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, 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)	225,310	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