Inventories, net and Notes Payable - Floor Plan, net - Floor Plan Outstanding (Details) - Floor Plan Facility - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 12, 2017
Floor Plan Payable
Total commitment	$ 1,415,000	$ 1,165,000	$ 1,415,000
Less: borrowings	(974,043)	(625,185)
Less: flooring line aggregate interest reduction account	(106,055)	(68,469)
Additional borrowing capacity	334,902	471,346
Less: accounts payable for sold inventory	(31,311)	(21,692)
Less: purchase commitments	(77,144)	(38,765)
Unencumbered borrowing capacity	$ 226,447	$ 410,889