CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net loss	$ (142,375)	$ (162,667)	$ (120,335)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	28,719	32,383	26,242
Reduction in related party contract asset due to Amended Collaboration Agreement with Merck	0	0	4,600
Depreciation	2,221	4,035	6,089
(Accretion of discount) amortization of premium on marketable securities	(5,552)	69	3,514
Noncash lease expense	2,096	1,949	1,810
Other noncash expenses	1,065	504	643
Changes in operating assets and liabilities:
Related party receivable from collaboration	7,522	(2,635)	(4,612)
Related party contract asset	0	0	1,500
Prepaid expenses and other assets	1,646	1,790	(4,145)
Accounts payable	(4,977)	3,207	(4,417)
Accrued and other liabilities	(16,965)	(589)	2,893
Operating lease liability	(5,385)	(5,077)	(4,785)
Contract liabilities	(366)	(17,408)	17,774
Other noncurrent liabilities	149	0	0
Net cash used in operating activities	(132,202)	(144,439)	(73,229)
Cash flows from investing activities
Purchase of marketable securities	(105,431)	(272,857)	(293,466)
Proceeds from maturities of marketable securities	220,970	289,037	223,500
Purchase of property and equipment	(1,252)	(1,858)	(1,684)
Net cash provided by (used in) investing activities	114,287	14,322	(71,650)
Cash flows from financing activities
Proceeds from Open Market Agreement, net	0	49,446	196
Proceeds from exercise of stock options	668	2,984	12,362
Proceeds from employee stock purchase plan	1,107	1,803	2,519
Proceeds from follow on offering, net	0	0	134,580
Net cash provided by financing activities	1,775	54,233	149,657
Net (decrease) increase in cash and cash equivalents	(16,140)	(75,884)	4,778
Cash, cash equivalents and restricted cash, at beginning of period	77,410	153,294	148,516
Cash, cash equivalents and restricted cash, at end of period	61,270	77,410	153,294
Supplemental disclosures of noncash investing and financing activities:
Property and equipment purchases not yet paid	122	606	0
Right of use asset acquired under operating lease on the adoption of ASC 842	$ 0	$ 0	$ 5,855