Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Balance at Dec. 31, 2018	$ 370,160	$ 782	$ 1,330,186	$ (87)	$ (960,721)
Balance (in shares) at Dec. 31, 2018		78,174,000
Conversion benefit included in Convertible Notes issued	102		102
Issuance of common stock on exercise of stock options	6,539	$ 7	6,532
Issuance of common stock on exercise of stock options (in shares)		732,000
Stock-based compensation expense	17,902		17,902
Net loss	(63,012)				(63,012)
Net unrealized gain on short-term investments	123			123
Comprehensive loss	(62,889)
Balance at Mar. 31, 2019	331,814	$ 789	1,354,722	36	(1,023,733)
Balance (in shares) at Mar. 31, 2019		78,906,000
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 on exercise of stock options	$ 504		504
Issuance of common stock on exercise of stock options (in shares)	70,000	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 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