Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	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, 2019	403,835	$ 903	1,568,317	85	(1,165,470)
Balance (in shares) at Dec. 31, 2019		90,304,000
Net loss	(164,997)
Net unrealized gain (loss) on short-term investments	455
Comprehensive loss	(164,542)
Balance at Sep. 30, 2020	277,147	$ 909	1,606,165	540	(1,330,467)
Balance (in shares) at Sep. 30, 2020		90,855,000
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
Conversion benefit included in Convertible Notes issued	109		109
Issuance of common stock under equity incentive plan	805	$ 1	804
Issuance of common stock under equity incentive plan (in shares)		54,000
Issuance of common stock on under the employee stock purchase plan	1,507	$ 1	1,506
Issuance of common stock on under the employee stock purchase plan (in shares)		124,000
Stock-based compensation expense	11,114		11,114
Net loss	(55,190)				(55,190)
Net unrealized gain (loss) on short-term investments	245			245
Comprehensive loss	(54,945)
Balance at Jun. 30, 2020	324,058	$ 908	1,594,436	953	(1,272,239)
Balance (in shares) at Jun. 30, 2020		90,820,000
Conversion benefit included in Convertible Notes issued	111		111
Issuance of common stock under equity incentive plan	524	$ 1	523
Issuance of common stock under equity incentive plan (in shares)		35,000
Stock-based compensation expense	11,095		11,095
Net loss	(58,228)				(58,228)
Net unrealized gain (loss) on short-term investments	(413)			(413)
Comprehensive loss	(58,641)
Balance at Sep. 30, 2020	277,147	$ 909	1,606,165	540	(1,330,467)
Balance (in shares) at Sep. 30, 2020		90,855,000
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
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
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
Issuance of common stock on exercise of stock options (in shares)	369,978
Net loss	$ (166,037)
Net unrealized gain (loss) on short-term investments	(261)
Comprehensive loss	(166,298)
Balance at Sep. 30, 2021	118,843	$ 1,019	1,676,613	(4)	(1,558,785)
Balance (in shares) at Sep. 30, 2021		101,912,000
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
Conversion benefit included in Convertible Notes issued	116		116
Issuance of common stock under equity incentive plan	4,218	$ 4	4,214
Issuance of common stock under equity incentive plan (in shares)		313,000
Issuance of common stock on under the employee stock purchase plan	1,529	$ 1	1,528
Issuance of common stock on under the employee stock purchase plan (in shares)		110,000
Issuance of common stock on exercise of warrants		$ 2	(2)
Issuance of common stock on exercise of warrants (in shares)		195,000
Issuance of common stock on conversion of notes	7,878	$ 98	7,780
Issuance of common stock on conversion of notes (in shares)		9,849,000
Stock-based compensation expense	11,241		11,241
Net loss	(61,015)				(61,015)
Net unrealized gain (loss) on short-term investments	(80)			(80)
Comprehensive loss	(61,095)
Balance at Jun. 30, 2021	160,112	$ 1,019	1,665,429	41	(1,506,377)
Balance (in shares) at Jun. 30, 2021		101,874,000
Issuance of common stock under equity incentive plan	(52)		(52)
Issuance of common stock under equity incentive plan (in shares)		38,000
Stock-based compensation expense	11,236		11,236
Net loss	(52,408)				(52,408)
Net unrealized gain (loss) on short-term investments	(45)			(45)
Comprehensive loss	(52,453)
Balance at Sep. 30, 2021	$ 118,843	$ 1,019	$ 1,676,613	$ (4)	$ (1,558,785)
Balance (in shares) at Sep. 30, 2021		101,912,000