NET REVENUES FROM COLLABORATIONS - Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	$ 125,853	$ 70,061	$ 329,291	$ 148,069
Net revenues from collaborations
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	26,647	23,995	80,614	37,481
Net revenues from collaborations | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	14,874	15,261	49,790	15,961
Net revenues from collaborations | Vir Biotechnology (Vir)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	8,512	5,869	21,476	7,888
Net revenues from collaborations | The Medicines Company (MDCO)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	1,091	528	6,029	2,273
Net revenues from collaborations | Sanofi Genzyme (Sanofi)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	420	1,882	793	10,382
Net revenues from collaborations | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	$ 1,750	$ 455	$ 2,526	$ 977