Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Direct Registered Offering	At The Market Offering Program	Convertible Preferred Stock	Convertible Preferred Stock Direct Registered Offering	Additional Paid-in Capital	Additional Paid-in Capital Direct Registered 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 Direct Registered Offering	Common Stock At The Market Offering Program	Accumulated Deficit	Accumulated Deficit Direct Registered Offering
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	3,571					3,555					$ 16
Issuance of common stock from option exercises, shares											1,607,108
Stock-based compensation	10,052					10,052
Issuance of common stock from releases of restricted stock units, shares											38,856
Issuance of common stock from warrant exercises						(1)					$ 1
Issuance of common stock from warrant exercises, shares											123,607
Issuance of common stock, value		$ 30,093	$ 162,141				$ 30,052	$ 161,954				$ 41	$ 187
Issuance of common stock, share												4,090,000	18,681,746
Issuance of preferred Series A stock upon direct registered offering, value		22,207			$ 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				$ (30)		22,177			$ (22,177)		$ 30
Conversion of Series A preferred stock into common stock, share				(3,000,581)							3,000,581
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	342					342
Net loss	(59,656)													(59,656)
Balance at Sep. 30, 2018	177,360					558,656					$ 941			(382,237)
Balance, shares at Sep. 30, 2018											95,195,872
Balance at Jun. 30, 2018	37,966					386,610					$ 741			(349,385)
Balance, shares at Jun. 30, 2018											75,188,832
Issuance of common stock from option exercises	2,856					2,844					$ 12
Issuance of common stock from option exercises, shares											1,210,169
Stock-based compensation	7,179					7,179
Issuance of common stock from releases of restricted stock units, shares											26,388
Issuance of common stock from warrant exercises						(1)					$ 1
Issuance of common stock from warrant exercises, shares											121,834
Issuance of common stock, value			$ 161,869					$ 161,682					$ 187
Issuance of common stock, share													18,648,649
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	342					342
Net loss	(32,852)													(32,852)
Balance at Sep. 30, 2018	177,360					558,656					$ 941			(382,237)
Balance, shares at Sep. 30, 2018											95,195,872
Balance at Dec. 31, 2018	167,309					565,334					$ 952			(398,977)
Balance, shares at Dec. 31, 2018											96,332,023
Issuance of common stock from option exercises	$ 9,641					9,618					$ 23
Issuance of common stock from option exercises, shares	2,219,251										2,219,251
Stock-based compensation	$ 14,369					14,369
Issuance of common stock from releases of restricted stock units						(4)					$ 4
Issuance of common stock from releases of restricted stock units, shares											390,332
Tax withholding paid on behalf of employees for stock-based awards	(2,410)					(2,410)
Issuance of common stock from warrant exercises						(5)					$ 5
Issuance of common stock from warrant exercises, shares											464,199
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	3,976					3,976
Net loss	(84,990)													(84,990)
Balance at Sep. 30, 2019	107,895					590,878					$ 984			(483,967)
Balance, shares at Sep. 30, 2019											99,405,805
Balance at Jun. 30, 2019	122,917					585,080					$ 973			(463,136)
Balance, shares at Jun. 30, 2019											98,259,203
Issuance of common stock from option exercises	1,438					1,433					$ 5
Issuance of common stock from option exercises, shares											517,624
Stock-based compensation	4,910					4,910
Issuance of common stock from releases of restricted stock units						(3)					$ 3
Issuance of common stock from releases of restricted stock units, shares											335,863
Tax withholding paid on behalf of employees for stock-based awards	(2,410)					(2,410)
Issuance of common stock from warrant exercises						(3)					$ 3
Issuance of common stock from warrant exercises, shares											293,115
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	1,871					1,871
Net loss	(20,831)													(20,831)
Balance at Sep. 30, 2019	$ 107,895					$ 590,878					$ 984			$ (483,967)
Balance, shares at Sep. 30, 2019											99,405,805