MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Sanofi S.A. (Detail) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 31, 2019 USD ($)	Jan. 31, 2014 USD ($) program	Sep. 30, 2021 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($) milestone product $ / shares	Sep. 30, 2020 USD ($) $ / shares	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues			$ 28,563	$ 57,763	$ 82,715	$ 92,392
Net loss			$ (47,688)	$ (1,508)	$ (140,801)	$ (80,447)
Earnings per share, diluted (in dollars per share) | $ / shares			$ (0.33)	$ (0.01)	$ (0.98)	$ (0.61)
Earnings per share, basic (in dollars per share) | $ / shares			$ (0.33)	$ (0.01)	$ (0.98)	$ (0.61)
Sanofi S.A.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs | program		2
Agreement termination, term					180 days
Revenues under agreement		$ 20,000
Milestone revenue receivable			$ 6,000		$ 6,000
Number of products approved | product					0
Revenues			(759)	$ 1,498	$ 1,258	$ 3,789
Sanofi S.A. | Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement		20,000
Development and sales-based milestone payments to be received		276,300
Number of milestones included in transaction price | milestone					0
Collaborative arrangement transaction price		98,100
Collaborative arrangement estimated reimbursable service costs		64,600
Deferred revenue			2,300		$ 2,300		$ 1,200
Cumulative milestone achieved					7,000
Sanofi S.A. | Collaboration And License Agreement | Milestone two
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable			7,500		7,500			$ 7,500
Milestone revenue reversal			400		300
Sanofi S.A. | Collaboration And License Agreement | Milestone three
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable			13,500		13,500
Sanofi S.A. | 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
Cumulative milestone achieved					5,600
Milestone revenue reversal			300		200
Sanofi S.A. | Collaboration agreement | Change in collaboration agreement scope
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues			2,500		2,500
Net loss			$ 2,500		$ 2,500
Earnings per share, diluted (in dollars per share) | $ / shares					$ 0.02	$ 0.02
Earnings per share, basic (in dollars per share) | $ / shares			$ 0.02	$ 0.02
Sanofi S.A. | Achievement of specified clinical development And regulatory milestones | Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received		115,800
Sanofi S.A. | Achievement of specified sales milestones | Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received		$ 160,500