Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,382.4	$ 2,109.3
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	1,760.0	946.6
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,622.4	$ 1,162.7