NET REVENUES FROM COLLABORATIONS - Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 169,571	$ 129,127
Clinical Trial and Manufacturing
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	5,949	6,732
Clinical Trial and Manufacturing | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	4,612	0
Clinical Trial and Manufacturing | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	339	294
Clinical Trial and Manufacturing | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	998	1,612
Clinical Trial and Manufacturing | Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	4,826
External Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	69	381
External Services | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	0
External Services | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	52	236
External Services | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	10
External Services | Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	17	135
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	13,681	238
Other | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	11,491	0
Other | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,693	129
Other | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	266	50
Other | Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 231	$ 59