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]
Beginning Balances at Dec. 31, 2014	$ 206,633	$ 690	$ 534,518	$ (328,550)	$ (25)
Beginning Balances, shares at Dec. 31, 2014		69,062,394
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax	4,348	$ 12	4,336
Issuance of common stock upon exercise of stock options and in connection with restricted stock units, net of tax, shares		1,164,033
Issuance of common stock under employee stock purchase plan	$ 911	$ 1	910
Issuance of common stock under employee stock purchase plan, shares	128,181	128,181
Stock-based compensation	$ 11,730		11,730
Claims settlement under Section 16(b), net of tax benefit	9,495		9,495
Comprehensive loss:
Net unrealized gain (loss) on marketable securities, net of tax	25				25
Net loss	(40,703)			(40,703)
Comprehensive loss	(40,678)
Ending Balances at Dec. 31, 2015	192,439	$ 703	560,989	(369,253)
Ending 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	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