Inventories (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Inventories
Retail automotive dealership new vehicles	$ 2,344.1	$ 2,153.0
Retail automotive dealership used vehicles	993.1	771.7
Retail automotive parts, accessories and other	141.7	119.1
Retail commercial truck dealership vehicles and parts	207.0	170.3
Commercial vehicle distribution vehicles, parts and engines	258.2	194.1
Total inventories	3,944.1	3,408.2
Interest credits and advertising assistance	$ 55.4	$ 51.6	$ 43.7