Inventory and Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,796.6	$ 3,709.7
New Vehicle Floorplan Facilities
Floorplan Payable [Line Items]
Vehicle floorplan facilities, weighted-average interest rate (percent)	5.90%	6.10%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,700.0
Vehicle floorplan facilities, amount outstanding	$ 3,200.0
Used Vehicle Floorplan Facilities
Floorplan Payable [Line Items]
Vehicle floorplan facilities, weighted-average interest rate (percent)	5.60%	5.80%
Vehicle floorplan facilities, maximum borrowing capacity	$ 710.5
Vehicle floorplan facilities, amount outstanding	629.1
Used vehicle floorplan facilities, unused borrowing capacity	81.4
Used vehicle floorplan facilities, remaining borrowing capacity	$ 0.2