Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Total Stockholders' Equity
Beginning Balance, (shares) at Dec. 31, 2016		80,466,735
Beginning Balance at Dec. 31, 2016		$ 80	$ 648,384	$ (1,579)	$ (421,040)	$ 225,845
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ (92,189)				(92,189)	(92,189)
Other comprehensive income (loss), net	$ 17,578			17,578		17,578
Recognition of stock-based compensation expense			14,227			14,227
Issuance of common stock to 401(k) plan for employees, (shares)		102,874
Issuance of common stock to 401(k) plan for employees			912			912
Issuance of common stock for employee stock purchase plan (shares)		203,229
Issuance of common stock for employee stock purchase plan			1,010			1,010
Issuance of common stock upon exercise of stock options, net (shares)	937,482	864,897
Issuance of common stock upon exercise of stock options, net		$ 1	5,477			5,478
Restricted stock award grants, net of forfeitures (shares)		548,394
Restricted stock award grants, net of forfeitures						0
Repurchase/retirement of restricted stock awards to satisfy employee tax withholding (shares)		(373,822)
Repurchase/retirement of restricted stock awards to satisfy employee tax withholding			(4,331)			(4,331)
Issuance of common shares under an at-the-market sales agreement (Note 5) (shares)		13,558,132
Issuance of common shares under an at-the-market sales agreement (Note 5)		$ 14	128,258			128,272
Common stock redeemed on 2018 Convertible Notes (Note 9) (shares)		5,372,296
Common stock redeemed on 2018 Convertible Notes (Note 9)		$ 5	43,410			43,415
Ending Balance, (shares) at Dec. 31, 2017		100,742,735
Ending Balance at Dec. 31, 2017		$ 100	837,347	15,999	(513,229)	340,217
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ (120,741)				(120,741)	(120,741)
Other comprehensive income (loss), net	$ (2,490)			(2,490)		(2,490)
Recognition of stock-based compensation expense			16,309			16,309
Issuance of common stock to 401(k) plan for employees, (shares)		70,379
Issuance of common stock to 401(k) plan for employees			1,175			1,175
Issuance of common stock for employee stock purchase plan (shares)		97,804
Issuance of common stock for employee stock purchase plan			1,122			1,122
Issuance of common stock upon exercise of stock options, net (shares)	7,858,141	7,858,141
Issuance of common stock upon exercise of stock options, net		$ 8	52,977			52,985
Restricted stock award grants, net of forfeitures (shares)		874,532
Restricted stock award grants, net of forfeitures		$ 1				1
Repurchase/retirement of restricted stock awards to satisfy employee tax withholding (shares)		(3,463,873)
Repurchase/retirement of restricted stock awards to satisfy employee tax withholding		$ (3)	(62,541)			(62,544)
Issuance of common stock upon vesting of restricted stock units (shares)		200,652
Issuance of common stock upon vesting of restricted stock units						0
Issuance of common stock upon exercise of warrants (shares)		292,575
Issuance of common stock upon exercise of warrants						0
Common stock redeemed on 2018 Convertible Notes (Note 9) (shares)		3,852,196
Common stock redeemed on 2018 Convertible Notes (Note 9)		$ 4	40,351			40,355
Ending Balance, (shares) at Dec. 31, 2018	110,525,141	110,525,141
Ending Balance at Dec. 31, 2018	$ 271,410	$ 110	886,740	(3,702)	(611,738)	271,410
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment of adopting ASUs | Accounting Standards Update 2016-01				17,200	17,200
Net loss	(112,689)				(112,689)	(112,689)
Other comprehensive income (loss), net	$ 204			204		204
Recognition of stock-based compensation expense			20,416			20,416
Issuance of common stock to 401(k) plan for employees, (shares)		225,780
Issuance of common stock to 401(k) plan for employees			1,422			1,422
Issuance of common stock for employee stock purchase plan (shares)		131,966
Issuance of common stock for employee stock purchase plan			663			663
Issuance of common stock upon exercise of stock options, net (shares)	1,121,403	1,121,403
Issuance of common stock upon exercise of stock options, net		$ 2	7,147			7,149
Restricted stock award grants, net of forfeitures (shares)		830,033
Restricted stock award grants, net of forfeitures		$ 1				1
Issuance of common stock upon vesting of restricted stock units (shares)		243,760
Issuance of common stock upon vesting of restricted stock units						0
Issuance of common shares under an at-the-market sales agreement (Note 5) (shares)		221,529
Issuance of common shares under an at-the-market sales agreement (Note 5)			1,817			1,817
Ending Balance, (shares) at Dec. 31, 2019	113,299,612	113,299,612
Ending Balance at Dec. 31, 2019	$ 190,393	$ 113	$ 918,205	$ (3,498)	$ (724,427)	$ 190,393