Inventories (Components of Inventories, net) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Inventory [Line Items]
Total inventories	$ 2,128.7	$ 2,433.7
New vehicles
Inventory [Line Items]
Total inventories	1,271.0	1,704.1
Used vehicles
Inventory [Line Items]
Total inventories	765.2	638.1
Parts and accessories
Inventory [Line Items]
Total inventories	$ 92.5	$ 91.5