Condensed Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive income (loss) [Member]	Accumulated deficit [Member]
Beginning balance at Dec. 31, 2017	$ 21,488	$ 15	$ 465,246	$ (1)	$ (443,772)
Beginning balance, shares at Dec. 31, 2017		150,837
Adjustment due to changes in accounting policies	470				470
Issuance of common stock upon equity financings, net of issuance costs	13,646	$ 1	13,645
Issuance of common stock upon equity financings, net of issuance cost, shares		8,171
Issuance of common stock upon exercise of stock options	565		565
Issuance of common stock upon exercise of stock options, shares		515
Stock-based compensation expense from stock options and ESPP shares	663		663
Fully vested options issued to settle accrued liabilities	1,860		1,860
Net loss	(8,297)				(8,297)
Ending balance at Mar. 31, 2018	30,395	$ 16	481,979	(1)	(451,599)
Ending balance, shares at Mar. 31, 2018		159,523
Beginning balance at Dec. 31, 2017	21,488	$ 15	465,246	(1)	(443,772)
Beginning balance, shares at Dec. 31, 2017		150,837
Net loss	(15,308)
Ending balance at Jun. 30, 2018	28,269	$ 16	486,863		(458,610)
Ending balance, shares at Jun. 30, 2018		162,001
Beginning balance at Mar. 31, 2018	30,395	$ 16	481,979	(1)	(451,599)
Beginning balance, shares at Mar. 31, 2018		159,523
Issuance of common stock upon equity financings, net of issuance costs	3,134		3,134
Issuance of common stock upon equity financings, net of issuance cost, shares		1,458
Issuance of common stock upon exercise of stock options	1,070		1,070
Issuance of common stock upon exercise of stock options, shares		959
Issuance of common stock upon ESPP purchases	41		41
Issuance of common stock upon ESPP purchases, shares		61
Stock-based compensation expense from stock options and ESPP shares	639		639
Net loss	(7,011)				(7,011)
Change in unrealized loss on available-for-sale securities, net of tax	1			1
Ending balance at Jun. 30, 2018	28,269	$ 16	486,863		(458,610)
Ending balance, shares at Jun. 30, 2018		162,001
Beginning balance at Dec. 31, 2018	20,000	$ 16	488,608		(468,624)
Beginning balance, shares at Dec. 31, 2018		162,060
Issuance of common stock upon equity financings, net of issuance costs	61		61
Issuance of common stock upon equity financings, net of issuance cost, shares		243
Stock-based compensation expense from stock options and ESPP shares	437		437
Fully vested options issued to settle accrued liabilities	994		994
Net loss	(7,130)				(7,130)
Change in unrealized loss on available-for-sale securities, net of tax	(4)			(4)
Ending balance at Mar. 31, 2019	14,358	$ 16	490,100	(4)	(475,754)
Ending balance, shares at Mar. 31, 2019		162,303
Beginning balance at Dec. 31, 2018	20,000	$ 16	488,608		(468,624)
Beginning balance, shares at Dec. 31, 2018		162,060
Net loss	(14,357)
Ending balance at Jun. 30, 2019	22,894	$ 19	505,863	(7)	(482,981)
Ending balance, shares at Jun. 30, 2019		191,931
Beginning balance at Mar. 31, 2019	14,358	$ 16	490,100	(4)	(475,754)
Beginning balance, shares at Mar. 31, 2019		162,303
Issuance of common stock upon equity financings, net of issuance costs	15,319	$ 3	15,316
Issuance of common stock upon equity financings, net of issuance cost, shares		29,571
Issuance of common stock upon ESPP purchases	27		27
Issuance of common stock upon ESPP purchases, shares		57
Stock-based compensation expense from stock options and ESPP shares	420		420
Net loss	(7,227)				(7,227)
Change in unrealized loss on available-for-sale securities, net of tax	(3)			(3)
Ending balance at Jun. 30, 2019	$ 22,894	$ 19	$ 505,863	$ (7)	$ (482,981)
Ending balance, shares at Jun. 30, 2019		191,931