Inventory And Vehicle Floorplan Payable (Components Of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 4,031.1	$ 3,997.7
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,453.7	2,388.0
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,577.4	$ 1,609.7