Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Secondary Offering [Member]	Common Stock [Member]	Common Stock [Member] Secondary Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Secondary Offering [Member]	Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning Balances at Dec. 31, 2014	$ 167,369		$ 2		$ 278,626			$ (111,259)
Beginning Balances, shares at Dec. 31, 2014			24,245,437
Issuance of stock		$ 80,209		$ 1		$ 80,208
Issuance of stock, shares				4,999,999
Issuance of common stock upon exercise of options	$ 121				121
Issuance of common stock upon exercise of options, shares	90,851		90,851
Issuance of common stock under employee benefit plans	$ 279				279
Issuance of common stock under employee benefit plans, shares			83,960
Stock-based compensation	10,699				10,699
Net loss	(82,177)							(82,177)
Ending Balances at Dec. 31, 2015	176,500		$ 3		369,933			(193,436)
Ending Balances, shares at Dec. 31, 2015			29,420,247
Issuance of stock		$ 59,136				$ 59,136
Issuance of stock, shares				5,176,545
Issuance of common stock upon exercise of options	$ 140				140
Issuance of common stock upon exercise of options, shares	85,646		85,646
Issuance of common stock under employee benefit plans	$ 535				535
Issuance of common stock under employee benefit plans, shares			161,447
Stock-based compensation	10,923				10,923
Unrealized loss on cash flow hedge	(350)						$ (350)
Net loss	(95,817)							(95,817)
Ending Balances at Dec. 31, 2016	151,067		$ 3		440,667		(350)	(289,253)
Ending Balances, shares at Dec. 31, 2016			34,843,885
Issuance of common stock upon exercise of options	$ 2,505		$ 1		2,504
Issuance of common stock upon exercise of options, shares	850,995		850,995
Issuance of common stock under employee benefit plans	$ 464				464
Issuance of common stock under employee benefit plans, shares			243,246
Stock-based compensation	13,349				13,349
Unrealized loss on cash flow hedge	350						$ 350
Net loss	(84,979)							(84,979)
Ending Balances at Dec. 31, 2017	$ 82,756		$ 4		$ 456,984			$ (374,232)
Ending Balances, shares at Dec. 31, 2017			35,938,126