Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	[1]	Total
Balance at Dec. 31, 2015	$ 5	$ 436,259	$ (1,416)	$ (208,420)		$ 226,428
Balance (in shares) at Dec. 31, 2015	50,346,000
Issuance of common stock upon exercise of stock options		3,595				3,595
Issuance of common stock upon exercise of stock options (in shares)	1,913,000
Issuance of common stock under employee stock purchase plan		2,589				2,589
Issuance of common stock under employee stock purchase plan (in shares)	341,000
Vesting of restricted stock (in shares)	65,000
Stock based compensation		10,601				10,601
Unrealized (loss) gain on available-for-sale investments			691			691
Net loss				(100,327)		(100,327)
Balance at Dec. 31, 2016	$ 5	453,044	(725)	(308,747)		143,577
Balance (in shares) at Dec. 31, 2016	52,665,000
Issuance of common stock upon exercise of stock options	$ 1	2,314				2,315
Issuance of common stock upon exercise of stock options (in shares)	565,000
Issuance of common stock under employee stock purchase plan		3,344				3,344
Issuance of common stock under employee stock purchase plan (in shares)	446,000
Issuance of common stock in exchange for commitment to provide debt financing		2,448				2,448
Issuance of common stock in exchange for commitment to provide debt financing (in shares)	300,000
Vesting of restricted stock (in shares)	64,000
Stock based compensation		11,402				11,402
Unrealized (loss) gain on available-for-sale investments			(41)			(41)
Net loss				(137,628)		(137,628)
Balance at Dec. 31, 2017	$ 6	472,552	(766)	(446,375)		$ 25,417
Balance (in shares) at Dec. 31, 2017	54,040,000					54,040,000
Issuance of common stock upon exercise of stock options		13,331				$ 13,331
Issuance of common stock upon exercise of stock options (in shares)	1,967,000
Issuance of common stock under employee stock purchase plan		3,617				3,617
Issuance of common stock under employee stock purchase plan (in shares)	391,000
Issuance of common stock upon exercise of warrants		6,762				6,762
Issuance of common stock upon exercise of warrants (in shares)	333,000
Sale of common stock through equity offering, net of issuance costs	$ 1	96,776				96,777
Sale of common stock through equity offering, net of issuance costs (in shares)	5,175,000
Vesting of restricted stock (in shares)	177,000
Stock based compensation		14,198				14,198
Unrealized (loss) gain on available-for-sale investments			214			214
Net loss				(128,154)		(128,154)
Balance at Dec. 31, 2018	$ 7	$ 607,236	$ (552)	$ (574,529)		$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083,000					62,082,348

[1] See Note 2 for a summary of adjustments. The cumulative-effect adjustment to Accumulated Deficit resulting from the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”) as of December 31, 2015 was $41.7 million.