Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,997.7	$ 3,806.9
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,388.0	2,179.1
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,609.7	$ 1,627.8