Inventory and Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,575.8	$ 3,997.7
Used vehicle floorplan facilities, remaining borrowing capacity	91.0
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)	3.70%	3.50%
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)	3.60%	3.50%
Vehicle floorplan facilities, maximum borrowing capacity	$ 515.0
Vehicle floorplan facilities, amount outstanding	$ 424.0