Inventories (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 02, 2018	Dec. 31, 2017
Inventories
Retail automotive dealership new vehicles	$ 2,345.3			$ 2,344.1
Retail automotive dealership used vehicles	1,066.2			993.1
Retail automotive parts, accessories and other	126.2			141.7
Retail commercial truck dealership vehicles and parts	196.6			207.0
Commercial vehicle distribution vehicles, parts and engines	237.9			258.2
Total inventories	3,972.2		$ 3,930.7	$ 3,944.1
Interest credits and advertising assistance	$ 12.0	$ 10.1