Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 1,248.4		$ 2,759.9
Used vehicle floorplan facilities, remaining borrowing capacity	33.5
Used vehicle floorplan facilities, current borrowing capacity	$ 0.2
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	1.70%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 700.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	1.80%	2.40%
Vehicle floorplan facilities, maximum borrowing capacity	$ 562.0
Vehicle floorplan facilities, amount outstanding	$ 528.5