CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 15,000	$ 0
Operating expenses:
Research and development	20,968	20,631
General and administrative	4,285	4,141
Total operating expenses	25,253	24,772
Loss from operations	(10,253)	(24,772)
Other income (expense), net:
Interest expense	0	(320)
Interest income	1,897	2,988
Other income, net	94	7
Total other income (expense), net	1,991	2,675
Loss before income taxes	(8,262)	(22,097)
Provision for income taxes	0	19
Net loss	(8,262)	(22,078)
Unrealized income on available for sale securities, net of tax of $0 and $115, respectively	807	427
Comprehensive loss	$ (7,455)	$ (21,651)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (0.30)	$ (0.82)
Weighted-average number of shares outstanding:
Basic and diluted (in shares)	27,264	26,981