Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Revenue from collaboration agreements	$ 11,072,000	$ 6,754,000	$ 17,495,000	$ 28,242,000
Operating expenses:
Research and development	28,347,000	29,663,000	87,315,000	87,189,000
General and administrative	10,533,000	9,579,000	31,784,000	32,294,000
Total operating expenses	38,880,000	39,242,000	119,099,000	119,483,000
Loss from operations	(27,808,000)	(32,488,000)	(101,604,000)	(91,241,000)
Other income (expense), net:
Interest expense and amortization of long-term debt − related party	(167,000)	(554,000)	(1,373,000)	(1,615,000)
Loss on early extinguishment of debt	(621,000)	0	(621,000)	0
Interest and other income, net	2,562,000	1,084,000	6,862,000	1,866,000
Total other income (expense), net	1,774,000	530,000	4,868,000	251,000
Loss before income taxes	(26,034,000)	(31,958,000)	(96,736,000)	(90,990,000)
Income tax expense	(1,003,000)	0	(1,003,000)	0
Net loss	$ (27,037,000)	$ (31,958,000)	$ (97,739,000)	$ (90,990,000)
Net loss per share − basic (in dollars per share)	$ (0.55)	$ (0.65)	$ (1.99)	$ (1.86)
Net loss per share − diluted (in dollars per share)	$ (0.55)	$ (0.65)	$ (1.99)	$ (1.86)
Weighted-average number of shares used in computed net loss per share − basic	49,212,126	48,921,928	49,103,351	48,827,503
Weighted-average number of shares used in computed net loss per share − diluted	49,212,126	48,921,928	49,103,351	48,827,503
Other comprehensive income (loss)
Unrealized gain (loss) on marketable securities	$ 870,000	$ (828,000)	$ 3,233,000	$ (4,634,000)
Comprehensive loss	$ (26,167,000)	$ (32,786,000)	$ (94,506,000)	$ (95,624,000)
Revenue from Contract with Customer, Product and Service [Extensible Enumeration]			Collaborative Arrangement [Member]	Collaborative Arrangement [Member]