Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,739.2		$ 3,806.9
Used vehicle floorplan facilities, remaining borrowing capacity	166.4
Used vehicle floorplan facilities, current borrowing capacity	$ 26.7
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.10%	2.30%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,800.0
Vehicle floorplan facilities, amount outstanding	$ 3,400.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.10%	2.20%
Vehicle floorplan facilities, maximum borrowing capacity	$ 500.0
Vehicle floorplan facilities, amount outstanding	$ 333.6