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, 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	1,573				1,573
Issuance of common stock in public offerings, net	(205)		(205)
Conversion benefit included in Convertible Notes issued	96		96
Issuance of common stock on exercise of stock options	4,202	$ 4	4,198
Issuance of common stock on exercise of stock options (in shares)		435,000
Stock-based compensation expense	7,701		7,701
Net loss	(52,265)				(52,265)
Net unrealized gain (loss) on short-term investments	(32)			(32)
Comprehensive loss	(52,297)
Balance at Mar. 31, 2018	92,206	$ 650	925,745	(42)	(834,147)
Balance (in shares) at Mar. 31, 2018		65,044,000
Cumulative effect of adoption of new accounting standard at Mar. 31, 2018	1				1
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	1,573				1,573
Net loss	(90,935)
Net unrealized gain (loss) on short-term investments	(15)
Comprehensive loss	(90,950)
Balance at Jun. 30, 2018	431,891	$ 775	1,303,957	(25)	(872,816)
Balance (in shares) at Jun. 30, 2018		77,560,000
Balance at Mar. 31, 2018	92,206	$ 650	925,745	(42)	(834,147)
Balance (in shares) at Mar. 31, 2018		65,044,000
Cumulative effect of adoption of new accounting standard at Mar. 31, 2018	1				1
Issuance of common stock in public offerings, net	363,332	$ 120	363,212
Issuance of common stock in public offerings, net (in shares)		11,963,000
Conversion benefit included in Convertible Notes issued	97		97
Issuance of common stock on exercise of stock options	6,439	$ 5	6,434
Issuance of common stock on exercise of stock options (in shares)		504,000
Issuance of common stock on under the employee stock purchase plan	648		648
Issuance of common stock on under the Employee Stock Purchase Plan (in shares)		49,000
Stock-based compensation expense	7,821		7,821
Net loss	(38,670)				(38,670)
Net unrealized gain (loss) on short-term investments	17			17
Comprehensive loss	(38,653)
Balance at Jun. 30, 2018	431,891	$ 775	1,303,957	(25)	(872,816)
Balance (in shares) at Jun. 30, 2018		77,560,000
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 (loss) 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, 2018	$ 370,160	$ 782	1,330,186	(87)	(960,721)
Balance (in shares) at Dec. 31, 2018		78,174,000
Issuance of common stock on exercise of stock options (in shares)	1,542,000
Net loss	$ (113,234)
Net unrealized gain (loss) on short-term investments	235
Comprehensive loss	(112,999)
Balance at Jun. 30, 2019	305,359	$ 798	1,378,368	148	(1,073,955)
Balance (in shares) at Jun. 30, 2019		79,779,000
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
Conversion benefit included in Convertible Notes issued	103		103
Issuance of common stock on exercise of stock options	9,676	$ 8	9,668
Issuance of common stock on exercise of stock options (in shares)		810,000
Issuance of common stock on under the employee stock purchase plan	1,170	$ 1	1,169
Issuance of common stock on under the Employee Stock Purchase Plan (in shares)		63,000
Stock-based compensation expense	12,706		12,706
Net loss	(50,222)				(50,222)
Net unrealized gain (loss) on short-term investments	112			112
Comprehensive loss	(50,110)
Balance at Jun. 30, 2019	$ 305,359	$ 798	$ 1,378,368	$ 148	$ (1,073,955)
Balance (in shares) at Jun. 30, 2019		79,779,000