ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues	$ 21,553	$ 17,548	$ 34,629	$ 25,619
Decrease in net loss	35,965	30,356	78,939	72,559
Letter of credit established as a deposit	1,500		1,500
Cash and cash equivalents	467,443	169,222	467,443	169,222	$ 80,428	$ 140,418
Current restricted cash	0	2,000	0	2,000	0	0
Non-current restricted cash	1,500	1,500	1,500	1,500	1,500	3,500
Cash, cash equivalents and restricted cash as reported within the accompanying Condensed Consolidated Statements of Cash Flows	468,943	172,722	468,943	172,722	$ 81,928	$ 143,918
Biogen MA, Inc.
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues	$ 8,178	$ 0	$ 8,178	$ 0
Biogen MA, Inc. | Customer Concentration Risk | Revenue Benchmark
Organization And Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	38.00%	0.00%	24.00%	0.00%
Kite Pharma Inc
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues	$ 7,422	$ 9,060	$ 14,641	$ 17,372
Kite Pharma Inc | Customer Concentration Risk | Revenue from Contract with Customer
Organization And Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	34.00%	52.00%	42.00%	61.00%
Pfizer | Customer Concentration Risk | Revenue from Contract with Customer
Organization And Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	18.00%	26.00%	21.00%	16.00%
Sanofi
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues	$ 1,792	$ 3,059	$ 2,291	$ 5,009
Sanofi | Customer Concentration Risk | Revenue from Contract with Customer
Organization And Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	8.00%	17.00%	7.00%	18.00%
Change in Collaboration Agreement Scope | Collaboration Agreement | Pfizer
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues	$ 3,000
Decrease in net loss	$ 3,000
Earnings per share, basic (in dollars per share)	$ 0.02
Change in Collaboration Agreement Scope | Collaboration Agreement | Pfizer and Sanofi
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase in revenues			$ 3,100
Decrease in net loss			$ 3,100
Earnings per share, basic (in dollars per share)			$ 0.02