Condensed Consolidated Statements of Redeemable Common Stock, Stock Options, Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total		Redeemable Common Stock	Notes Receivable for Redeemable Common Stock Issued	Redeemable Stock Options	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Notes Receivable for Common Stock Issued	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2019			$ 56,422	$ 855	$ 59,631	$ 327,581
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2019			3,563,884			14,374,455
Stockholders' Equity, Beginning Balance, Amount at Dec. 31, 2019	$ (419,809)									$ 46	$ (419,855)
Stockholders' Equity, Beginning Balance, Shares at Dec. 31, 2019							1
Exercise of stock options			$ 303		(257)
Exercise of stock options, shares			11,351
Stock-based compensation expense	623							623
Accretion to redemption value of redeemable stock options	2,924				(2,924)						2,924
Accretion to redemption value of redeemable stock	(10,663)		$ 3,493			$ 7,170		(623)			(10,040)
Unrealized gain on short-term investments and cash equivalents, net of tax	77									77
Foreign currency translation adjustment	(1)									(1)
Change in notes receivable for common stock issued				84
Net loss	(16,502)										(16,502)
Temporary Equity, Ending Balance, Amount at Mar. 31, 2020			$ 60,218	(771)	56,450	$ 334,751
Temporary Equity, Ending Balance, Shares at Mar. 31, 2020			3,575,235			14,374,455
Stockholders' Equity, Ending Balance, Amount at Mar. 31, 2020	(443,351)									122	(443,473)
Stockholders' Equity, Ending Balance, Shares at Mar. 31, 2020							1
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2019			$ 56,422	855	59,631	$ 327,581
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2019			3,563,884			14,374,455
Stockholders' Equity, Beginning Balance, Amount at Dec. 31, 2019	(419,809)									46	(419,855)
Stockholders' Equity, Beginning Balance, Shares at Dec. 31, 2019							1
Net loss	1,280
Temporary Equity, Ending Balance, Amount at Sep. 30, 2020			$ 76,463	790	50,680
Temporary Equity, Ending Balance, Amount at Sep. 30, 2020						$ 347,697
Temporary Equity, Ending Balance, Shares at Sep. 30, 2020			3,809,574			14,374,455
Stockholders' Equity, Ending Balance, Amount at Sep. 30, 2020	(445,860)							(0)		(4)	(445,856)
Stockholders' Equity, Ending Balance, Shares at Sep. 30, 2020							1
Temporary Equity, Beginning Balance, Amount at Mar. 31, 2020			$ 60,218	(771)	56,450	$ 334,751
Temporary Equity, Beginning Balance, Shares at Mar. 31, 2020			3,575,235			14,374,455
Stockholders' Equity, Beginning Balance, Amount at Mar. 31, 2020	(443,351)									122	(443,473)
Stockholders' Equity, Beginning Balance, Shares at Mar. 31, 2020							1
Exercise of stock options	5,128				(4,293)
Exercise of stock options, shares			211,590
Stock-based compensation expense	1,083							1,083
Accretion to redemption value of redeemable stock options	2,259				(2,259)						2,259
Accretion to redemption value of redeemable stock	(11,330)		$ 4,975			$ 6,355		(1,083)			(10,247)
Unrealized gain on short-term investments and cash equivalents, net of tax	(112)									(112)
Foreign currency translation adjustment	3									3
Change in notes receivable for common stock issued				(6)
Net loss	(12,882)										(12,882)
Temporary Equity, Ending Balance, Amount at Jun. 30, 2020			$ 70,321	777
Temporary Equity, Ending Balance, Amount at Jun. 30, 2020					49,899	$ 341,106
Temporary Equity, Ending Balance, Shares at Jun. 30, 2020			3,786,825			14,374,455
Stockholders' Equity, Ending Balance, Amount at Jun. 30, 2020	(464,330)									13	(464,343)
Stockholders' Equity, Ending Balance, Shares at Jun. 30, 2020							1
Exercise of stock options			$ 551		(457)
Exercise of stock options, shares			22,749
Stock-based compensation expense	1,242							1,242
Accretion to redemption value of redeemable stock options					1,238						(1,238)
Accretion to redemption value of redeemable stock	(1,238)		$ 5,590		(12,181)	$ 6,591		(1,242)			(10,939)
Unrealized gain on short-term investments and cash equivalents, net of tax	(8)									(8)
Foreign currency translation adjustment	(9)									(9)
Change in notes receivable for common stock issued				(13)
Net loss	30,664										30,664
Temporary Equity, Ending Balance, Amount at Sep. 30, 2020			$ 76,463	790	50,680
Temporary Equity, Ending Balance, Amount at Sep. 30, 2020						$ 347,697
Temporary Equity, Ending Balance, Shares at Sep. 30, 2020			3,809,574			14,374,455
Stockholders' Equity, Ending Balance, Amount at Sep. 30, 2020	(445,860)							(0)		(4)	(445,856)
Stockholders' Equity, Ending Balance, Shares at Sep. 30, 2020							1
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2020	353,300	[1]	$ 80,780		53,085	$ 353,300
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2020	(803)			(803)
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2020			3,813,450			14,376,272
Stockholders' Equity, Beginning Balance, Amount at Dec. 31, 2020	(430,591)									(3)	(430,588)
Stockholders' Equity, Beginning Balance, Shares at Dec. 31, 2020							1
Exercise of stock options			$ 6,922		(5,715)
Exercise of stock options, shares			280,545
Stock-based compensation expense	1,239							1,239
Unrealized gain on short-term investments and cash equivalents, net of tax	7									7
Foreign currency translation adjustment	(4)									(4)
Change in notes receivable for common stock issued				(14)
Reclassification of shares of redeemable common stock to shares of common stock	86,885		$ (87,702)	817			$ 4	87,698	(817)
Reclassification of shares of redeemable common stock to shares of common stock, shares			(4,093,995)				4,093,995
Reclassification of redeemable common stock options to common stock options	47,370				(47,370)			47,370
Net loss											(6,805)
Temporary Equity, Ending Balance, Amount at Mar. 31, 2021			$ 0		0	$ 353,300
Temporary Equity, Ending Balance, Amount at Mar. 31, 2021				0
Temporary Equity, Ending Balance, Shares at Mar. 31, 2021			0			14,376,272
Stockholders' Equity, Ending Balance, Amount at Mar. 31, 2021	(301,899)						$ 4	136,307	(817)		(437,393)
Stockholders' Equity, Ending Balance, Shares at Mar. 31, 2021							4,093,996
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2020	353,300	[1]	$ 80,780		53,085	$ 353,300
Temporary Equity, Beginning Balance, Amount at Dec. 31, 2020	(803)			(803)
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2020			3,813,450			14,376,272
Stockholders' Equity, Beginning Balance, Amount at Dec. 31, 2020	$ (430,591)									(3)	(430,588)
Stockholders' Equity, Beginning Balance, Shares at Dec. 31, 2020							1
Conversion of preferred stock to common stock upon initial public offering, net of fractional shares settled (in shares)	4,093,995
Net loss	$ (32,954)
Temporary Equity, Ending Balance, Amount at Sep. 30, 2021			$ 0	0	0	$ 0
Temporary Equity, Ending Balance, Amount at Sep. 30, 2021	0
Temporary Equity, Ending Balance, Shares at Sep. 30, 2021			0			0
Stockholders' Equity, Ending Balance, Amount at Sep. 30, 2021	$ 150,891						$ 27	614,418	0	(11)	(463,543)
Stockholders' Equity, Ending Balance, Shares at Sep. 30, 2021	27,324,054						27,324,054
Temporary Equity, Beginning Balance, Amount at Mar. 31, 2021			$ 0		0	$ 353,300
Temporary Equity, Beginning Balance, Amount at Mar. 31, 2021				0
Temporary Equity, Beginning Balance, Shares at Mar. 31, 2021			0			14,376,272
Stockholders' Equity, Beginning Balance, Amount at Mar. 31, 2021	$ (301,899)						$ 4	136,307	(817)		(437,393)
Stockholders' Equity, Beginning Balance, Shares at Mar. 31, 2021							4,093,996
Exercise of stock options	125							125
Exercise of stock options, shares							30,215
Stock-based compensation expense	1,406							1,406
Unrealized gain on short-term investments and cash equivalents, net of tax	(4)									(4)
Foreign currency translation adjustment	1									1
Change in notes receivable for common stock issued	452			452
Net loss	(13,477)										(13,477)
Temporary Equity, Ending Balance, Amount at Jun. 30, 2021			$ 0		0	$ 353,300
Temporary Equity, Ending Balance, Amount at Jun. 30, 2021				0
Temporary Equity, Ending Balance, Shares at Jun. 30, 2021			0			14,376,272
Stockholders' Equity, Ending Balance, Amount at Jun. 30, 2021	(313,396)						$ 4	137,838	(365)	(3)	(450,870)
Stockholders' Equity, Ending Balance, Shares at Jun. 30, 2021							4,124,211
Exercise of stock options	46							46
Exercise of stock options, shares							11,051
Stock-based compensation expense	2,013							2,013
Proceeds from notes receivable	136								136
Surrender of common stock in exchange for cancellation of note receivable, Shares							(11,011)
Surrender of common stock in exchange for cancellation of note receivable								(229)	229
Exercise of preferred stock warrants, net of shares withheld for exercise price	(2,011)					$ 3,912		(2,011)
Exercise of preferred stock warrants, net of shares withheld for exercise price, Shares						100,261
Issuance of common stock in connection with initial public offering, net of issuance costs (in shares)							8,248,549
Issuance of common stock in connection with initial public offering, net of issuance costs	119,582						$ 8	119,574
Conversion of preferred stock to common stock upon initial public offering, net of fractional shares settled						$ (357,212)
Conversion of preferred stock to common stock upon initial public offering, net of fractional shares settled (in shares)						(14,476,533)
Conversion of preferred stock to common stock upon initial public offering, net of fractional shares settled (in shares)							14,951,254
Conversion of preferred stock to common stock upon initial public offering, net of fractional shares settled	357,202						$ 15	357,187
Unrealized gain on short-term investments and cash equivalents, net of tax	(5)									(5)
Foreign currency translation adjustment	(3)									(3)
Net loss	(12,673)										(12,673)
Temporary Equity, Ending Balance, Amount at Sep. 30, 2021			$ 0	$ 0	$ 0	$ 0
Temporary Equity, Ending Balance, Amount at Sep. 30, 2021	0
Temporary Equity, Ending Balance, Shares at Sep. 30, 2021			0			0
Stockholders' Equity, Ending Balance, Amount at Sep. 30, 2021	$ 150,891						$ 27	$ 614,418	$ 0	$ (11)	$ (463,543)
Stockholders' Equity, Ending Balance, Shares at Sep. 30, 2021	27,324,054						27,324,054

[1] The carrying value reflects the gross proceeds received from the sale of the preferred stock less issuance costs and the fair value at issuance of preferred stock warrants classified as a liability, plus accretion of redemption value.