Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2017	$ 13,464		$ 94,339	$ (80,855)	$ (20)
Balance (in shares) at Dec. 31, 2017		35,817,104
Employee stock-based compensation	1,142		1,142
Employee stock-based compensation (in shares)		(22,341)
Issuance of common stock under employee stock plans	313		313
Issuance of common stock under employee stock plans (in shares)		235,685
Issuance of common stock from warrant exercises	4,826		4,826
Issuance of common stock from warrant exercises (in shares)		3,347,346
Sale of common stock, net of issuance costs	130,968	$ 1	130,967
Sale of common stock, net of issuance costs (in shares)		21,275,000
Unrealized gain (loss) on investments	(127)				(127)
Net loss	(10,223)			(10,223)
Balance at Jun. 30, 2018	140,363	$ 1	231,587	(91,078)	(147)
Balance (in shares) at Jun. 30, 2018		60,652,794
Balance at Mar. 31, 2018	72,632	$ 1	157,146	(84,406)	(109)
Balance (in shares) at Mar. 31, 2018		50,933,195
Employee stock-based compensation	531		531
Employee stock-based compensation (in shares)		(22,341)
Issuance of common stock under employee stock plans	242		242
Issuance of common stock under employee stock plans (in shares)		191,292
Issuance of common stock from warrant exercises	1,381		1,381
Issuance of common stock from warrant exercises (in shares)		925,648
Sale of common stock, net of issuance costs	72,287		72,287
Sale of common stock, net of issuance costs (in shares)		8,625,000
Unrealized gain (loss) on investments	(38)				(38)
Net loss	(6,672)			(6,672)
Balance at Jun. 30, 2018	140,363	$ 1	231,587	(91,078)	(147)
Balance (in shares) at Jun. 30, 2018		60,652,794
Balance at Dec. 31, 2018	297,750	$ 1	401,090	(102,918)	(423)
Balance (in shares) at Dec. 31, 2018		71,742,043
Employee stock-based compensation	1,834		1,834
Employee stock-based compensation (in shares)		(14,044)
Issuance of common stock under employee stock plans	171		171
Issuance of common stock under employee stock plans (in shares)		89,127
Issuance of common stock from warrant exercises	535		535
Issuance of common stock from warrant exercises (in shares)		356,363
Unrealized gain (loss) on investments	662				662
Net loss	(12,598)			(12,598)
Balance at Jun. 30, 2019	288,354	$ 1	403,630	(115,516)	239
Balance (in shares) at Jun. 30, 2019		72,173,489
Balance at Mar. 31, 2019	294,574	$ 1	402,464	(107,842)	(49)
Balance (in shares) at Mar. 31, 2019		72,027,657
Employee stock-based compensation	884		884
Employee stock-based compensation (in shares)		(4,464)
Issuance of common stock under employee stock plans	107		107
Issuance of common stock under employee stock plans (in shares)		33,693
Issuance of common stock from warrant exercises	175		175
Issuance of common stock from warrant exercises (in shares)		116,603
Unrealized gain (loss) on investments	288				288
Net loss	(7,674)			(7,674)
Balance at Jun. 30, 2019	$ 288,354	$ 1	$ 403,630	$ (115,516)	$ 239
Balance (in shares) at Jun. 30, 2019		72,173,489