Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,894.4		$ 3,849.2
Used vehicle floorplan facilities, remaining borrowing capacity	92.2
Used vehicle floorplan facilities, current borrowing capacity	$ 0.4
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.40%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 3,500.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	2.30%	2.00%
Vehicle floorplan facilities, maximum borrowing capacity	$ 490.0
Vehicle floorplan facilities, amount outstanding	$ 397.8