Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,806.9	$ 3,849.2
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,179.1	2,308.8
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,627.8	$ 1,540.4