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, 2017	$ 38,940	$ 115	$ 760,225	$ (97)	$ (721,303)
Balance (in shares) at Dec. 31, 2017		115,555
Issuance of common stock from public offerings, net of offering costs	85,085	$ 18	85,067
Issuance of common stock from public offerings, net of offering costs (in shares)		18,202
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	7,848	$ 3	7,845
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (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 offerings, net of offering costs	26,860	$ 6	26,854
Issuance of common stock from public offerings, net of offering costs (in shares)		5,648
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	3,210	$ 2	3,208
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (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
Issuance of common stock from public offerings, net of offering costs	76,272	$ 19	76,253
Issuance of common stock from public offerings, net of offering costs (in shares)		19,338
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	11,750	$ 5	11,745
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		4,541
Stock-based compensation	18,029		18,029
Other comprehensive income (loss)	560			560
Net loss	(59,857)				(59,857)
Balance at Dec. 31, 2020	$ 103,806	$ 168	$ 1,012,932	$ 674	$ (909,968)
Balance (in shares) at Dec. 31, 2020		168,170