CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Alliance revenue	$ 1,448	$ 662	$ 902
Revenue, Product and Service [Extensible List]	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax	us_gaap:RevenueFromContractWithCustomerIncludingAssessedTax
Operating expenses:
Research and development	$ 34,853	$ 41,841	$ 50,653
General and administrative	12,481	15,420	15,588
Merger-related costs, net		1,245	1,128
Restructuring costs	181	3,112
Total operating expenses	47,515	61,618	67,369
Loss from operations	(46,067)	(60,956)	(66,467)
Other income (expense):
Interest income	1,150	1,089	574
Interest expense		(11)	(50)
Warrant revaluation expense	(598)
Future tranche right revaluation expense	(10,964)
Foreign currency exchange loss	(36)	(3)	(41)
Net loss	(56,515)	(59,881)	(65,984)
Deemed dividend related to December 2019 Private Placement (see Note 7)	(28,043)
Net loss attributable to common stockholders	$ (84,558)	$ (59,881)	$ (65,984)
Net loss per share applicable to common stockholders - basic and diluted (Note 17)	$ (2.96)	$ (2.25)	$ (3.35)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders - basic and diluted	28,545	26,601	19,675
Comprehensive loss:
Net loss	$ (56,515)	$ (59,881)	$ (65,984)
Unrealized gain on available-for-sale securities			17
Total other comprehensive income			17
Comprehensive loss	$ (56,515)	$ (59,881)	$ (65,967)