Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive income [Member]	Accumulated deficit [Member]
Beginning balance at Dec. 31, 2013	$ 30,721	$ 11	$ 391,504	$ 1	$ (360,795)
Beginning balance, shares at Dec. 31, 2013		110,409
Issuance of common stock upon exercise of stock options and purchases of ESPP shares	399		399
Issuance of common stock upon exercise of stock options and purchases of ESPP shares, shares		416
Issuance of common stock upon equity financings, net of issuance costs	4,618		4,618
Issuance of common stock upon equity financings, net of issuance cost, shares		2,908
Stock-based compensation expense from stock options and ESPP shares	4,801		4,801
Net loss	(22,110)				(22,110)
Change in unrealized gain on available-for-sale securities, net of tax	86			86
Ending balance at Dec. 31, 2014	18,515	$ 11	401,322	87	(382,905)
Ending balance, shares at Dec. 31, 2014		113,733
Issuance of common stock upon exercise of stock options and purchases of ESPP shares	1,175		1,175
Issuance of common stock upon exercise of stock options and purchases of ESPP shares, shares		1,039
Issuance of common stock upon equity financings, net of issuance costs	14,290	$ 1	14,289
Issuance of common stock upon equity financings, net of issuance cost, shares		7,067
Stock-based compensation expense from stock options and ESPP shares	3,667		3,667
Net loss	(22,663)				(22,663)
Change in unrealized gain on available-for-sale securities, net of tax	(101)			(101)
Ending balance at Dec. 31, 2015	14,883	$ 12	420,453	(14)	(405,568)
Ending balance, shares at Dec. 31, 2015		121,839
Issuance of common stock upon exercise of stock options and purchases of ESPP shares	410		410
Issuance of common stock upon exercise of stock options and purchases of ESPP shares, shares		413
Issuance of common stock upon equity financings, net of issuance costs	23,745	$ 2	23,743
Issuance of common stock upon equity financings, net of issuance cost, shares		19,045
Stock-based compensation expense from stock options and ESPP shares	3,798		3,798
Net loss	(34,509)				(34,509)
Change in unrealized gain on available-for-sale securities, net of tax	11			11
Ending balance at Dec. 31, 2016	$ 8,338	$ 14	$ 448,404	$ (3)	$ (440,077)
Ending balance, shares at Dec. 31, 2016		141,297