Inventories (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Inventories
Retail automotive dealership new vehicles	$ 2,297.7		$ 2,297.7		$ 2,153.0
Retail automotive dealership used vehicles	892.1		892.1		771.7
Retail automotive parts, accessories and other	127.3		127.3		119.1
Retail commercial truck dealership vehicles and parts	169.4		169.4		170.3
Commercial vehicle distribution vehicles and parts	225.4		225.4		194.1
Total inventories	3,711.9		3,711.9		$ 3,408.2
Interest credits and advertising assistance	$ 10.6	$ 10.4	$ 20.7	$ 21.8