Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Balance at Dec. 31, 2019	$ 403,835	$ 903	$ 1,568,317	$ 85	$ (1,165,470)
Balance (in shares) at Dec. 31, 2019		90,304,000
Conversion benefit included in Convertible Notes issued	108		108
Issuance of common stock under equity incentive plan	504		504
Issuance of common stock under equity incentive plan (in shares)		70,000
Issuance of common stock on exercise of warrants	3	$ 3
Issuance of common stock on exercise of warrants (in shares)		268,000
Stock-based compensation expense	11,974		11,974
Net loss	(51,579)				(51,579)
Net unrealized gain (loss) on short-term investments	623			623
Comprehensive loss	(50,956)
Balance at Mar. 31, 2020	365,468	$ 906	1,580,903	708	(1,217,049)
Balance (in shares) at Mar. 31, 2020		90,642,000
Balance at Dec. 31, 2020	236,492	$ 913	1,628,070	257	(1,392,748)
Balance (in shares) at Dec. 31, 2020		91,310,000
Conversion benefit included in Convertible Notes issued	114		114
Issuance of common stock under equity incentive plan	$ 883	$ 1	882
Issuance of common stock under equity incentive plan (in shares)		97,000
Issuance of common stock on exercise of stock options (in shares)	70,885
Stock-based compensation expense	$ 11,486		11,486
Net loss	(52,614)				(52,614)
Net unrealized gain (loss) on short-term investments	(136)			(136)
Comprehensive loss	(52,750)
Balance at Mar. 31, 2021	$ 196,225	$ 914	$ 1,640,552	$ 121	$ (1,445,362)
Balance (in shares) at Mar. 31, 2021		91,407,000