Inventory and Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,806.9	$ 3,849.2
Used vehicle floorplan facilities, remaining borrowing capacity	60.2
Used vehicle floorplan facilities, current borrowing capacity	$ 0.4
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates	2.50%	2.10%
Vehicle floorplan facilities, maximum borrowing capacity	$ 500.0
Vehicle floorplan facilities, amount outstanding	$ 439.8
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates	2.60%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 3,400.0