Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 2,540.2	$ 1,898.8
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	1,766.3	1,362.3
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 773.9	$ 536.5