Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
General and administrative	$ 5,072	$ 4,879
Research and development	4,598	16,096
Restructuring charges	23
Total operating expenses	9,693	20,975
Loss from operations	(9,693)	(20,975)
Other income (expense):
Change in fair value of warrant liability		35
Net gain on asset disposals	223
Miscellaneous income	928	628
Interest income	199	174
Interest expense	(678)	(576)
Total other income	672	261
Net loss attributable to common stockholders	(9,021)	(20,714)
Other comprehensive loss, net:
Unrealized loss on marketable securities	(4)	(51)
Other comprehensive loss	(4)	(51)
Comprehensive loss	$ (9,025)	$ (20,765)
Per share information:
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.14)	$ (0.36)
Weighted average common shares outstanding, basic and diluted (in shares)	64,562,236	56,894,672