Inventories and Floor Plan Payable - Floor Plan Payable (Details) $ in Thousands		6 Months Ended
	Jul. 01, 2016 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 12, 2017 USD ($)	Jun. 30, 2016 USD ($)
Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity		$ 1,415,000	$ 1,415,000	$ 1,415,000
Quarterly reduction in maximum borrowing capacity				1,750
Letters of credit | Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity				15,000
Line of Credit | Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity		$ 35,000	$ 35,000	$ 35,000
Line of Credit | Notes Payable to Banks | Floor Plan Facility
Floor Plan Payable
Maximum borrowing capacity	$ 1,180,000				$ 880,000
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)		2.15%
Variable rate basis (as a percent)		1.98	1.36
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%
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%
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%
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%