Stockholders' Equity (Deficit) - Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	63,939,109	142,693,665
Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	98,514,932
Redeemable convertible preferred stock warrants
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,682,847
Shares available for grant under employee stock purchase plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	3,900,000	0
Shares available for grant under stock option plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	20,067,705	1,687,947
Restricted stock units
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	155,422	0
Options issued and outstanding under stock option plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	39,815,982	40,807,939