Collaboration Agreements - Revenue from Collaborators (Detail) - USD ($) $ in Thousands			3 Months Ended		9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			$ 70,061	$ 2,069	$ 148,069	$ 53,875
Deferred revenue	$ 107,359		107,359		107,359		$ 3,496
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			23,995	1,609	37,481	53,415
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			1,882	(1,560)	10,382	40,370
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			5,869	2,957	7,888	10,313
Net Revenues from Collaborators | The Medicines Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			528	0	2,273	1,957
Net Revenues from Collaborators | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			15,261	0	15,961	0
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			455	$ 212	977	$ 775
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	402,100		402,100		402,100
Transaction price	540,100	$ 521,600
Revenues			14,700		15,400
Global Strategic Collaboration | Research Services Obligation | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			11,100		11,800
Deferred revenue	90,600		90,600		90,600
Transaction price	182,400	178,500
Global Strategic Collaboration | C5 License Obligation | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues			0		0
Deferred revenue	66,500		66,500		66,500
Transaction price	109,200	93,500
Global Strategic Collaboration | C5 Co-Co Obligation | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	245,000		245,000		245,000
Transaction price	$ 248,500	$ 249,600
Revenue recognized under ASC 808			$ 3,600		$ 3,600