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	715					711					$ 4
Issuance of common stock from option exercises, shares											396,939
Stock-based compensation	2,873					2,873
Issuance of common stock from releases of restricted stock units, shares											12,468
Issuance of common stock from warrant exercises, shares											1,773
Issuance of common stock, value		$ 30,093	$ 272				$ 30,052	$ 272				$ 41
Issuance of common stock, share			0									4,090,000	33,097
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
Net loss	(26,804)													(26,804)
Balance at Jun. 30, 2018	37,966					386,610					$ 741			(349,385)
Balance, shares at Jun. 30, 2018											75,188,832
Balance at Mar. 31, 2018	57,664					362,142			22,177		$ 710			(327,395)
Temporary equity balance , shares at Mar. 31, 2018				3,000,581
Temporary equity balance at Mar. 31, 2018				$ 30
Balance, shares at Mar. 31, 2018											72,022,089
Issuance of common stock from option exercises	337					336					$ 1
Issuance of common stock from option exercises, shares											131,292
Stock-based compensation	1,683					1,683
Issuance of common stock from warrant exercises, shares											1,773
Issuance of common stock, value			$ 272					$ 272
Issuance of common stock, share													33,097
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
Net loss	(21,990)													(21,990)
Balance at Jun. 30, 2018	37,966					386,610					$ 741			(349,385)
Balance, shares at Jun. 30, 2018											75,188,832
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	$ 8,203					8,185					$ 18
Issuance of common stock from option exercises, shares	1,701,627										1,701,627
Stock-based compensation	$ 9,459					9,459
Issuance of common stock from releases of restricted stock units						(1)					$ 1
Issuance of common stock from releases of restricted stock units, shares											54,469
Issuance of common stock from warrant exercises						(2)					$ 2
Issuance of common stock from warrant exercises, shares											171,084
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	2,105					2,105
Net loss	(64,159)													(64,159)
Balance at Jun. 30, 2019	122,917					585,080					$ 973			(463,136)
Balance, shares at Jun. 30, 2019											98,259,203
Balance at Mar. 31, 2019	141,476					572,855					$ 959			(432,338)
Balance, shares at Mar. 31, 2019											96,933,098
Issuance of common stock from option exercises	5,319					5,308					$ 11
Issuance of common stock from option exercises, shares											1,113,507
Stock-based compensation	4,907					4,907
Issuance of common stock from releases of restricted stock units						(1)					$ 1
Issuance of common stock from releases of restricted stock units, shares											54,469
Issuance of common stock from warrant exercises						(2)					$ 2
Issuance of common stock from warrant exercises, shares											158,129
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	2,013					2,013
Net loss	(30,798)													(30,798)
Balance at Jun. 30, 2019	$ 122,917					$ 585,080					$ 973			$ (463,136)
Balance, shares at Jun. 30, 2019											98,259,203