Redeemable Convertible Preferred Stock, Preferred Stock Warrants, and Stockholders’ Equity (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
Preferred Units [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 exercise of warrants for redeemable convertible preferred stock							$ 183,007	$ 0
Redeemable convertible preferred stock, shares outstanding (in shares)							0			102,074,703
Initial Public Offering
Preferred Units [Line Items]
Number of redeemable convertible preferred stock converted (in shares)	120,534,419
Series E Redeemable Convertible Preferred Stock
Preferred Units [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
Accretion of redeemable convertible preferred stock to redemption value				$ 4,100
Series E Redeemable Convertible Preferred Stock | Navistar, Inc.
Preferred Units [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 exercise of warrants for redeemable convertible preferred stock		$ 134,000
Series E-2 Redeemable Convertible Preferred Stock | Traton
Preferred Units [Line Items]
Warrants to purchase preferred stock (in shares)			4,331,644
Exercise price per share (in dollars per share)			$ 11.31
Proceeds from exercise of warrants for redeemable convertible preferred stock			$ 49,000
Redeemable convertible preferred stock
Preferred Units [Line Items]
Accretion of redeemable convertible preferred stock 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
Preferred Units [Line Items]
Redeemable convertible preferred stock, shares issued (in shares)							0
Redeemable convertible preferred stock, shares outstanding (in shares)							0