Inventories and Floor Plan Payable - Floor Plan Outstanding (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 12, 2017
Floor Plan Payable
Less: borrowings	$ (24,403)
Floor Plan Facility
Floor Plan Payable
Total commitment	1,415,000	$ 1,415,000	$ 1,415,000
Less: borrowings, net	(734,038)	(974,043)
Less: flooring line aggregate interest reduction account	(147,481)	(106,055)
Additional borrowing capacity	533,481	334,902
Less: accounts payable for sold inventory	(59,236)	(31,311)
Less: purchase commitments	(39,723)	(77,144)
Unencumbered borrowing capacity	434,522	226,447
Line of Credit | Floor Plan Facility
Floor Plan Payable
Total commitment	35,000	35,000	$ 35,000
Less: borrowings	(24,403)
Additional borrowing capacity	10,597	35,000
Letters of credit | Floor Plan Facility
Floor Plan Payable
Total commitment	15,000	15,000
Less: outstanding letters of credit	(9,369)	(9,369)
Additional borrowing capacity	$ 5,631	$ 5,631