Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Convertible Preferred Stock
Balance at Dec. 31, 2014	$ 28,340	$ 21	$ 140,711		$ (112,392)
Balance (in shares) at Dec. 31, 2014		20,569,399
Exercise of stock options, net of issuance costs	420		420
Exercise of stock options, net of issuance costs (in shares)		227,215
Repurchase liability for unvested equity awards	44		44
Stock–based compensation	2,400		2,400
Senior executive incentive bonuses paid in common stock	97		97
Senior executive incentive bonuses paid in common stock (in Shares)		19,956
Public offering of common stock, net of offering costs	32,149	$ 7	32,142
Public offering of common stock, net of offering costs (in shares)		6,900,000
Issuance of common stock to collaboration partner	4,580	$ 1	4,579
Issuance of common stock to collaboration partner (in Shares)		1,000,000
Net loss	(29,992)				(29,992)
Balance at Dec. 31, 2015	38,038	$ 29	180,393		(142,384)
Balance (in shares) at Dec. 31, 2015		28,716,570
Exercise of stock options, net of issuance costs	187		187
Exercise of stock options, net of issuance costs (in shares)		136,368
Repurchase liability for unvested equity awards	1		1
Stock–based compensation	3,184		3,184
Private placement issuances of common stock, net of offering costs	28,797	$ 12	28,785
Private placement issuances of common stock, net of offering costs (in shares)		12,486,837
Private placement issuance of Series A convertible preferred stock, net of offering costs	36,289		36,286			$ 3
Private placement issuance of Series A convertible preferred stock, net of offering costs (in shares)						2,819,549
Senior executive incentive bonuses paid in common stock	121		121
Senior executive incentive bonuses paid in common stock (in Shares)		46,731
Unrealized loss on short-term investments	(1)			$ (1)
Net loss	(33,462)				(33,462)
Balance at Dec. 31, 2016	73,154	$ 41	248,957	(1)	(175,846)	$ 3
Balance (in shares) at Dec. 31, 2016		41,386,506				2,819,549
Exercise of stock options, net of issuance costs	226		226
Exercise of stock options, net of issuance costs (in shares)		83,220
Issuance of common stock upon vesting of restricted stock units		$ 1	(1)
Issuance of common stock upon vesting of restricted stock units (in shares)		225,125
Stock–based compensation	3,606		3,606
Issuance of warrants for common stock	217		217
Private placement issuances of common stock, net of offering costs	(13)		(13)
Private placement issuance of Series A convertible preferred stock, net of offering costs	(26)		(26)
Public offering of common stock, net of offering costs	42,979	$ 11	42,968
Public offering of common stock, net of offering costs (in shares)		10,953,750
Unrealized loss on short-term investments	(2)			(2)
Net loss	(42,952)				(42,952)
Balance at Dec. 31, 2017	$ 77,189	$ 53	$ 295,934	$ (3)	$ (218,798)	$ 3
Balance (in shares) at Dec. 31, 2017		52,648,601				2,819,549