Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	AOCI Attributable to Parent [Member]	Accumulated Deficit [Member]
Beginning Balance, (shares) at Dec. 31, 2017		100,742,735
Beginning Balance at Dec. 31, 2017	$ 351,339	$ 100	$ 837,347	$ 15,999	$ (502,107)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(15,816)				(15,816)
Other comprehensive income, net	393			393
Employee stock-based compensation expense	4,144		4,144
Issuance of common stock to 401(k) plan for employee match (shares)		16,834
Issuance of common stock to 401(k) plan for employee match	334		334
Issuance of common stock upon exercise of stock options (shares)		5,793,413
Issuance of common stock upon exercise of stock options	41,423	$ 6	41,417
RSA grants, net of forfeitures (shares)		614,035
RSA grants, net of forfeitures	0
Retirement of RSAs and shares as part of stock option cashless exercises to satisfy employee tax withholdings (shares)		(3,463,873)
Retirement of RSAs and shares as part of stock option cashless exercises to satisfy employee tax withholdings	(62,544)	$ (3)	(62,541)
Issuance of common stock upon vesting of RSUs (shares)		200,652
Issuance of common stock upon vesting of RSUs	0
Issuance of common stock upon exercise of warrants (shares)		31,602
Issuance of common stock upon exercise of warrants	0
Ending Balance, (shares) at Mar. 31, 2018		103,935,398
Ending Balance at Mar. 31, 2018	324,293	$ 103	820,701	(819)	(495,692)
Beginning Balance, (shares) at Dec. 31, 2017		100,742,735
Beginning Balance at Dec. 31, 2017	351,339	$ 100	837,347	15,999	(502,107)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(2,074)
Ending Balance, (shares) at Jun. 30, 2018		105,130,603
Ending Balance at Jun. 30, 2018	344,119	$ 103	829,052	(3,088)	(481,948)
Beginning Balance, (shares) at Mar. 31, 2018		103,935,398
Beginning Balance at Mar. 31, 2018	324,293	$ 103	820,701	(819)	(495,692)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	13,744
Other comprehensive income, net	(2,269)			(2,269)
Employee stock-based compensation expense	4,461		4,461
Issuance of common stock to 401(k) plan for employee match (shares)		14,736
Issuance of common stock to 401(k) plan for employee match	272		272
Issuance of common stock for ESPP (shares)		45,543
Issuance of common stock for ESPP	734		734
Issuance of common stock upon exercise of stock options (shares)		732,694
Issuance of common stock upon exercise of stock options	2,884	$ 0	2,884
RSA grants, net of forfeitures (shares)		176,954
RSA grants, net of forfeitures	0
Issuance of common stock upon exercise of warrants (shares)		225,278
Issuance of common stock upon exercise of warrants	0
Ending Balance, (shares) at Jun. 30, 2018		105,130,603
Ending Balance at Jun. 30, 2018	344,119	$ 103	829,052	(3,088)	(481,948)
Beginning Balance, (shares) at Dec. 31, 2018		110,525,141
Beginning Balance at Dec. 31, 2018	283,262	$ 110	886,740	(3,702)	(599,886)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(19,155)				(19,155)
Other comprehensive income, net	(390)			(390)
Employee stock-based compensation expense	7,481		7,481
Issuance of common stock to 401(k) plan for employee match (shares)		47,347
Issuance of common stock to 401(k) plan for employee match	519		519
Issuance of common stock upon exercise of stock options (shares)		146,785
Issuance of common stock upon exercise of stock options	831	$ 0	831
RSA grants, net of forfeitures (shares)		259,539
RSA grants, net of forfeitures	1	$ 1
Issuance of common stock upon vesting of RSUs (shares)		233,760
Issuance of common stock upon vesting of RSUs	0
Ending Balance, (shares) at Mar. 31, 2019		111,212,572
Ending Balance at Mar. 31, 2019	272,549	$ 111	895,571	(4,092)	(619,041)
Beginning Balance, (shares) at Dec. 31, 2018		110,525,141
Beginning Balance at Dec. 31, 2018	283,262	$ 110	886,740	(3,702)	(599,886)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(47,550)
Ending Balance, (shares) at Jun. 30, 2019		112,684,387
Ending Balance at Jun. 30, 2019	254,783	$ 112	905,871	(3,764)	(647,436)
Beginning Balance, (shares) at Mar. 31, 2019		111,212,572
Beginning Balance at Mar. 31, 2019	272,549	$ 111	895,571	(4,092)	(619,041)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(28,395)				(28,395)
Other comprehensive income, net	328			328
Employee stock-based compensation expense	4,814		4,814
Issuance of common stock to 401(k) plan for employee match (shares)		24,382
Issuance of common stock to 401(k) plan for employee match	205		205
Issuance of common stock for ESPP (shares)		60,606
Issuance of common stock for ESPP	444		444
Issuance of common stock upon exercise of stock options (shares)		504,226
Issuance of common stock upon exercise of stock options	3,023	$ 0	3,023
RSA grants, net of forfeitures (shares)		651,072
RSA grants, net of forfeitures	1	$ 1
Issuance of common stock upon vesting of RSUs (shares)		10,000
Issuance of common stock upon vesting of RSUs	$ 0
Issuance of common shares under an at-the-market sales agreement (Note 13) (shares)	221,529	221,529
Issuance of common shares under an at-the-market sales agreement (Note 13)	$ 1,814		1,814
Ending Balance, (shares) at Jun. 30, 2019		112,684,387
Ending Balance at Jun. 30, 2019	$ 254,783	$ 112	$ 905,871	$ (3,764)	$ (647,436)