Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 0	$ 0	$ 15,000	$ 5,000
Operating expenses:
Research and development	19,542	29,931	58,458	77,912
General and administrative	4,794	3,814	13,766	12,262
Total operating expenses	24,336	33,745	72,224	90,174
Loss from operations	(24,336)	(33,745)	(57,224)	(85,174)
Other income (expense), net:
Interest expense	0	(240)	0	(841)
Interest income	625	2,757	3,583	8,702
Other (expense) income, net	(56)	144	64	110
Total other income (expense), net	569	2,661	3,647	7,971
Loss before income taxes	(23,767)	(31,084)	(53,577)	(77,203)
Provision for income taxes	0	51	0	130
Net loss	(23,767)	(31,033)	(53,577)	(77,073)
Unrealized (loss) income on available for sale securities, net of tax of $0, $(25), $0 and $189, respectively	(494)	(94)	(79)	703
Comprehensive loss	$ (24,261)	$ (31,127)	$ (53,656)	$ (76,370)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (0.87)	$ (1.15)	$ (1.96)	$ (2.85)
Weighted-average number of shares outstanding:
Basic and diluted (in shares)	27,316	27,058	27,286	27,022