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, 2019		46,639,000
Beginning of the period at Dec. 31, 2019	$ 54,202	$ 47	$ 420,170	$ 287	$ (366,302)
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	(10,418)				(10,418)
Other comprehensive income (loss)	(9)			(9)
Shares, Outstanding, Ending at Mar. 31, 2020		47,045,000
End of period at Mar. 31, 2020	58,786	$ 47	435,181	278	(376,720)
Shares, Outstanding, Beginning at Dec. 31, 2019		46,639,000
Beginning of the period at Dec. 31, 2019	54,202	$ 47	420,170	287	(366,302)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(30,030)
Other comprehensive income (loss)	381
Shares, Outstanding, Ending at Jun. 30, 2020		47,620,000
End of period at Jun. 30, 2020	60,390	$ 47	456,007	668	(396,332)
Shares, Outstanding, Beginning at Mar. 31, 2020		47,045,000
Beginning of the period at Mar. 31, 2020	58,786	$ 47	435,181	278	(376,720)
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	(19,612)				(19,612)
Other comprehensive income (loss)	390			390
Shares, Outstanding, Ending at Jun. 30, 2020		47,620,000
End of period at Jun. 30, 2020	60,390	$ 47	456,007	668	(396,332)
Shares, Outstanding, Beginning at Dec. 31, 2020		48,789,000
Beginning of the period at Dec. 31, 2020	64,277	$ 49	478,698	230	(414,700)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	11,623		11,623
Issuance of common stock upon exercise of stock options (in shares)		312,000
Issuance of common stock upon exercise of stock options	4,138	$ 1	4,137
Issuance of common stock under employee stock purchase plan (in shares)		93,000
Issuance of common stock under employee stock purchase plan	4,237		4,237
Issuance of restricted stock units (in shares)		803,000
Tax withholding related to net share settlements of stock-based compensation awards (in shares)		(70,000)
Tax withholding related to net share settlements of stock-based compensation awards	(7,146)		(7,146)
Net loss	(7,324)				(7,324)
Other comprehensive income (loss)	(49)			(49)
Shares, Outstanding, Ending at Mar. 31, 2021		49,927,000
End of period at Mar. 31, 2021	69,756	$ 50	491,549	181	(422,024)
Shares, Outstanding, Beginning at Dec. 31, 2020		48,789,000
Beginning of the period at Dec. 31, 2020	$ 64,277	$ 49	478,698	230	(414,700)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	429,696
Net loss	$ (16,838)
Other comprehensive income (loss)	(26)
Shares, Outstanding, Ending at Jun. 30, 2021		50,354,000
End of period at Jun. 30, 2021	72,066	$ 50	503,350	204	(431,538)
Shares, Outstanding, Beginning at Mar. 31, 2021		49,927,000
Beginning of the period at Mar. 31, 2021	69,756	$ 50	491,549	181	(422,024)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	11,052		11,052
Issuance of common stock upon exercise of stock options (in shares)		117,000
Issuance of common stock upon exercise of stock options	1,480	$ 0	1,480
Issuance of restricted stock units (in shares)		318,000
Tax withholding related to net share settlements of stock-based compensation awards (in shares)		(8,000)
Tax withholding related to net share settlements of stock-based compensation awards	(731)		(731)
Net loss	(9,514)				(9,514)
Other comprehensive income (loss)	23			23
Shares, Outstanding, Ending at Jun. 30, 2021		50,354,000
End of period at Jun. 30, 2021	$ 72,066	$ 50	$ 503,350	$ 204	$ (431,538)