Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 0	$ 5,000	$ 15,000	$ 5,000
Operating expenses:
Research and development	17,948	27,350	38,916	47,981
General and administrative	4,687	4,307	8,972	8,448
Total operating expenses	22,635	31,657	47,888	56,429
Loss from operations	(22,635)	(26,657)	(32,888)	(51,429)
Other income (expense), net:
Interest expense	0	(281)	0	(601)
Interest income	1,061	2,957	2,958	5,945
Other income (expense), net	26	(41)	120	(34)
Total other income (expense), net	1,087	2,635	3,078	5,310
Loss before income taxes	(21,548)	(24,022)	(29,810)	(46,119)
Provision for income taxes	0	60	0	79
Net loss	(21,548)	(23,962)	(29,810)	(46,040)
Unrealized (loss) income on available for sale securities, net of tax of $0, $99, $0 and $214, respectively	(392)	370	415	797
Comprehensive loss	$ (21,940)	$ (23,592)	$ (29,395)	$ (45,243)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (0.79)	$ (0.89)	$ (1.09)	$ (1.70)
Weighted-average number of shares outstanding:
Basic and diluted (in shares)	27,279	27,026	27,271	27,004