Inventories (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 02, 2018	Dec. 31, 2017
Inventories
Retail automotive dealership new vehicles	$ 2,298.8		$ 2,298.8			$ 2,344.1
Retail automotive dealership used vehicles	1,014.8		1,014.8			993.1
Retail automotive parts, accessories and other	124.8		124.8			141.7
Retail commercial truck dealership vehicles and parts	226.3		226.3			207.0
Commercial vehicle distribution vehicles, parts and engines	230.4		230.4			258.2
Total inventories	3,895.1		3,895.1		$ 3,930.7	$ 3,944.1
Interest credits and advertising assistance	$ 13.6	$ 10.6	$ 25.6	$ 20.7