Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Private Placement	Direct Registered Offering	Public Offering	At The Market Offering Program	Convertible Preferred Stock	Convertible Preferred Stock Direct Registered Offering	Additional Paid-in Capital	Additional Paid-in Capital Private Placement	Additional Paid-in Capital Direct Registered Offering	Additional Paid-in Capital Public Offering	Additional Paid-in Capital At The Market Offering Program	Additional Paid-in Capital Convertible Preferred Stock	Additional Paid-in Capital Convertible Preferred Stock Direct Registered Offering	Common Stock	Common Stock Private Placement	Common Stock Direct Registered Offering	Common Stock Public Offering	Common Stock At The Market Offering Program	Accumulated Deficit	Accumulated Deficit Direct Registered Offering
Balance at Dec. 31, 2015	$ (6,957)							$ 189,712							$ 372					$ (197,041)
Balance, shares at Dec. 31, 2015															38,204,960
Issuance of common stock from option exercises	539							531							$ 8
Issuance of common stock from option exercises, shares															773,718
Stock-based compensation	2,907							2,907
Issuance of common stock, value		$ 10,494							$ 10,448							$ 46
Issuance of common stock, shares																4,602,506
Net loss	(50,636)																			(50,636)
Balance at Dec. 31, 2016	(43,653)							203,598							$ 426					(247,677)
Balance, shares at Dec. 31, 2016															43,581,184
Issuance of common stock from option exercises	665							661							$ 4
Issuance of common stock from option exercises, shares															420,377
Issuance of common stock from releases of restricted stock units, shares															57,626
Stock-based compensation	5,319							5,319
Issuance of common stock, value		$ 22,616	$ 49,860		$ 38,979				$ 22,530	$ 49,776		$ 38,913				$ 86	$ 84		$ 66
Issuance of common stock, shares																8,602,589	8,382,643		6,575,062
Issuance of common stock from warrant exercises	103							103
Issuance of common stock from warrant exercises, shares															34,493
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	274							274
Net loss	(72,176)																			(72,176)
Balance at Dec. 31, 2017	1,987							321,174							$ 666					(319,853)
Balance, shares at Dec. 31, 2017															67,653,974
Issuance of common stock from option exercises	$ 5,285							5,259							$ 26
Issuance of common stock from option exercises, shares	2,608,812														2,608,812
Issuance of common stock from releases of restricted stock units, shares															59,437
Stock-based compensation	$ 14,169							14,169
Issuance of common stock, value			30,093	$ 161,869	$ 272					30,052	$ 161,682	$ 272					$ 41	$ 187
Issuance of common stock, shares																	4,090,000	18,648,649	33,097
Issuance of preferred Series A stock upon direct registered offering, value							$ 30			2,728				$ 22,177							$ (2,728)
Issuance of preferred Series A stock upon direct registered offering, share							3,000,581
Issuance of common stock warrants in connection with direct registered offering			$ 6,623							$ 6,623
Conversion of Series A preferred stock into common stock, value	22,207					$ (30)		22,177					$ (22,177)		$ 30
Conversion of Series A preferred stock into common stock, share						(3,000,581)									3,000,581
Issuance of common stock from warrant exercises	3							1							$ 2
Issuance of common stock from warrant exercises, shares															237,473
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	1,197							1,197
Net loss	(76,396)																			(76,396)
Balance at Dec. 31, 2018	$ 167,309							$ 565,334							$ 952					$ (398,977)
Balance, shares at Dec. 31, 2018															96,332,023