Inventories (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Inventories
Retail automotive dealership new vehicles	$ 1,064.9		$ 1,064.9		$ 1,810.9
Retail automotive dealership used vehicles	1,106.2		1,106.2		1,019.7
Retail automotive parts, accessories and other	114.0		114.0		117.2
Retail commercial truck dealership vehicles and parts	383.2		383.2		223.1
Commercial vehicle distribution vehicles, parts and engines	268.6		268.6		254.7
Total inventories	2,936.9		2,936.9		$ 3,425.6
Interest credits and advertising assistance	$ 21.3	$ 12.7	$ 37.1	$ 24.2