Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,029.0	$ 2,540.2
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,130.1	1,766.3
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 898.9	$ 773.9