Detail of Certain Balance Sheet Accounts - Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Summary of inventories
New vehicles	$ 1,170,844		$ 1,194,632
Used vehicles	359,997		350,760
Rental vehicles	132,113		144,213
Parts, accessories and other	80,133		82,755
Total inventories	1,743,087		1,772,360
Less lower of cost or net realizable value allowance	9,331		9,067
Inventories, net	$ 1,733,756	$ 1,759,633	$ 1,763,293