Credit Facilities (Details)	3 Months Ended
	Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Funds immediately available under facility	$ 119,562,000	$ 86,547,000
Immediately available funds under facility	24,500,000	$ 22,500,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,800,000,000
Maturity date of credit facility	Jun. 17, 2021
Number of financial institutions | loan	24
Number of manufacturer-affiliated finance companies | loan	6
Maximum restricted payments	$ 208,500,000
Restricted payment	162,100,000
Floorplan Line
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,750,000,000
Outstanding balance	995,600,000
Available amount under borrowing capacity	444,400,000
Funds immediately available under facility	$ 119,600,000
Weighted average interest rate	3.20%	2.70%
Acquisition Line
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 360,000,000
Minimum borrowing capacity	50,000,000.0
Outstanding balance	39,636,000	$ 26,988,000
Available amount under borrowing capacity	$ 295,100,000
Interest rate	2.24%
Letters of credit outstanding	$ 25,000,000
FMCC Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	300,000,000
Available amount under borrowing capacity	$ 155,900,000
Interest rate	6.50%
Outstanding balance	$ 144,100,000
Basis spread on variable rate	1.50%
UK Credit Facilities
Line of Credit Facility [Line Items]
Outstanding balance	$ 139,500,000
UK Credit Facilities | Minimum
Line of Credit Facility [Line Items]
Interest rate	1.65%
UK Credit Facilities | Maximum
Line of Credit Facility [Line Items]
Interest rate	3.45%
Brazilian Credit Facilities
Line of Credit Facility [Line Items]
Outstanding balance	$ 20,100,000
Brazilian Credit Facilities | Minimum
Line of Credit Facility [Line Items]
Interest rate	10.92%
Grace period	0 days
Brazilian Credit Facilities | Maximum
Line of Credit Facility [Line Items]
Interest rate	16.63%
Grace period	90 days
Rental Vehicles Financed through Credit Facility
Line of Credit Facility [Line Items]
Outstanding balance	$ 108,700,000
Rental Vehicles Financed through Credit Facility | Maximum
Line of Credit Facility [Line Items]
Interest rate	6.50%