Inventories and Floor Plan Payable - Floor Plan Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Floor Plan Payable
Less: borrowings	$ (42,610)	$ (38,739)
Floor Plan Facility
Floor Plan Payable
Total commitment	1,415,000	1,415,000
Less: borrowings, net	(882,346)	(885,980)
Less: flooring line aggregate interest reduction account	(152,346)	(97,757)
Additional borrowing capacity	380,308	431,263
Less: accounts payable for sold inventory	(76,534)	(33,928)
Less: purchase commitments	(37,671)	(22,530)
Unencumbered borrowing capacity	266,103	374,805
Line of Credit | Floor Plan Facility
Floor Plan Payable
Total commitment	60,000	60,000
Less: borrowings	(42,610)	(38,739)
Additional borrowing capacity	17,390	21,261
Letters of credit | Floor Plan Facility
Floor Plan Payable
Total commitment	15,000	15,000
Less: outstanding letters of credit	(10,380)	(10,380)
Additional letters of credit capacity	$ 4,620	$ 4,620