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Inventory And Vehicle Floorplan Payable (Components Of Inventory) (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Inventory [Line Items]    
Inventory $ 3,692.6 $ 3,520.1
New Vehicle [Member]    
Inventory [Line Items]    
Inventory 2,931.6 2,761.5
Used Vehicle [Member]    
Inventory [Line Items]    
Inventory 562.8 559.1
Parts and Service [Member]    
Inventory [Line Items]    
Inventory $ 198.2 $ 199.5