ORGANIZATION, BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIIES - Additional Information (Details)	12 Months Ended
	Dec. 31, 2022 USD ($) segment $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Organization and Summary of Significant Accounting Policies [Line Items]
Capital resources	$ 307,500,000
Net loss	192,278,000	$ 178,286,000	$ 120,996,000
Accumulated deficit	1,148,545,000	956,267,000
Revenues	111,299,000	110,701,000	118,192,000
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)
Goodwill and intangible asset impairment	$ 0
Impairment of long-lived assets held-for-use	0
Letter of credit	$ 1,500,000
Stock options and RSUs outstanding (in shares) | shares	18,560,755	15,159,908	14,237,871
Number of operating segments | segment	1
Pfizer SB-525
Organization and Summary of Significant Accounting Policies [Line Items]
Revenues	$ 0
Pfizer SB-525 | Change in collaboration agreement scope | Collaborative arrangement
Organization and Summary of Significant Accounting Policies [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
Minimum
Organization and Summary of Significant Accounting Policies [Line Items]
Estimated useful lives of related assets	3 years
Maximum
Organization and Summary of Significant Accounting Policies [Line Items]
Estimated useful lives of related assets	5 years