Debt Instruments - Floor Plan Facility (Details) - USD ($)	1 Months Ended	9 Months Ended
	Nov. 30, 2017	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of credit, outstanding		$ 349,392,000		$ 248,792,000
Restricted cash		$ 18,471,000		14,443,000	$ 11,755,000	$ 10,266,000
Floor Plan Facility | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity			$ 350,000,000.0	275,000,000.0
Deposit required under floor plan facility, percentage of principal balance		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		5.91%
Line of credit, outstanding		$ 349,400,000
Line of credit facility, remaining borrowing capacity		600,000
Restricted cash		$ 17,500,000		$ 12,400,000
Floor Plan Facility | Line of Credit | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		3.65%