MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Additional Information (Detail)		1 Months Ended										3 Months Ended		16 Months Ended	34 Months Ended
	Apr. 05, 2018 USD ($)	Apr. 30, 2020 USD ($) $ / shares shares	Apr. 30, 2020 USD ($) $ / 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 ($) program	Mar. 31, 2020 USD ($) option milestone product	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Feb. 28, 2020
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue reduction due to decrease in measure of proportional cumulative performance, percentage													38.00%
Revenues												$ 13,076,000	$ 8,071,000
Other long-term liabilities				$ 5,880,000		$ 5,711,000						5,880,000			$ 5,880,000
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				5,900,000		5,700,000						5,900,000			5,900,000
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. | 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
Biogen MA, Inc. | Collaboration and License Agreement | Subsequent Event
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Proceeds from collaborators			$ 125,000,000.0
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]
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%
Biogen MA, Inc. | Stock Purchase Agreement | Subsequent Event
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	$ 9.2137
Consideration received on transaction		$ 225,000,000.0	$ 225,000,000.0
Kite Pharma Inc
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												7,219,000	8,312,000
Kite Pharma Inc | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												$ 6,227,000	6,159,000
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				100,300,000		106,500,000						$ 100,300,000			100,300,000
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				$ 70,000,000.0		1,000,000.0			25,000,000.0
Deferred revenue				1,000,000.0		4,000,000.0						1,000,000.0			1,000,000.0
Contract to perform for others, cumulative compensation earned				24,700,000								24,700,000			24,700,000
Revenue reduction due to decrease in measure of proportional cumulative performance													3,000,000.0
Increase (decrease) in revenue				2,400,000
Increase (decrease) in net loss				$ 2,400,000
Increase (decrease) in basic net loss per share (in dollars per share) | $ / shares				$ 0.02
Pfizer | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												360,000	615,000
Pfizer | C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue				$ 7,600,000		8,000,000.0						7,600,000			7,600,000
Number of milestones included in transaction price | milestone									0
Pfizer | Maximum | SB-525
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
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
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
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
Number of products approved | product												0
Research Service Fees												$ 79,000,000.0
Agreement termination, term												15 years
Revenues												$ 3,183,000	(3,040,000)
Pfizer SB-525 | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												$ 2,190,000	(3,040,000)
Pfizer SB-525 | SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of milestones included in transaction price | milestone												0
Pfizer SB-525 | Amended Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue				300,000								$ 300,000			300,000
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												$ 499,000	1,949,000
Number of research programs | program											2
Sanofi | Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												(729,000)	$ 752,000
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				$ 2,900,000		1,700,000						$ 2,900,000			2,900,000
Number of milestones included in transaction price | milestone												0
Cumulative milestone achieved				6,800,000								$ 6,800,000			6,800,000
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				300,000								300,000			300,000
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				5,500,000								5,500,000			5,500,000
Milestone revenue reversal				$ 200,000								$ 200,000			$ 200,000
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