Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,575.8	$ 3,997.7
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,120.6	2,388.0
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,455.2	$ 1,609.7