Collaboration Agreements - Net Revenue from Collaborators (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 2,900	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 21,033	$ 2,069	$ 29,907	$ 21,899	$ 219,750	$ 74,908	$ 89,912
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										53,213	62,373	89,912
Net Revenues from Collaborators | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										26,075	0	0
Net Revenues from Collaborators | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										12,809	12,778	1,464
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										10,976	46,000	54,625
Net Revenues from Collaborators | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										2,315	2,789	30,217
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										$ 1,038	$ 806	$ 3,606