CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2016	$ 57,787	$ 103	$ 718,299	$ 103	$ (660,718)
Balance (in shares) at Dec. 31, 2016		103,475
Issuance of common stock from public offering, net of offering costs	30,156	$ 11	30,145
Issuance of common stock from public offering, net of offering costs (in shares)		10,986
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP	2,427	$ 1	2,426
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP (In Shares)		1,094
Stock-based compensation	9,355		9,355
Other comprehensive income (loss)	(200)			(200)
Net loss	(60,585)				(60,585)
Balance at Dec. 31, 2017	38,940	$ 115	760,225	(97)	(721,303)
Balance (in shares) at Dec. 31, 2017		115,555
Issuance of common stock from public offering, net of offering costs	85,085	$ 18	85,067
Issuance of common stock from public offering, net of offering costs (in shares)		18,202
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP	7,848	$ 3	7,845
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP (In Shares)		3,096
Stock-based compensation	10,394		10,394
Other comprehensive income (loss)	(184)			(184)
Net loss	(57,564)				(57,564)
Balance at Dec. 31, 2018	84,519	$ 136	863,531	(281)	(778,867)
Balance (in shares) at Dec. 31, 2018		136,853
Issuance of common stock from public offering, net of offering costs	26,860	$ 6	26,854
Issuance of common stock from public offering, net of offering costs (in shares)		5,648
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP	3,210	$ 2	3,208
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and purchases from ESPP (In Shares)		1,790
Stock-based compensation	13,312		13,312
Other comprehensive income (loss)	395			395
Net loss	(71,244)				(71,244)
Balance at Dec. 31, 2019	$ 57,052	$ 144	$ 906,905	$ 114	$ (850,111)
Balance (in shares) at Dec. 31, 2019		144,291