Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,481.1		$ 3,997.7
Used vehicle floorplan facilities, remaining borrowing capacity	98.3
Used vehicle floorplan facilities, current borrowing capacity	$ 0.6
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.80%	3.40%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,800.0
Vehicle floorplan facilities, amount outstanding	$ 3,100.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.80%	3.40%
Vehicle floorplan facilities, maximum borrowing capacity	$ 515.0
Vehicle floorplan facilities, amount outstanding	$ 416.7