MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Revenues Recognized under Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	$ 21,553	$ 17,548	$ 34,629	$ 25,619
Biogen MA, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	8,178	0	8,178	0
Biogen MA, Inc. | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	6,744	0	6,744	0
Biogen MA, Inc. | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	1,434	0	1,434	0
Kite Pharma Inc
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	7,422	9,060	14,641	17,372
Kite Pharma Inc | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	6,227	6,227	12,454	12,386
Kite Pharma Inc | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	1,195	2,833	2,187	4,986
Pfizer SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	429	4,635	3,612	1,595
Pfizer SB-525 | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	326	4,635	2,516	1,595
Pfizer SB-525 | Milestone Achievement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	103	0	1,096	0
Pfizer | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	3,425	455	3,785	1,070
Sanofi
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	1,792	3,059	2,291	5,009
Sanofi | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	380	901	(349)	1,654
Sanofi | Research services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	1,155	2,158	2,875	3,355
Sanofi | Milestone Achievement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues	$ 257	$ 0	$ (235)	$ 0