MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Revenues Recognized under Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	$ 28,563	$ 57,763	$ 82,715	$ 92,392
Novartis Institutes for BioMedical Research, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	11,514	0	28,956	0
Novartis Institutes for BioMedical Research, Inc. | Recognition of upfront fee
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	9,093	0	22,852	0
Novartis Institutes for BioMedical Research, Inc. | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	2,421	0	6,104	0
Biogen MA, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	10,967	9,621	32,184	17,799
Biogen MA, Inc. | Recognition of upfront fee
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	7,306	7,306	21,918	14,050
Biogen MA, Inc. | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	3,661	2,315	10,266	3,749
Kite Pharma, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	6,409	7,294	19,021	21,935
Kite Pharma, Inc. | Recognition of upfront fee
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	6,296	6,296	18,682	18,750
Kite Pharma, Inc. | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	113	998	339	3,185
Sanofi
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	(759)	1,498	1,258	3,789
Sanofi | Recognition of upfront fee
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	(1,125)	321	(692)	(28)
Sanofi | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	1,125	961	2,417	3,836
Sanofi | Milestone achievement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	$ (759)	$ 216	$ (467)	$ (19)