Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Controlled Equity Offering Sales Agreement [Member]	Common Stock [Member]	Common Stock [Member] Controlled Equity Offering Sales Agreement [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Controlled Equity Offering Sales Agreement [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2013	$ 154,934		$ 3		$ 217,500			$ (62,569)
Beginning balance,Shares at Dec. 31, 2013			29,587,258
Vesting of restricted stock	305				305
Vesting of restricted stock (in shares)			155,058
Exercise of stock options and shares issued under the employee stock purchase plan	$ 284				284
Exercise of stock options and shares issued under the employee stock purchase plan, (shares)	6,202		112,730
Stock-based compensation expense	$ 14,240				14,240
Issuance of Stock	112,837				112,837
Issuance of Stock (in shares)			2,844,334
Unrealized gain (loss) on investments	(22)						$ (22)
Foreign currency translation adjustment	(7)						(7)
Net loss	(75,777)							(75,777)
Ending balance at Dec. 31, 2014	206,794		$ 3		345,166		(29)	(138,346)
Ending balance,Shares at Dec. 31, 2014			32,699,380
Vesting of restricted stock (in shares)			11,410
Exercise of stock options and shares issued under the employee stock purchase plan	$ 616				616
Exercise of stock options and shares issued under the employee stock purchase plan, (shares)	25,421		82,661
Stock-based compensation expense	$ 17,057				17,057
Issuance of Stock	90,830	$ 1,502		$ 1	90,830	$ 1,501
Issuance of Stock (in shares)			2,950,000	121,314
Unrealized gain (loss) on investments	(167)						(167)
Foreign currency translation adjustment	(86)						(86)
Net loss	(118,181)							(118,181)
Ending balance at Dec. 31, 2015	198,365		$ 4		455,170		(282)	(256,527)
Ending balance,Shares at Dec. 31, 2015			35,864,765
Vesting of restricted stock (in shares)			262,125
Settlements of restricted stock units for tax withholding obligations	(39)				(39)
Settlements of restricted stock units for tax withholding obligations (in shares)			(6,526)
Exercise of stock options and shares issued under the employee stock purchase plan	$ 630				630
Exercise of stock options and shares issued under the employee stock purchase plan, (shares)	46,815		122,383
Stock-based compensation expense	$ 22,283				22,283
Issuance of Stock	50,573				50,573
Issuance of Stock (in shares)			5,645,082
Unrealized gain (loss) on investments	34						34
Foreign currency translation adjustment	(26)						(26)
Net loss	(109,577)							(109,577)
Ending balance at Dec. 31, 2016	$ 162,243		$ 4		$ 528,617		$ (274)	$ (366,104)
Ending balance,Shares at Dec. 31, 2016			41,887,829