Inventory And Vehicle Floorplan Payable (Components Of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,739.2	$ 3,806.9
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,233.1	2,179.1
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,506.1	$ 1,627.8