Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Beginning balance at Dec. 31, 2017	$ 237,473	$ 8	$ 519,435	$ 1,893	$ (283,863)
Beginning balance, Shares at Dec. 31, 2017		77,736,201
Issuance of common stock upon exercise of stock options	638		638
Issuance of common stock upon exercise of stock options, Shares		712,973
Issuance of common stock upon exercise of warrants, Shares		3,317
Release of restricted stock units		26,175
Stock-based compensation	4,924		4,924
Other comprehensive income (loss)	728			728
Cumulative effect of changes in accounting principles related to revenue recognition	(25,312)				(25,312)
Other	2				2
Net loss	(21,494)				(21,494)
Ending balance at Mar. 31, 2018	196,959	$ 8	524,997	2,621	(330,667)
Ending balance, Shares at Mar. 31, 2018		78,478,666
Beginning balance at Dec. 31, 2017	237,473	$ 8	519,435	1,893	(283,863)
Beginning balance, Shares at Dec. 31, 2017		77,736,201
Unrealized gain/loss on investments	16
Cumulative change in translation adjustment	(642)
Net loss	(69,037)
Ending balance at Sep. 30, 2018	157,695	$ 8	534,632	1,267	(378,212)
Ending balance, Shares at Sep. 30, 2018		79,369,891
Beginning balance at Mar. 31, 2018	196,959	$ 8	524,997	2,621	(330,667)
Beginning balance, Shares at Mar. 31, 2018		78,478,666
Issuance of common stock upon exercise of stock options	622		622
Issuance of common stock upon exercise of stock options, Shares		528,320
Issuance of common stock under Employee Stock Purchase Plan	366		366
Issuance of common stock under Employee Stock Purchase Plan, Shares		52,778
Stock-based compensation	4,327		4,327
Other comprehensive income (loss)	(1,442)			(1,442)
Other	(1)				(1)
Net loss	(24,397)				(24,397)
Ending balance at Jun. 30, 2018	176,434	$ 8	530,312	1,179	(355,065)
Ending balance, Shares at Jun. 30, 2018		79,059,764
Issuance of common stock upon exercise of stock options	99		99
Issuance of common stock upon exercise of stock options, Shares		73,446
Release of restricted stock units		236,681
Stock-based compensation	4,221		4,221
Other comprehensive income (loss)	87			87
Unrealized gain/loss on investments	111
Cumulative change in translation adjustment	(24)
Other				1	(1)
Net loss	(23,146)				(23,146)
Ending balance at Sep. 30, 2018	157,695	$ 8	534,632	1,267	(378,212)
Ending balance, Shares at Sep. 30, 2018		79,369,891
Beginning balance at Dec. 31, 2018	$ 135,311	$ 8	538,895	940	(404,532)
Beginning balance, Shares at Dec. 31, 2018	79,571,714	79,571,714
Issuance of common stock upon exercise of stock options	$ 251		251
Issuance of common stock upon exercise of stock options, Shares		254,481
Release of restricted stock units		25,850
Stock-based compensation	3,703		3,703
Other comprehensive income (loss)	(449)			(449)
Net loss	(23,427)				(23,427)
Ending balance at Mar. 31, 2019	115,389	$ 8	542,849	491	(427,959)
Ending balance, Shares at Mar. 31, 2019		79,852,045
Beginning balance at Dec. 31, 2018	$ 135,311	$ 8	538,895	940	(404,532)
Beginning balance, Shares at Dec. 31, 2018	79,571,714	79,571,714
Issuance of common stock upon exercise of stock options, Shares	548,523
Unrealized gain/loss on investments	$ 324
Cumulative change in translation adjustment	(1,442)
Net loss	(62,957)
Ending balance at Sep. 30, 2019	$ 82,046	$ 8	549,705	(178)	(467,489)
Ending balance, Shares at Sep. 30, 2019	80,514,459	80,514,459
Beginning balance at Mar. 31, 2019	$ 115,389	$ 8	542,849	491	(427,959)
Beginning balance, Shares at Mar. 31, 2019		79,852,045
Issuance of common stock upon exercise of stock options	188		188
Issuance of common stock upon exercise of stock options, Shares		173,925
Issuance of common stock under Employee Stock Purchase Plan	164		164
Issuance of common stock under Employee Stock Purchase Plan, Shares		58,748
Release of restricted stock units		45,556
Stock-based compensation	3,336		3,336
Other comprehensive income (loss)	551			551
Net loss	(18,578)				(18,578)
Ending balance at Jun. 30, 2019	101,050	$ 8	546,537	1,042	(446,537)
Ending balance, Shares at Jun. 30, 2019		80,130,274
Issuance of common stock upon exercise of stock options	93		93
Issuance of common stock upon exercise of stock options, Shares		120,117
Release of restricted stock units		264,068
Stock-based compensation	3,075		3,075
Other comprehensive income (loss)	(1,220)			(1,220)
Unrealized gain/loss on investments	(7)
Cumulative change in translation adjustment	(1,213)
Net loss	(20,952)				(20,952)
Ending balance at Sep. 30, 2019	$ 82,046	$ 8	$ 549,705	$ (178)	$ (467,489)
Ending balance, Shares at Sep. 30, 2019	80,514,459	80,514,459