Credit Facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Floorplan Notes Payable Credit Facility	$ 1,133,296	$ 1,072,092
Floorplan Offset	86,547	59,626
Other floorplan notes payable	20,852	4,936
Total floorplan notes payable - credit facility and other net	1,154,148	1,077,028
FMCC Offset	22,500	25,500
Floorplan notes payable - manufacturer affiliates	397,183	392,661
Floorplan Notes Payable Manufacturer Affiliates Net	374,683	367,161
New Vehicles [Member]
Line of Credit Facility [Line Items]
Floorplan Notes Payable Credit Facility	1,019,511	941,913
Used Vehicles [Member]
Line of Credit Facility [Line Items]
Floorplan Notes Payable Credit Facility	176,802	160,070
Rental Vehicle Credit Facility [Member]
Line of Credit Facility [Line Items]
Floorplan Notes Payable Credit Facility	23,530	29,735
FMCC Facility [Member]
Line of Credit Facility [Line Items]
Floorplan Notes Payable Manufacturer Affiliates Gross	152,984	175,244
Floorplan notes payable - manufacturer affiliates	130,484	149,744
Other and Rental Vehicles [Member]
Line of Credit Facility [Line Items]
Floorplan notes payable - manufacturer affiliates	$ 244,199	$ 217,417