Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2021	Feb. 28, 2021	Jun. 30, 2020	Mar. 31, 2020
Class Of Stock [Line Items]
Convertible preferred stock shares issued		134,113,000			0
Gross proceeds from issuance of convertible preferred stock	$ 435,538
Outstanding shares of convertible preferred stock converted into shares of common stock						134,100,000
Convertible preferred stock outstanding		134,113,000			0
Series A-1 Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock shares issued				11,500,000
Share price				$ 4.00
Gross proceeds from issuance of convertible preferred stock				$ 45,900
Series A-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock shares issued			56,000,000.0
Share price			$ 4.00
Gross proceeds from issuance of convertible preferred stock			$ 224,000
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock shares issued		27,200,000
Share price		$ 16.00
Gross proceeds from issuance of convertible preferred stock		$ 435,600
Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock outstanding	134,113,000	134,113,000	106,890,000		0		134,113,000	106,890,000