Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,682.5		$ 3,575.8
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)	2.80%	4.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 3,300.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.10%	3.90%
Vehicle floorplan facilities, maximum borrowing capacity	$ 482.0
Vehicle floorplan facilities, amount outstanding	$ 383.7