Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,630.5		$ 3,849.2
Used vehicle floorplan facilities, remaining borrowing capacity	115.7
Used vehicle floorplan facilities, current borrowing capacity	$ 0.5
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.70%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 3,200.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.70%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 510.0
Vehicle floorplan facilities, amount outstanding	$ 394.3