Inventory and Vehicle Floorplan Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 2,540.2	1,898.8
Used vehicle floorplan facilities, current borrowing capacity	92.9
Used vehicle floorplan facilities, remaining borrowing capacity	155.5
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	119.5
Vehicle floorplan facilities, maximum borrowing capacity	275.0
Vehicle floorplan facilities, average LIBOR-based interest rates	2.00%	2.30%
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	2,400.0
Vehicle floorplan facilities, maximum borrowing capacity	$ 2,900.0
Vehicle floorplan facilities, average LIBOR-based interest rates	2.10%	2.40%