Inventories (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Inventories
Retail automotive dealership new vehicles	$ 2,228.4		$ 2,228.4		$ 2,153.0
Retail automotive dealership used vehicles	926.6		926.6		771.7
Retail automotive parts, accessories and other	129.4		129.4		119.1
Retail commercial truck dealership vehicles and parts	227.8		227.8		170.3
Commercial vehicle distribution vehicles and parts	258.1		258.1		194.1
Total inventories	3,770.3		3,770.3		$ 3,408.2
Interest credits and advertising assistance	$ 15.7	$ 12.9	$ 40.2	$ 38.2