Inventories (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Inventories
Retail automotive dealership new vehicles	$ 2,057.1		$ 2,057.1		$ 2,218.6
Retail automotive dealership used vehicles	741.8		741.8		719.0
Retail automotive parts, accessories and other	118.6		118.6		113.6
Retail commercial truck dealership vehicles and parts	230.6		230.6		208.8
Commercial vehicle distribution vehicles and parts	203.7		203.7		203.5
Total inventories	3,351.8		3,351.8		$ 3,463.5
Interest credits and advertising assistance	$ 11.0	$ 14.4	$ 22.4	$ 23.5