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, 2018	$ 370,160	$ 782	$ 1,330,186	$ (87)	$ (960,721)
Balance (in shares) at Dec. 31, 2018		78,174
Issuance of common stock in public offerings, net	162,151	$ 99	162,052
Issuance of common stock in public offerings, net (in shares)		9,857
Conversion benefit included in Convertible Notes issued	416		416
Issuance of common stock under Employee Stock Purchase Plan	2,109	$ 1	2,108
Issuance of common stock under Employee Stock Purchase Plan (in shares)		126
Issuance of common stock under equity incentive plan	22,164	$ 20	22,144
Issuance of common stock under equity incentive plan (in shares)		1,983
Issuance of common stock on exercise of warrants	1	$ 1
Issuance of common stock on exercise of warrants (in shares)		132
Issuance of common stock on conversion of notes (in shares)		32
Stock-based compensation expense	51,411		51,411
Net loss	(204,749)				(204,749)
Net unrealized gain (loss) on short-term investments	172			172
Comprehensive loss	(204,577)
Balance at Dec. 31, 2019	403,835	$ 903	1,568,317	85	(1,165,470)
Balance (in shares) at Dec. 31, 2019		90,304
Conversion benefit included in Convertible Notes issued	440		440
Issuance of common stock under Employee Stock Purchase Plan	2,317	$ 2	2,315
Issuance of common stock under Employee Stock Purchase Plan (in shares)		194
Issuance of common stock under equity incentive plan	6,759	$ 5	6,754
Issuance of common stock under equity incentive plan (in shares)		545
Issuance of common stock on exercise of warrants	3	$ 3
Issuance of common stock on exercise of warrants (in shares)		267
Issuance of common stock on conversion of Convertible Notes	26		26
Stock-based compensation expense	50,218		50,218
Net loss	(227,278)				(227,278)
Net unrealized gain (loss) on short-term investments	172			172
Comprehensive loss	(227,106)
Balance at Dec. 31, 2020	236,492	$ 913	1,628,070	257	(1,392,748)
Balance (in shares) at Dec. 31, 2020		91,310
Conversion benefit included in Convertible Notes issued	230		230
Issuance of common stock under Employee Stock Purchase Plan	2,139	$ 2	2,137
Issuance of common stock under Employee Stock Purchase Plan (in shares)		175
Issuance of common stock under equity incentive plan	4,926	$ 5	4,921
Issuance of common stock under equity incentive plan (in shares)		476
Issuance of common stock on exercise of warrants		$ 2	(2)
Issuance of common stock on exercise of warrants (in shares)		195
Issuance of common stock on conversion of Convertible Notes	7,878	$ 98	7,780
Issuance of common stock on conversion of notes (in shares)		9,849
Stock-based compensation expense	46,851		46,851
Net loss	(220,683)				(220,683)
Net unrealized gain (loss) on short-term investments	(263)			(263)
Comprehensive loss	(220,946)
Balance at Dec. 31, 2021	$ 77,570	$ 1,020	$ 1,689,987	$ (6)	$ (1,613,431)
Balance (in shares) at Dec. 31, 2021		102,005