Vehicle Floorplan Facilities - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2020	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Vehicle Floorplan Facilities
Line Of Credit Facility [Line Items]
Outstanding borrowings			$ 249,000,000		$ 249,000,000		$ 173,500,000
Interest expense			$ 2,000,000	$ 3,000,000	$ 5,700,000	$ 7,400,000
Weighted average interest rate			4.41%		4.41%		6.00%
Debt instrument, covenant compliance					As of December 31, 2019 and September 30, 2020, the Company was in compliance with all covenants related to the vehicle floorplan facilities.
Vehicle Floorplan Facilities | Credit Balance Agreements
Line Of Credit Facility [Line Items]
Interest credits earned			$ 1,200,000	$ 1,200,000	$ 3,600,000	$ 4,000,000
Ally Bank and Ally Financial | Line Of Credit | 2020 Vehicle Floorplan Facility
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 450,000,000
Expiration term		2021-03
Line of credit facility, description					The amount of credit available is determined on a monthly basis based on a calculation that considers average outstanding borrowings and vehicle units paid off by the Company within the immediately preceding three-month period.
Additional borrowing capacity to be increase in monthly credit line of three month of each year		$ 25,000,000
Line of credit facility, current borrowing capacity			261,100,000		$ 261,100,000
Line of credit facility, unutilized borrowing capacity			$ 12,100,000		$ 12,100,000
Period of LIBOR measurement		1 month
Basis points		4.25%
Debt instrument, covenant description					The 2020 Vehicle Floorplan Facility is collateralized by the Company’s vehicle inventory and certain other assets and the Company is subject to covenants that require it to maintain a certain level of equity in the vehicles that are financed, to maintain at least 10% of the outstanding borrowings in cash and cash equivalents, to maintain 10% of the monthly credit line availability on deposit with Ally Bank and to maintain a minimum tangible adjusted net worth of $167.0 million, which is defined as shareholder (deficit) equity plus redeemable convertible preferred stock as determined under U.S. GAAP.
Debt instrument covenant to maintain minimum percentage of outstanding borrowings in cash and cash equivalents		10.00%
Debt instrument covenant percentage of deposit in basis of monthly credit line availability		10.00%
Debt instrument covenant to maintain minimum tangible adjusted net worth		$ 167,000,000
Debt instrument upfront commitment fee		$ 1,100,000
Ally Bank and Ally Financial | Line Of Credit | Amended 2020 Vehicle Floorplan Facility | Subsequent Event
Line Of Credit Facility [Line Items]
Line of credit facility expiration date	Sep. 30, 2022
Availability fee payable each quarter on average unused capacity from prior quarter floorplan allowance percentage	50.00%
Ally Bank and Ally Financial | Line Of Credit | Amended 2020 Vehicle Floorplan Facility | Subsequent Event | Minimum [Member]
Line Of Credit Facility [Line Items]
Debt instrument covenant to maintain minimum liquidity percentage of outstanding borrowings in cash and cash equivalents	10.00%
Ally Bank and Ally Financial | Line Of Credit | Amended 2020 Vehicle Floorplan Facility | Subsequent Event | Maximum
Line Of Credit Facility [Line Items]
Debt instrument covenant to maintain minimum liquidity percentage of outstanding borrowings in cash and cash equivalents	7.50%
Ally Bank and Ally Financial | Revolving Line of Credit | 2016 Vehicle Floorplan Facility
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity							$ 220,000,000
Period of LIBOR measurement							1 month
Basis points							4.25%