NET REVENUES FROM COLLABORATIONS - Net Revenue from Collaborators (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 2,900	$ 163,562	$ 125,853	$ 103,962	$ 99,476	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 492,853	$ 219,750	$ 74,908
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										131,333	53,213	62,373
Net Revenues from Collaborators | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										74,072	26,075	0
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										31,396	12,809	12,778
Net Revenues from Collaborators | Novartis AG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										22,208	2,315	2,789
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										995	10,976	46,000
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										$ 2,662	$ 1,038	$ 806