CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock (Class A and B)	Additional Paid-in-Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Shares, Outstanding, Beginning at Dec. 31, 2018		44,044,000
Beginning of the period at Dec. 31, 2018	$ (9,740)	$ 44	$ 297,145	$ 98	$ (307,027)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	8,193		8,193
Issuance of common stock upon exercise of stock options (in shares)		961,000
Issuance of common stock upon exercise of stock options	11,055	$ 1	11,054
Issuance of common stock under employee stock purchase plan (in shares)		101,000
Issuance of common stock under employee stock purchase plan	2,149		2,149
Issuance of restricted stock units (in shares)		25,000
Tax withholding related to net share settlements of stock-based compensation awards (in shares)		(10,000)
Tax withholding related to net share settlements of stock-based compensation awards	(390)		(390)
Net loss	(7,463)				(7,463)
Other comprehensive income (loss)	52			52
End of period at Mar. 31, 2019	3,856	$ 45	318,151	150	(314,490)
Shares, Outstanding, Ending at Mar. 31, 2019		45,121,000
Shares, Outstanding, Beginning at Dec. 31, 2018		44,044,000
Beginning of the period at Dec. 31, 2018	(9,740)	$ 44	297,145	98	(307,027)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(31,836)
Other comprehensive income (loss)	196
End of period at Sep. 30, 2019	70,134	$ 47	408,656	294	(338,863)
Shares, Outstanding, Ending at Sep. 30, 2019		46,422,000
Shares, Outstanding, Beginning at Mar. 31, 2019		45,121,000
Beginning of the period at Mar. 31, 2019	3,856	$ 45	318,151	150	(314,490)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	8,513		8,513
Issuance of common stock upon exercise of stock options (in shares)		455,000
Issuance of common stock upon exercise of stock options	5,498	$ 1	5,497
Issuance of restricted stock units (in shares)		323,000
Net loss	(8,322)				(8,322)
Other comprehensive income (loss)	82			82
End of period at Jun. 30, 2019	9,627	$ 46	332,161	232	(322,812)
Shares, Outstanding, Ending at Jun. 30, 2019		45,899,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	9,223		9,223
Issuance of common stock upon exercise of stock options (in shares)		401,000
Issuance of common stock upon exercise of stock options	5,940	$ 1	5,939
Issuance of common stock under employee stock purchase plan (in shares)		87,000
Issuance of common stock under employee stock purchase plan	2,773		2,773
Issuance of restricted stock units (in shares)		35,000
Equity component of convertible senior notes, net	58,560		58,560
Net loss	(16,051)				(16,051)
Other comprehensive income (loss)	62			62
End of period at Sep. 30, 2019	70,134	$ 47	408,656	294	(338,863)
Shares, Outstanding, Ending at Sep. 30, 2019		46,422,000
Shares, Outstanding, Beginning at Dec. 31, 2019		46,639,000
Beginning of the period at Dec. 31, 2019	65,343	$ 47	420,170	287	(355,161)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	9,936		9,936
Issuance of common stock upon exercise of stock options (in shares)		225,000
Issuance of common stock upon exercise of stock options	2,794		2,794
Issuance of common stock under employee stock purchase plan (in shares)		94,000
Issuance of common stock under employee stock purchase plan	3,660		3,660
Issuance of restricted stock units (in shares)		117,000
Tax withholding related to net share settlements of stock-based compensation awards (in shares)		(30,000)
Tax withholding related to net share settlements of stock-based compensation awards	(1,379)		(1,379)
Net loss	(10,573)				(10,573)
Other comprehensive income (loss)	(9)			(9)
End of period at Mar. 31, 2020	69,772	$ 47	435,181	278	(365,734)
Shares, Outstanding, Ending at Mar. 31, 2020		47,045,000
Shares, Outstanding, Beginning at Dec. 31, 2019		46,639,000
Beginning of the period at Dec. 31, 2019	$ 65,343	$ 47	420,170	287	(355,161)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	1,038,807
Net loss	$ (40,845)
Other comprehensive income (loss)	200
End of period at Sep. 30, 2020	79,498	$ 48	474,969	487	(396,006)
Shares, Outstanding, Ending at Sep. 30, 2020		48,131,000
Shares, Outstanding, Beginning at Mar. 31, 2020		47,045,000
Beginning of the period at Mar. 31, 2020	69,772	$ 47	435,181	278	(365,734)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	14,894		14,894
Issuance of common stock upon exercise of stock options (in shares)		443,000
Issuance of common stock upon exercise of stock options	6,664	$ 0	6,664
Issuance of restricted stock units (in shares)		153,000
Tax withholding related to net share settlements of stock-based compensation awards (in shares)		(21,000)
Tax withholding related to net share settlements of stock-based compensation awards	(732)		(732)
Net loss	(19,765)				(19,765)
Other comprehensive income (loss)	390			390
End of period at Jun. 30, 2020	71,223	$ 47	456,007	668	(385,499)
Shares, Outstanding, Ending at Jun. 30, 2020		47,620,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	10,601		10,601
Issuance of common stock upon exercise of stock options (in shares)		371,000
Issuance of common stock upon exercise of stock options	4,795	$ 1	4,794
Issuance of common stock under employee stock purchase plan (in shares)		93,000
Issuance of common stock under employee stock purchase plan	3,567		3,567
Issuance of restricted stock units (in shares)		47,000
Net loss	(10,507)				(10,507)
Other comprehensive income (loss)	(181)			(181)
End of period at Sep. 30, 2020	$ 79,498	$ 48	$ 474,969	$ 487	$ (396,006)
Shares, Outstanding, Ending at Sep. 30, 2020		48,131,000