Inventories (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Inventories
Retail automotive dealership new vehicles	$ 2,037.5		$ 2,037.5		$ 2,218.6
Retail automotive dealership used vehicles	756.8		756.8		719.0
Retail automotive parts, accessories and other	118.6		118.6		113.6
Retail commercial truck dealership vehicles and parts	180.5		180.5		208.8
Commercial vehicle distribution vehicles and parts	186.3		186.3		203.5
Total inventories	3,279.7		3,279.7		$ 3,463.5
Interest credits and advertising assistance	$ 12.9	$ 12.5	$ 38.2	$ 32.7