Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue:
Collaboration revenue	$ 0	$ 1,875
Total revenue	0	1,875
Operating expenses:
General and administrative	4,879	3,918
Research and development	16,096	15,753
Total operating expenses	20,975	19,671
Loss from operations	(20,975)	(17,796)
Other income (expense):
Change in fair value of warrant liability	35	42
Miscellaneous income	628	221
Interest income	174	193
Interest expense	(576)	(439)
Total other income	261	17
Net loss attributable to common stockholders	(20,714)	(17,779)
Other comprehensive (loss) income, net:
Unrealized gain (loss) on marketable securities	(51)	236
Other comprehensive (loss) income	(51)	236
Comprehensive loss	$ (20,765)	$ (17,543)
Per share information:
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.36)	$ (0.35)
Weighted average common shares outstanding, basic and diluted (in shares)	56,894,672	51,350,365