Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Private Placement [Member]	Common Stock [Member]	Common Stock [Member] Private Placement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Private Placement [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 184,028		$ 5		$ 439,048		$ (963)	$ (254,062)
Balance, shares at Dec. 31, 2017			49,015
Issuance of common stock net of issuance costs	70,187		$ 1		70,186
Issuance of common stock net of issuance costs, shares			11,642
Adoption of Topic 606	6,146							6,146
Stock-based compensation expense	13,432				13,432
Common stock issued upon exercise of stock options	688				688
Common stock issued upon exercise of stock options, shares			1,606
Common stock issued under employee stock purchase plan	340				340
Common stock issued under employee stock purchase plan, shares			120
Common stock issued upon release of restricted stock units, shares			774
Restricted stock surrendered for taxes	(1,191)				(1,191)
Restricted stock surrendered for taxes, shares			(192)
Net unrealized gain on marketable securities	168						168
Foreign currency translation adjustments	(4)						(4)
Net loss	(72,627)							(72,627)
Balance at Dec. 31, 2018	201,167		$ 6		522,503		(799)	(320,543)
Balance, shares at Dec. 31, 2018			62,965
Issuance of common stock net of issuance costs	25,755	$ 134	$ 1		25,754	$ 134
Issuance of common stock net of issuance costs, shares			2,436	20
Stock-based compensation expense	9,899				9,899
Common stock issued upon exercise of stock options	3,442				3,442
Common stock issued upon exercise of stock options, shares			1,397
Common stock issued under employee stock purchase plan	442				442
Common stock issued under employee stock purchase plan, shares			86
Common stock issued upon release of restricted stock units, shares			663
Restricted stock surrendered for taxes	(1,470)				(1,470)
Restricted stock surrendered for taxes, shares			(238)
Net unrealized gain on marketable securities	33						33
Foreign currency translation adjustments	41						41
Net loss	(64,486)							(64,486)
Balance at Dec. 31, 2019	$ 174,957		$ 7		$ 560,704		$ (725)	$ (385,029)
Balance, shares at Dec. 31, 2019			67,329