Collaboration Agreements - Additional Information (Detail)						1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($)	May 21, 2019 USD ($) $ / shares	Apr. 25, 2019 shares	Apr. 08, 2019 USD ($) Program $ / shares shares	Jan. 31, 2019 $ / shares shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	May 03, 2019 shares	Jan. 17, 2019 shares	Dec. 31, 2018 USD ($) shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares						5,000,000
Offering proceeds, net of costs									$ 381,900,000	$ 0
Common stock at a price per share | $ / shares						$ 77.50
Change in Transaction Price							$ 18,500,000
Deferred revenue	$ 158,446,000						$ 158,446,000		$ 158,446,000				$ 3,954,000
Common stock shares owned | shares	111,325,000						111,325,000		111,325,000				101,177,000
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares owned | shares											0	10,554,134
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares				4,444,445,000	4,444,445
Offering proceeds, net of costs			$ 400,000,000.0		$ 400,000,000.0
Common stock at a price per share | $ / shares			$ 90.00		$ 90.00
Discovery period of programs development					5 years
Extended additional discovery period of programs development					2 years
Maximum royalties and commercial milestone payments upon potential product sale					$ 325,000,000.0
Upfront fee received					400,000,000.0
Maximum additional milestone payments to be receive upon achievement of certain criteria					$ 200,000,000.0
Number of targeted programs | Program					30
Maximum percentage of royalty payments					20.00%
Transaction price	$ 540,100,000	$ 521,600,000
Transactional price remaining performance obligation	$ 279,800,000						$ 279,800,000		$ 279,800,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Program Initiation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement					$ 2,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Lead Candidate Identification
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement					2,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Steady State
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement					30,000,000.0
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research term extension fee					400,000,000.0
Collaborative arrangement milestone payments					$ 150,000,000.0
Royalty rate					20.00%
Collaboration Amendment | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investor agreement description								until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us.
Collaboration Amendment | Minimum | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners	5.00%						5.00%		5.00%