Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue:
Total revenues	$ 379	$ 379	$ 1,138	$ 1,138
Type of revenue [extensible list]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 9,923	$ 14,095	$ 33,492	$ 44,286
General and administrative	3,605	4,125	10,980	13,395
Total operating expenses	13,528	18,220	44,472	57,681
Loss from operations	(13,149)	(17,841)	(43,334)	(56,543)
Other income (expense):
Interest income, net	395	488	1,349	1,422
Other (expense) income, net	(75)	15	(62)	(3)
Total other income	320	503	1,287	1,419
Net loss	(12,829)	(17,338)	(42,047)	(55,124)
Other comprehensive income (loss):
Unrealized (loss) gain on marketable securities	(33)	54	50	108
Comprehensive loss	(12,862)	(17,284)	(41,997)	(55,016)
Net loss attributable to common stockholders	$ (12,829)	$ (17,338)	$ (42,047)	$ (55,124)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.41)	$ (0.68)	$ (1.40)	$ (2.21)
Weighted-average number of common shares used to compute net loss per share attributable to common stockholders —basic and diluted	31,630,639	25,471,587	30,103,338	24,888,738