Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 30,067	$ 42,846	$ 34,504
General and administrative	17,078	15,094	13,112
Total operating expenses	47,145	57,940	47,616
Loss from operations	(47,145)	(57,940)	(47,616)
Other income (expense), net
Other income (expense), net	(1,062)	65	(386)
Loss on extinguishment of debt		(29)
Change in fair value of warrant liabilities			29
Total other income (expense)	(1,062)	36	(357)
Loss before income taxes	(48,207)	(57,904)	(47,973)
Income tax benefit		49
Net loss	(48,207)	(57,855)	(47,973)
Accretion to redemption value of redeemable convertible preferred stock			(15)
Net loss per share attributable to common stockholders, basic and diluted	$ (48,207)	$ (57,855)	$ (47,988)
Net loss per share, basic and diluted	$ (1.87)	$ (2.44)	$ (3.03)
Weighted average common shares outstanding, basic and diluted	25,799,853	23,681,019	15,838,353