Inventories (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 02, 2018	Dec. 31, 2017
Inventories
Retail automotive dealership new vehicles	$ 2,143.2		$ 2,143.2			$ 2,344.1
Retail automotive dealership used vehicles	1,042.4		1,042.4			993.1
Retail automotive parts, accessories and other	126.3		126.3			141.7
Retail commercial truck dealership vehicles and parts	245.9		245.9			207.0
Commercial vehicle distribution vehicles, parts and engines	225.6		225.6			258.2
Total inventories	3,783.4		3,783.4		$ 3,930.7	$ 3,944.1
Interest credits and advertising assistance	$ 15.1	$ 15.7	$ 40.7	$ 40.2