Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income/ (Loss) [Member]	Non-Controlling Interest [Member]
Beginning Balances at Dec. 31, 2015	$ 192,439	$ 703	$ 560,989	$ (369,253)
Beginning Balances, shares at Dec. 31, 2015		70,354,608
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax	(481)	$ 3	(484)
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax, shares		314,583
Issuance of common stock under employee stock purchase plan	$ 818	$ 3	815
Issuance of common stock under employee stock purchase plan, shares	202,711	202,711
Stock-based compensation	$ 15,057		15,057
Comprehensive loss:
Net unrealized gain (loss) on marketable securities, net of tax	20				$ 20
Net loss	(71,658)			(71,658)
Comprehensive loss	(71,638)
Ending Balances at Dec. 31, 2016	136,195	$ 709	576,377	(440,911)	20
Ending Balances, shares at Dec. 31, 2016		70,871,902
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax	15,099	$ 21	15,078
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax, shares		2,101,489
Issuance of common stock under employee stock purchase plan	$ 818	$ 2	816
Issuance of common stock under employee stock purchase plan, shares	253,994	253,994
Issuance of common stock under public offering, net of issuance costs	$ 81,573	$ 124	81,449
Issuance of common stock under public offering, net of issuance costs, shares		12,371,149
Stock-based compensation	9,089		9,089
Comprehensive loss:
Net unrealized gain (loss) on marketable securities, net of tax	(306)				(306)
Net loss	(54,568)			(54,568)
Comprehensive loss	(54,874)
Ending Balances at Dec. 31, 2017	$ 187,900	$ 856	682,809	(495,479)	(286)
Ending Balances, shares at Dec. 31, 2017	85,598,534	85,598,534
Cumulative-effect adjustment of ASC Topic606 on January 1, 2018	$ 1,117			1,117
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax	14,467	$ 20	14,447
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax, shares		2,103,727
Issuance of common stock under employee stock purchase plan	$ 1,484	$ 4	1,480
Issuance of common stock under employee stock purchase plan, shares	328,710	328,710
Issuance of common stock under public offering, net of issuance costs	$ 215,758	$ 142	215,616
Issuance of common stock under public offering, net of issuance costs, shares		14,156,500
Stock-based compensation	14,677		14,677
Additional Paid in Capital for Acquisition of TxCell	603		603
Non-Controlling Interest upon Acquisition of TxCell	1,313					$ 1,313
Comprehensive loss:
Foreign currency translation adjustment	(1,148)				(1,129)	(19)
Net pension losses	(21)				(21)
Net unrealized gain (loss) on marketable securities, net of tax	(4)				(4)
Net loss	(68,889)			(68,334)		(555)
Comprehensive loss	(70,062)
Ending Balances at Dec. 31, 2018	$ 367,257	$ 1,022	$ 929,632	$ (562,696)	$ (1,440)	$ 739
Ending Balances, shares at Dec. 31, 2018	102,187,471	102,187,471