STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
STATEMENTS OF OPERATIONS
Alliance revenue		$ 1,448	$ 662
Revenue, Product and Service [Extensible List]	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax
Operating expenses:
Research and development	$ 24,772	$ 34,853	$ 41,841
General and administrative	11,915	12,481	15,420
Merger-related costs, net			1,245
Restructuring costs		181	3,112
Total operating expenses	36,687	47,515	61,618
Loss from operations	(36,687)	(46,067)	(60,956)
Other income (expense):
Interest income	165	1,150	1,089
Interest expense	(3)		(11)
Warrant revaluation loss	(3,742)	(598)
Future tranche right revaluation loss	(72,367)	(10,964)
Foreign currency exchange loss	(28)	(36)	(3)
Net loss	(112,662)	(56,515)	(59,881)
Deemed dividend related to December 2019 Private Placement (see Note 7)		(28,043)
Net loss attributable to common stockholders	$ (112,662)	$ (84,558)	$ (59,881)
Net loss per share applicable to common stockholders - basic and diluted (Note 17)	$ (3.33)	$ (2.96)	$ (2.25)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders - basic and diluted	33,821	28,545	26,601