale of shares								$ 17,400,000
Fair value recorded in statement of stockholders’ equity upon issuance of shares								12,600,000
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions	$ 70,000,000.0							$ 70,000,000.0
Collaboration agreement further option extension period								twenty-four months
Additional regeneron target cap | Targetcap	5
Fee to extend collaboration arrangement	$ 25,000,000.0
Collaboration term extension period	2024-04
One time collaboration payment	$ 30,000,000.0
Collaboration term extension period	2 years
Royalty payment obligation expiration period	12 years
Termination period of agreement	180 days
Number of performance obligations | PerformanceObligation								3
Transaction price								$ 110,900,000
Amount allocated to licenses to targets and associated research activities and evaluation plans								91,900,000
Amount allocated to technology collaboration and associated research activities								3,700,000
Cumulative catch up adjustment								8,400,000
Remaining transaction price transferred at inception of arrangement								23,500,000
Transaction price allocated to combined performance obligation								110,900,000
Amount allocated to transfer license to develop factor VIII target for hemophilia A								15,300,000
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member] | Option Exercised to Increase Regeneron Target Cap [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions	$ 30,000,000.0
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member] | Hemophilia Co Co Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment of royalty percentage on net product sales	65.00%
Number of agreements entered | Agreement	2
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in regeneron target cap | Targetcap	15
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member] | Maximum [Member] | Non-exclusive License to Regeneron under Certain CRISPR/Cas Platform IP [Member] | CRISPR Product [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of unique ex vivo products commercialized | Product	10
Regeneron Pharmaceuticals Inc. [Member] | 2020 Regeneron Amendment [Member] | Minimum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in regeneron target cap | Targetcap	10
Regeneron Pharmaceuticals Inc. [Member] | 2020 Stock Purchase Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares sold | shares	925,218
Common stock, par value | $ / shares	$ 0.0001
Cash consideration received on sale of common stock	$ 30,000,000.0
Sale of stock, price per share | $ / shares	$ 32.42
Percentage of premium over volume-weighted average trading price of common stock during 30-day period prior to closing of equity transaction	100.00%
Regeneron Pharmaceuticals Inc. [Member] | Co-Development and Co-Promotion Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment of royalty percentage on net product sales		25.00%	50.00%	50.00%
Termination period of agreement				180 days
Regeneron Pharmaceuticals Inc. [Member] | Co-Development and Co-Promotion Agreements [Member] | Minimum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Options exercisable, number of shares exercisable | shares				4
Collaborative arrangement obligation to be fund in development costs				$ 5,000,000.0
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions										145,000,000.0
Collaboration revenue						16,300,000	6,300,000	$ 24,200,000	12,000,000.0	94,400,000
Aggregate transaction price remaining to be recognized, period								Through June 30, 2020
Payments due						3,800,000	3,200,000	$ 8,600,000	5,800,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue										32,600,000
Regeneron Pharmaceuticals Inc. [Member] | Regeneron Amendment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate transaction price remaining to be recognized						85,300,000		85,300,000		85,300,000	85,300,000
Accounts receivable						3,900,000		3,900,000		3,900,000	3,900,000	3,600,000
Deferred revenue						100,700,000		$ 100,700,000		100,700,000	100,700,000	28,800,000
Novartis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Strategic collaboration agreement amended date								2018-12
Novartis [Member] | Regeneron Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue								$ 5,000,000.0
Deferred revenue						0		0		0	0	0
Accounts receivable						0		$ 0		$ 0	0	$ 1,000,000.0
Novartis [Member] | Novartis Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue additions											62,400,000
Collaboration revenue						$ 0	$ 4,800,000		$ 9,500,000		$ 62,400,000
Aggregate transaction price remaining to be recognized, period								Through June 30, 2020