Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Balance at Dec. 31, 2016	$ (21,251)	$ 394	$ 564,343	$ (17)	$ (585,971)
Balance (in shares) at Dec. 31, 2016		39,355,000
Issuance of common stock in public offerings, net	306,279	$ 238	306,041
Issuance of common stock in public offerings, net (in shares)		23,822,000
Conversion benefit included in Convertible Notes issued	369		369
Issuance of common stock under Employee Stock Purchase Plan	989	$ 1	988
Issuance of common stock under Employee Stock Purchase Plan (in shares)		77,000
Issuance of common stock on exercise of stock options	$ 11,463	$ 13	11,450
Issuance of common stock on exercise of stock options (in shares)	1,350,605	1,351,000
Issuance of common stock on exercise of warrants (in shares)		4,000
Issuance of warrants	$ 226		226
Stock-based compensation expense	30,538		30,538
Net loss	(197,484)				(197,484)
Net unrealized gain (loss) on short-term investments	7			7
Comprehensive loss	(197,477)
Balance at Dec. 31, 2017	131,136	$ 646	913,955	(10)	(783,455)
Balance (in shares) at Dec. 31, 2017		64,609,000
Cumulative effect of adoption of new accounting standard at Dec. 31, 2017	1,574				1,574
Issuance of common stock in public offerings, net	363,128	$ 120	363,008
Issuance of common stock in public offerings, net (in shares)		11,963,000
Conversion benefit included in Convertible Notes issued	392		392
Issuance of common stock under Employee Stock Purchase Plan	1,179	$ 1	1,178
Issuance of common stock under Employee Stock Purchase Plan (in shares)		72,000
Issuance of common stock on exercise of stock options	$ 18,301	$ 15	18,286
Issuance of common stock on exercise of stock options (in shares)	1,529,509	1,530,000
Stock-based compensation expense	$ 33,367		33,367
Net loss	(178,840)				(178,840)
Net unrealized gain (loss) on short-term investments	(77)			(77)
Comprehensive loss	(178,917)
Balance at Dec. 31, 2018	370,160	$ 782	1,330,186	(87)	(960,721)
Balance (in shares) at Dec. 31, 2018		78,174,000
Issuance of common stock in public offerings, net	162,151	$ 99	162,052
Issuance of common stock in public offerings, net (in shares)		9,857,000
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)		126,000
Issuance of common stock on exercise of stock options	$ 22,164	$ 20	22,144
Issuance of common stock on exercise of stock options (in shares)	1,983,221	1,983,000
Issuance of common stock on exercise of warrants	$ 1	$ 1
Issuance of common stock on exercise of warrants (in shares)		132,000
Issuance of common stock on conversion of convertible notes (in shares)		32,000
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		90,304,000