Debt Instruments - Floor Plan Facility (Details) - USD ($)	1 Months Ended		3 Months Ended	10 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Mar. 31, 2019	Oct. 31, 2018	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Line of credit, outstanding			$ 434,777,000		$ 196,963,000
Restricted cash			$ 21,734,000		$ 9,848,000	$ 18,428,000	$ 14,443,000
Floor Plan Facility | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 650,000,000.0			$ 350,000,000.0
Deposit required under floor plan facility, percentage of principal balance			5.00%		5.00%	5.00%	5.00%
Term of principal payments			5 days
Principal repayment, term threshold after sale of used vehicle		15 days	15 days
Principal repayment, term threshold after sale of related finance receivable		2 days	1 day
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			5.90%		5.90%
Line of credit, outstanding			$ 434,800,000		$ 197,000,000.0
Line of credit facility, remaining borrowing capacity			215,200,000
Current borrowing capacity			181,800,000
Restricted cash			$ 21,700,000		$ 9,800,000
Floor Plan Facility | Line of Credit | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.40%			3.65%