Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 250	$ 833	$ 250	$ 1,542
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 9,944	$ 14,480	$ 29,045	$ 38,491
General and administrative	7,389	4,826	20,097	19,373
Impairment of intangible asset		5,000		5,000
Total operating expenses	17,333	24,306	49,142	62,864
Operating loss	(17,083)	(23,473)	(48,892)	(61,322)
Other income, net	965	1,265	3,331	3,104
Net loss before income tax benefit	(16,118)	(22,208)	(45,561)	(58,218)
Income tax benefit		1,250		1,250
Net loss	(16,118)	(20,958)	(45,561)	(56,968)
Other comprehensive loss:
Net unrealized gain on marketable securities	4	53	27	129
Foreign currency translation adjustment	(11)		27	(21)
Comprehensive loss	$ (16,125)	$ (20,905)	$ (45,507)	$ (56,860)
Net loss per share — basic and diluted	$ (0.25)	$ (0.34)	$ (0.71)	$ (0.94)
Weighted-average common shares used to compute net loss per share - basic and diluted	64,484	62,454	63,764	60,856