Inventories (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Inventories
Retail automotive dealership new vehicles	$ 1,671.2		$ 1,810.9
Retail automotive dealership used vehicles	991.5		1,019.7
Retail automotive parts, accessories and other	113.7		117.2
Retail commercial truck dealership vehicles and parts	231.3		223.1
Commercial vehicle distribution vehicles, parts and engines	270.1		254.7
Total inventories	3,277.8		$ 3,425.6
Interest credits and advertising assistance	$ 15.8	$ 11.5