MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Additional Information (Detail)		1 Months Ended										3 Months Ended		6 Months Ended		16 Months Ended	34 Months Ended
	Apr. 05, 2018 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Apr. 30, 2020 USD ($) $ / shares shares	Mar. 31, 2020 USD ($) $ / shares	Feb. 29, 2020 USD ($) product_target	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	May 31, 2018 USD ($)	Dec. 31, 2017 USD ($) milestone	May 31, 2017 USD ($)	Jan. 31, 2014 USD ($) program	Jun. 30, 2020 USD ($) product_target	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) milestone product option	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Feb. 28, 2020
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum number of product targets replaced | product_target												10
Recognized portion of equity issued													$ 18,000		$ 276,000
Revenue reduction due to decrease in measure of proportional cumulative performance, percentage													38.00%
Revenues												$ 21,553,000	$ 17,548,000	$ 34,629,000	25,619,000
Other long-term liabilities		$ 6,055,000				$ 5,711,000						6,055,000		6,055,000
California Institute For Regenerative Medicine Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement								$ 8,000,000.0
Other long-term liabilities		6,100,000				5,700,000						6,100,000		6,100,000
California Institute For Regenerative Medicine Agreement | Research Grants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																$ 5,200,000
Biogen MA, Inc.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												8,178,000	0	8,178,000	0
Biogen MA, Inc. | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												$ 6,744,000	0	6,744,000	0
Biogen MA, Inc. | Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Proceeds from collaborators					$ 125,000,000.0
Maximum milestone payment receivable					$ 2,370,000,000
Number of product targets | product_target					12
Number of product targets selected | product_target					3
Number of additional product targets | product_target					9
Target selection period					5 years
Percent of initial recognition												2.00%
Portion of contract asset recognized		4,100,000										$ 4,100,000		4,100,000
Collaboration Arrangement, Commission Fee, Portion Of Gross Proceeds, Value		7,000,000.0										7,000,000.0		7,000,000.0
Biogen MA, Inc. | Collaboration and License Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research period					7 years
Biogen MA, Inc. | Collaboration and License Agreement | Pre-approval Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum milestone payment receivable					$ 925,000,000.0
Biogen MA, Inc. | Collaboration and License Agreement | Sales-based Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum milestone payment receivable					$ 1,450,000,000
Biogen MA, Inc. | Stock Purchase Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of shares issued in transaction (in shares) | shares			24,420,157
Sale of stock, price per share (in dollars per share) | $ / shares			$ 9.2137
Consideration received on transaction			$ 225,000,000.0
Standstill restriction period					3 years
Standstill restriction, ownership threshold percentage ownership percentage																		5.00%
Agreement restriction, percentage of shares held																		50.00%
Voting provisions expiration period					2 years
Voting provisions, ownership threshold percentage																		5.00%
Excess consideration received on transaction														79,600,000
Recognized portion of equity issued												2,900,000
Collaboration agreement, equity issued		145,400,000										145,400,000		145,400,000
Deferred revenue		197,800,000										197,800,000		197,800,000
Kite Pharma Inc
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												7,422,000	9,060,000	14,641,000	17,372,000
Kite Pharma Inc | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												6,227,000	6,227,000	$ 12,454,000	12,386,000
Kite Pharma Inc | Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received	$ 150,000,000.0
Initial research term of agreement	6 years
Number of options to extend initial research term | option														2
Extended research term of agreement														1 year
Separate upfront fee	$ 10,000,000.0
Collaborative arrangement estimated reimbursable service costs for new research plan							$ 3,400,000
Collaborative arrangement transaction price	189,300,000
Revenues under agreement	150,000,000.0
Collaborative arrangement estimated reimbursable service costs	39,300,000
Deferred revenue		94,000,000.0				106,500,000						94,000,000.0		$ 94,000,000.0
Kite Pharma Inc | Collaboration and License Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received	3,010,000,000.00
Kite Pharma Inc | Collaboration and License Agreement | Achievement of Specified Research, Clinical Development, Regulatory and First Commercial Sale Milestones
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received	1,260,000,000
Kite Pharma Inc | Collaboration and License Agreement | Achievement of Specified Sales-based Milestones if Annual Worldwide Net Sales of Licensed Products Reach Specified Levels
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received	$ 1,750,000,000
Pfizer
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received									$ 12,000,000.0
Collaborative arrangement transaction price									$ 12,000,000.0
Revenues under agreement						25,000,000.0								1,100,000			$ 25,000,000.0
Deferred revenue		700,000				4,000,000.0						700,000		700,000
Contract to perform for others, cumulative compensation earned		24,800,000										24,800,000		24,800,000
Revenue reduction due to decrease in measure of proportional cumulative performance													3,000,000.0
Increase (decrease) in revenue		3,000,000.0		$ 2,400,000
Increase (decrease) in net loss		$ 3,000,000.0		$ (2,400,000)
Increase (decrease) in basic net loss per share (in dollars per share) | $ / shares		$ 0.02		$ (0.02)
Pfizer | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												3,425,000	455,000	3,785,000	1,070,000
Pfizer | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement										$ 70,000,000.0
Pfizer | C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue		$ 4,200,000				8,000,000.0						4,200,000		4,200,000
Number of milestones included in transaction price | milestone									0
Pfizer | Maximum | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount to be funded for achievement of specified commercialized and sales milestones										266,500,000
Pfizer | Maximum | SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable										300,000,000.0
Pfizer | Maximum | Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable										175,000,000.0
Pfizer | Achievement of Specified Clinical Development Intellectual Property and Regulatory Milestones | Maximum | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received										208,500,000
Pfizer | Achievement of First Commercial Sale Milestones | Maximum | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received										$ 475,000,000.0
Pfizer | Achievement of Specified Preclinical Development Clinical Development and First Commercial Sale Milestones | Maximum | C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received									$ 60,000,000.0
Pfizer | Achievement of Commercial Milestones | Maximum | C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received									$ 90,000,000.0
Pfizer SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price														104,000,000.0
Research Service Fees														$ 79,000,000.0
Agreement termination, term														15 years
Revenues												429,000	4,635,000	$ 3,612,000	1,595,000
Pfizer SB-525 | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												326,000	4,635,000	$ 2,516,000	1,595,000
Pfizer SB-525 | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of products approved | product														0
Number of milestones included in transaction price | milestone														0
Pfizer SB-525 | Milestone Achievement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												103,000	0	$ 1,096,000	0
Pfizer SB-525 | Amended Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue		200,000										200,000		200,000
Pfizer SB-525 | Amended Collaboration And License Agreement | SB-525 and Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		25,000,000.0										25,000,000.0		$ 25,000,000.0
Sanofi
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement											$ 20,000,000.0
Number of products approved | product														0
Agreement termination, term														180 days
Increase (decrease) in revenue				$ (2,200,000)
Increase (decrease) in net loss				$ (2,200,000)
Increase (decrease) in basic net loss per share (in dollars per share) | $ / shares				$ (0.02)
Revenues												1,792,000	3,059,000	$ 2,291,000	5,009,000
Number of research programs | program											2
Sanofi | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												380,000	901,000	(349,000)	1,654,000
Sanofi | Milestone Achievement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												257,000	$ 0	(235,000)	$ 0
Sanofi | Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price											$ 93,300,000
Revenues under agreement											20,000,000.0
Collaborative arrangement estimated reimbursable service costs											59,800,000
Deferred revenue		2,300,000				1,700,000						2,300,000		$ 2,300,000
Number of milestones included in transaction price | milestone														0
Cumulative milestone achieved		7,000,000.0										7,000,000.0		$ 7,000,000.0
Sanofi | Collaboration and License Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received											276,300,000
Sanofi | Collaboration and License Agreement | Milestone Two
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable						$ 7,500,000
Milestone revenue reversal		100,000										100,000		100,000
Sanofi | Collaboration and License Agreement | Milestone Three
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		13,500,000										13,500,000		13,500,000
Sanofi | Collaboration and License Agreement | ST-40 Beta Thalassemia Phase 1 Clinical Trial Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received														6,000,000.0
Cumulative milestone achieved		5,600,000										5,600,000		5,600,000
Milestone revenue reversal		$ 100,000										$ 100,000		$ 100,000
Sanofi | Collaboration and License Agreement | Achievement Of Specified Clinical Development And Regulatory Milestones | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received											115,800,000
Sanofi | Collaboration and License Agreement | Achievement of Specified Sales Milestones | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received											$ 160,500,000