Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2017		81,468,891
Beginning balance at Dec. 31, 2017	$ 235,430	$ 81	$ 364,352	$ (129,003)	$ 0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		187,076
Issuance of common stock pursuant to employee stock purchase program	3,672		3,672
Issuance of common stock pursuant to exercise of stock options (in shares)		2,653,008
Issuance of common stock pursuant to exercise of stock options	13,728	$ 3	13,725
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		127,746
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(38,238)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(705)		(705)
Stock-based compensation	14,835		14,835
Equity component of convertible senior notes, net	27,951		27,951
Net loss	(29,756)			(29,756)
Ending balance (in shares) at Sep. 30, 2018		89,234,819
Ending balance at Sep. 30, 2018	372,748	$ 89	531,418	(158,759)	0
Increase (Decrease) in Stockholders' Equity
Stock Issued During Period, Shares, New Issues		4,836,336
Stock Issued During Period, Value, New Issues	107,593	$ 5	107,588
Beginning balance (in shares) at Jun. 30, 2018		83,785,251
Beginning balance at Jun. 30, 2018	225,613	$ 84	387,764	(162,235)	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to exercise of stock options (in shares)		544,523
Issuance of common stock pursuant to exercise of stock options	2,976		2,976
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		96,724
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(28,015)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(478)		(478)
Stock-based compensation	5,617		5,617
Equity component of convertible senior notes, net	27,951		27,951
Net loss	3,475			3,475
Ending balance (in shares) at Sep. 30, 2018		89,234,819
Ending balance at Sep. 30, 2018	$ 372,748	$ 89	531,418	(158,759)	0
Increase (Decrease) in Stockholders' Equity
Stock Issued During Period, Shares, New Issues	4,836,336
Stock Issued During Period, Value, New Issues	$ 107,593	$ 5	107,588
Beginning balance (in shares) at Dec. 31, 2018	90,151,341	90,151,341
Beginning balance at Dec. 31, 2018	$ 371,938	$ 90	542,829	(170,981)	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		262,110
Issuance of common stock pursuant to employee stock purchase program	$ 3,246		3,246
Issuance of common stock pursuant to exercise of stock options (in shares)	1,397,074	1,397,074
Issuance of common stock pursuant to exercise of stock options	$ 7,667	$ 2	7,665
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		562,755
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(160,964)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(2,856)		(2,856)
Stock-based compensation	20,723		20,723
Other comprehensive income (loss)	26				26
Net loss	$ (73,023)			(73,023)
Ending balance (in shares) at Sep. 30, 2019	92,212,316	92,212,316
Ending balance at Sep. 30, 2019	$ 327,721	$ 92	571,607	(244,004)	26
Beginning balance (in shares) at Jun. 30, 2019		91,777,537
Beginning balance at Jun. 30, 2019	312,245	$ 92	562,894	(250,785)	44
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to exercise of stock options (in shares)		297,475
Issuance of common stock pursuant to exercise of stock options	1,897		1,897
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		195,840
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(58,536)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(1,064)		(1,064)
Stock-based compensation	7,880		7,880
Other comprehensive income (loss)	(18)				(18)
Net loss	$ 6,782			6,782
Ending balance (in shares) at Sep. 30, 2019	92,212,316	92,212,316
Ending balance at Sep. 30, 2019	$ 327,721	$ 92	$ 571,607	$ (244,004)	$ 26