Inventory and Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 1,457.6	$ 2,759.9
Used vehicle floorplan facilities, remaining borrowing capacity	10.6
Used vehicle floorplan facilities, current borrowing capacity	$ 0.1
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, weighted average interest rates on amounts outstanding	1.60%	1.60%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,800.0
Vehicle floorplan facilities, amount outstanding	$ 900.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, weighted average interest rates on amounts outstanding	1.80%	1.80%
Vehicle floorplan facilities, maximum borrowing capacity	$ 592.0
Vehicle floorplan facilities, amount outstanding	$ 581.4