MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Sanofi S.A. (Details)		1 Months Ended	12 Months Ended			68 Months Ended	102 Months Ended	108 Months Ended
	Sep. 06, 2022 USD ($)	Jan. 31, 2014 USD ($) program	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2022 USD ($)	Jun. 28, 2022 USD ($)	Dec. 31, 2022 USD ($) milestone product
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues			$ 111,299,000	$ 110,701,000	$ 118,192,000
Accounts receivable			3,678,000	6,013,000		$ 3,678,000		$ 3,678,000
Decrease in research and development expense			2,100,000
Net loss			$ (192,278,000)	$ (178,297,000)	$ (121,122,000)
Earnings per share, basic (in dollars per share) | $ / shares			$ (1.25)	$ (1.23)	$ (0.90)
Prepaid Expenses and Other Current Assets
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Decrease in research and development expense			$ 1,100,000
Sanofi S.A.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs | program		2
Revenues under agreement		$ 20,000,000
Revenues			3,260,000	$ 3,114,000	$ 5,322,000		$ 13,500,000
Number of products approved | product								0
Expenses reimbursable through December 31, 2023 (up to)	$ 7,000,000
Expenses reimbursable after December 31, 2023 (up to)	$ 5,300,000
Collaborative arrangement decrease in revenue					2,200,000
Collaborative arrangement, increase in net loss					$ 2,200,000
Collaborative arrangement, net loss per share (in dollars per share) | $ / shares					$ 0.02
Sanofi S.A. | Collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement							20,000,000
Number of milestones included in transaction price | milestone								0
Collaborative arrangement transaction price							96,300,000
Collaborative arrangement estimated reimbursable service costs							62,800,000
Accounts receivable			0	600,000		0		$ 0
Deferred revenue			0	1,100,000		0		$ 0
Sanofi S.A. | Collaboration and license agreement | Milestone Three
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable							$ 13,500,000
Pfizer SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement						55,000,000
Revenues			$ 0
Collaborative arrangement transaction price						$ 134,000,000
Pfizer SB-525 | Collaborative arrangement | Change in collaboration agreement scope
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues				1,600,000	$ 8,900,000
Net loss				$ 1,600,000	$ 8,900,000
Earnings per share, basic (in dollars per share) | $ / shares				$ 0.01	$ 0.06