Redeemable Convertible Preferred Stock, Preferred Stock Warrants, and Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended		9 Months Ended
	Apr. 19, 2021	Mar. 31, 2021	Feb. 28, 2021	Jan. 31, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Capitalization Equity [Line Items]
Redeemable convertible preferred stock, shares issued (in shares)							0			102,074,703
Redeemable convertible preferred stock, par value (in dollars per share)							$ 0.0001			$ 0.0001
Proceeds from warrant exercises							$ 183,007	$ 0
Redeemable convertible preferred stock, shares outstanding (in shares)							0			102,074,703
Initial Public Offering
Schedule Of Capitalization Equity [Line Items]
Number of redeemable convertible preferred stock converted (in shares)	120,534,419
Series E Redeemable Convertible Preferred Stock
Schedule Of Capitalization Equity [Line Items]
Redeemable convertible preferred stock, shares issued (in shares)				4,650,999
Redeemable convertible preferred stock, par value (in dollars per share)				$ 14.14
Proceeds from Issuance of convertible preferred stock, net of issuance costs				$ 61,600
Stock issuance costs				4,100
Temporary equity accretion to redemption value				$ 4,100
Series E Redeemable Convertible Preferred Stock | Navistar, Inc.
Schedule Of Capitalization Equity [Line Items]
Warrants to purchase preferred stock (in shares)		9,477,073			9,477,073
Exercise price per share (in dollars per share)		$ 14.14			$ 14.14
Proceeds from warrant exercises		$ 134,000
Series E-2 Redeemable Convertible Preferred Stock | Traton
Schedule Of Capitalization Equity [Line Items]
Warrants to purchase preferred stock (in shares)			4,331,644
Exercise price per share (in dollars per share)			$ 11.31
Proceeds from warrant exercises			$ 49,000
Redeemable convertible preferred stock
Schedule Of Capitalization Equity [Line Items]
Temporary equity accretion to redemption value					$ 4,135	$ 11,943
Redeemable convertible preferred stock, shares outstanding (in shares)		120,534,419			120,534,419	76,788,483	0	76,788,483	0	102,074,703	74,939,388	74,939,388	74,939,388
Redeemable convertible preferred stock | Initial Public Offering
Schedule Of Capitalization Equity [Line Items]
Redeemable convertible preferred stock, shares issued (in shares)							0
Redeemable convertible preferred stock, shares outstanding (in shares)							0