Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 2,308.6		$ 3,575.8
Used vehicle floorplan facilities, remaining borrowing capacity	288.1
Used vehicle floorplan facilities, current borrowing capacity	$ 251.1
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.00%	3.80%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,800.0
Vehicle floorplan facilities, amount outstanding	$ 2,100.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.40%	3.80%
Vehicle floorplan facilities, maximum borrowing capacity	$ 482.0
Vehicle floorplan facilities, amount outstanding	$ 193.9