Inventories and Floor Plan Payable - Floor Plan Payable (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity	$ 1,415,000	$ 1,415,000
Quarterly reduction in maximum borrowing capacity	3,000	3,000
Letters of credit | Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity	15,000	15,000
Line of Credit | Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity	$ 60,000	$ 60,000
Line of Credit | Notes Payable to Banks | Floor Plan Facility | London Interbank Offered Rate (LIBOR)
Floor Plan Payable
Variable rate spread (as a percent)		2.15%
Line of Credit | Notes Payable to Banks | Floor Plan Facility, floor plan notes | London Interbank Offered Rate (LIBOR)
Floor Plan Payable
Variable rate spread (as a percent)	2.35%
Variable rate basis (as a percent)	2.44	2.35
Line of Credit | Notes Payable to Banks | Floor Plan Facility, floor plan notes | London Interbank Offered Rate (LIBOR) | Minimum
Floor Plan Payable
Variable rate spread (as a percent)	2.05%	2.05%
Line of Credit | Notes Payable to Banks | Floor Plan Facility, floor plan notes | London Interbank Offered Rate (LIBOR) | Maximum
Floor Plan Payable
Variable rate spread (as a percent)	2.50%	2.50%
Line of Credit | Notes Payable to Banks | Floor Plan Facility, floor plan notes | Base Rate | Minimum
Floor Plan Payable
Variable rate spread (as a percent)	0.55%	0.55%
Line of Credit | Notes Payable to Banks | Floor Plan Facility, floor plan notes | Base Rate | Maximum
Floor Plan Payable
Variable rate spread (as a percent)	1.00%	1.00%