Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Initial Public Offering [Member]	Follow-on Public Offering [Member]	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member] Series A Preferred Stock [Member]	Convertible Preferred Stock [Member] Series B Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Series A Preferred Stock [Member]	Common Stock [Member] Initial Public Offering [Member]	Common Stock [Member] Follow-on Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Series A Preferred Stock [Member]	Additional Paid-in Capital [Member] Series B Preferred Stock [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Additional Paid-in Capital [Member] Follow-on Public Offering [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Series A Preferred Stock [Member]
Beginning balance at Dec. 31, 2013	$ (73,726)					$ 75,890							$ 670						$ (74,396)
Beginning balance, shares at Dec. 31, 2013						70,463,000			421,904
Exercise of stock options	177								$ 1				176
Exercise of stock options, shares									98,401
Stock-based compensation expense	274												274
Issuance of common stock upon completion of public offering, net of offering costs							$ 4,932
Issuance of common stock upon completion of public offering, net of offering costs, shares							5,000,000
Modification of preferred stock		$ (6,037)					$ 6,037							$ (1,027)						$ (5,010)
Net loss	(15,772)																		(15,772)
Ending balance at Dec. 31, 2014	(95,084)					$ 86,859			$ 1				93						(95,178)
Ending balance, shares at Dec. 31, 2014						75,463,000			520,305
Exercise of stock options	95												95
Exercise of stock options, shares									50,832
Stock-based compensation expense	472												472
Issuance of common stock upon completion of public offering, net of offering costs								$ 21,090
Issuance of common stock upon completion of public offering, net of offering costs, shares								17,107,303
Conversion of convertible preferred stock to common stock								$ 15,798
Conversion of convertible preferred stock to common stock, shares								12,284,466
Modification of preferred stock		$ 11,481	$ (26)				$ (11,481)	$ 26						11,481	$ (26)
Net loss	(25,040)																		(25,040)
Ending balance at Dec. 31, 2015	$ (108,102)					$ 112,292			$ 1				12,115						(120,218)
Ending balance, shares at Dec. 31, 2015	571,137					104,854,769			571,137
Exercise of stock options	$ 268												268
Exercise of stock options, shares	101,816								101,816
Stock-based compensation expense	$ 1,805												1,805
Issuance of common stock upon completion of public offering, net of offering costs				$ 42,529	$ 69,469						$ 6	$ 6				$ 42,523	$ 69,463
Issuance of common stock upon completion of public offering, net of offering costs, shares											6,250,000	5,750,000
Conversion of convertible preferred stock to common stock	112,292					$ (112,292)			$ 10				112,282
Conversion of convertible preferred stock to common stock, shares						(104,854,769)			9,699,600
Issuance of common stock to settle accruing dividends										$ 3				$ (3)
Issuance of common stock to settle accruing dividends, Shares										2,590,742
Issuance of common stock for partial payment of accrued bonus	62								$ 0				62
Issuance of common stock for partial payment of accrued bonus, shares									10,218
Conversion of preferred stock warrant to common stock warrant	28												28
Issuance of common stock for payment of consulting services	359												359
Issuance of common stock for payment of consulting services, shares									27,221
Other comprehensive loss	(22)																	$ (22)
Net loss	(37,232)																		(37,232)
Ending balance at Dec. 31, 2016	$ 81,456								$ 26				$ 238,902					$ (22)	$ (157,450)
Ending balance, shares at Dec. 31, 2016	25,000,734								25,000,734