Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Total revenue	$ 31	$ 5,732	$ 0
Revenue, Product and Service [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
General and administrative	$ 13,212	$ 18,979	$ 19,639
Research and development	13,291	15,824	48,974
Restructuring charges		1,427	1,774
Impairment of property and equipment	108
Total operating expenses	26,611	36,230	70,387
Loss from operations	(26,580)	(30,498)	(70,387)
Other income (expense):
Change in fair value of warrant liability	(4)	9	65
Net (loss) gain on asset disposals	(12)	107	(56)
Other income, net	2,254	1,567	614
Interest income	418	1,001	679
Interest expense	(931)	(2,231)	(2,780)
(Loss) gain on foreign currency exchange	(16)	6
Total other income (expense)	1,709	459	(1,478)
Loss before income tax expense	(24,871)	(30,039)	(71,865)
Foreign income tax expense		(745)
Net loss attributable to common stockholders	(24,871)	(30,784)	(71,865)
Other comprehensive gain (loss), net:
Unrealized gain (loss) on marketable securities	9	33	(44)
Other comprehensive gain (loss), net	9	33	(44)
Comprehensive loss	$ (24,862)	$ (30,751)	$ (71,909)
Per share information:
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.27)	$ (0.42)	$ (1.21)
Weighted average common shares outstanding, basic and diluted (in shares)	91,677,963	73,558,548	59,436,649