Consolidated Statements of Changes in Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	IPO	Common Stock	Common Stock IPO	Additional Paid-In Capital	Additional Paid-In Capital IPO	Accumulated Deficit	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2015	$ (85,035)		$ 4		$ 100,276		$ (185,315)
Beginning balance, shares at Dec. 31, 2015			4,382
Issuance of common stock, net of issuance costs	453	$ 107,192		$ 7	453	$ 107,185
Issuance of common stock, net of issuance costs, shares			197	7,047
Stock-based compensation	2,765				2,765
Accretion of preferred stock	(6,258)				(6,258)
Conversion of Preferred Stock Warrants Into Common Stock Warrants Initial Public Offering	505				505
Common Stock Issued in Connection with Net Exercises of Common Stock Warrants			55
Conversion of preferred stock into common stock at initial public offering	106,084		$ 9		106,075
Conversion of preferred stock into common stock at initial public offering, shares			8,655
Net cumulative effect adjustment upon adoption of share-based payment guidance to account for forfeitures as they occur					215		(215)
Net loss	(1,673)						(1,673)
Other comprehensive income (loss)	(10)							$ (10)
Ending balance at Dec. 31, 2016	124,023		$ 20		311,216		(187,203)	(10)
Ending balance, shares at Dec. 31, 2016			20,336
Issuance of common stock, net of issuance costs	4,839		$ 1		4,838
Issuance of common stock, net of issuance costs, shares			637
Stock-based compensation	7,428				7,428
Net loss	(17,322)						(17,322)
Other comprehensive income (loss)	(26)							(26)
Ending balance at Dec. 31, 2017	118,942		$ 21		323,482		(204,525)	(36)
Ending balance, shares at Dec. 31, 2017			20,973
Issuance of common stock, net of issuance costs	2,828				2,828
Issuance of common stock, net of issuance costs, shares			519
Stock-based compensation	11,317				11,317
Net loss	(35,231)						(35,231)
Other comprehensive income (loss)	27							27
Ending balance at Dec. 31, 2018	$ 97,883		$ 21		$ 337,627		$ (239,756)	$ (9)
Ending balance, shares at Dec. 31, 2018			21,492