Debt Instruments - Floor Plan Facility (Details) - USD ($)		1 Months Ended		12 Months Ended
	Nov. 01, 2019	Nov. 30, 2019	Nov. 30, 2017	Dec. 31, 2019	Oct. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Line of credit, outstanding				$ 568,840,000		$ 196,963,000
Restricted cash				42,443,000		$ 9,848,000	$ 14,443,000
Line of Credit | Floor Plan Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity (up to)		$ 950,000,000.0		$ 950,000,000.0	$ 650,000,000.0
Debt instrument, average monthly outstanding balance threshold		$ 500,000,000.0
Deposit required under floor plan facility, percentage of principal balance		7.50%			5.00%
Term of principal payments				5 days
Principal repayment, term threshold after sale of used vehicle			15 days
Principal repayment, term threshold after sale of related finance receivable			2 days
Outstanding balance, days held in inventory threshold				180 days
Outstanding balance, held in inventory, percentage of original principal amount due				10.00%
Outstanding balance, held in inventory, original principal amount, threshold				50.00%
Outstanding balance, held in inventory, wholesale value, threshold				50.00%
Interest rate				4.91%		5.90%
Line of credit, outstanding				$ 515,500,000		$ 197,000,000.0
Line of credit facility, remaining borrowing capacity				434,500,000
Restricted cash				$ 38,700,000		$ 9,800,000
Line of Credit | Floor Plan Facility | Purchase And Sale Agreement And Master Transfer Agreement
Line of Credit Facility [Line Items]
Principal repayment, term threshold after sale of used vehicle				15 days
Principal repayment, term threshold after sale of related finance receivable				1 day
Line of Credit | Floor Plan Facility | LIBOR
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate if average monthly outstanding balance threshold is exceeded		3.15%
Debt instrument, basis spread on variable rate	3.40%