Convertible Preferred Stock and Stockholders' Deficit - Summary of Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2020		Sep. 30, 2021
Temporary Equity [Line Items]
Par Value		$ 0.001		$ 0.001
Shares Authorized		16,572,792	[1]	100,000,000
Preferred Shares Issued		14,376,272	[1]	0
Liquidation Preference		$ 196,528
Carrying Value (2) Share Capital	[2]	$ 353,300
Shares Outstanding		14,376,272	[1]	0
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Feb. 29, 2000
Common stock par or stated value per share		$ 40.81
Shares Authorized	[1]	355,921
Preferred Shares Issued	[1]	355,903
Liquidation Preference		$ 14,523
Carrying Value (2) Share Capital	[2]	$ 13,535
Shares Outstanding	[1]	355,903
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		May 31, 2003
Common stock par or stated value per share		$ 9.07
Shares Authorized	[1]	1,741,452
Preferred Shares Issued	[1]	1,741,399
Liquidation Preference		$ 15,795
Carrying Value (2) Share Capital	[2]	$ 39,715
Shares Outstanding	[1]	1,741,399
Series C Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Feb. 28, 2000
Common stock par or stated value per share		$ 12.92
Shares Authorized	[1]	1,168,344
Preferred Shares Issued	[1]	1,168,311
Liquidation Preference		$ 15,086
Carrying Value (2) Share Capital	[2]	$ 28,136
Shares Outstanding	[1]	1,168,311
Series D Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Aug. 31, 2009
Common stock par or stated value per share		$ 18.08
Shares Authorized	[1]	663,808
Preferred Shares Issued	[1]	663,728
Liquidation Preference		$ 12,000
Carrying Value (2) Share Capital	[2]	$ 18,503
Shares Outstanding	[1]	663,728
Series E Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Oct. 31, 2011
Common stock par or stated value per share		$ 20.66
Shares Authorized	[1]	353,339
Preferred Shares Issued	[1]	353,327
Liquidation Preference		$ 7,300
Carrying Value (2) Share Capital	[2]	$ 10,350
Shares Outstanding	[1]	353,327
Series F Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		May 31, 2012
Common stock par or stated value per share		$ 25.83
Shares Authorized	[1]	507,744
Preferred Shares Issued	[1]	499,159
Liquidation Preference		$ 12,891
Carrying Value (2) Share Capital	[2]	$ 18,305
Shares Outstanding	[1]	499,159
Series G Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Jun. 30, 2015
Common stock par or stated value per share		$ 12.40
Shares Authorized	[1]	5,832,685
Preferred Shares Issued	[1]	5,788,878
Liquidation Preference		$ 71,759
Carrying Value (2) Share Capital	[2]	$ 135,682
Shares Outstanding	[1]	5,788,878
Series H Preferred Stock [Member]
Temporary Equity [Line Items]
Par Value		$ 0.001
Date of Issuance		Feb. 28, 2017
Common stock par or stated value per share		$ 12.40
Shares Authorized	[1]	5,949,499
Preferred Shares Issued	[1]	3,805,567
Liquidation Preference		$ 47,174
Carrying Value (2) Share Capital	[2]	$ 89,074
Shares Outstanding	[1]	3,805,567

[1] The shares authorized, issued and outstanding do not reflect any anti-dilution provisions of Series C, Series D, Series E and Series F as a result of the Series G financing.

[2] 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.