Inventories and Floor Plan Payable - Floor Plan Outstanding (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Oct. 08, 2019
Floor Plan Payable
Less: borrowings	$ (46,340)	$ (38,739)
Floor Plan Facility
Floor Plan Payable
Total commitment	1,415,000	1,415,000	$ 1,380,000
Less: borrowings, net	(693,889)	(885,980)
Less: flooring line aggregate interest reduction account	(156,751)	(97,757)
Additional borrowing capacity	564,360	431,263
Less: accounts payable for sold inventory	(55,804)	(33,928)
Less: purchase commitments	(33,962)	(22,530)
Unencumbered borrowing capacity	474,594	374,805
Line of Credit | Floor Plan Facility
Floor Plan Payable
Total commitment	60,000	60,000
Less: borrowings	(46,340)	(38,739)
Additional borrowing capacity	13,660	21,261
Letters of credit | Floor Plan Facility
Floor Plan Payable
Total commitment	15,000	15,000
Less: outstanding letters of credit	(10,280)	(10,380)
Additional letters of credit capacity	$ 4,720	$ 4,620