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Inventory and Vehicle Floorplan Payable (Components of Inventory) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Inventory [Line Items]    
Inventory $ 3,520.1 $ 3,612.0
New Vehicle [Member]    
Inventory [Line Items]    
Inventory 2,761.5 2,888.1
Used Vehicle [Member]    
Inventory [Line Items]    
Inventory 559.1 539.7
Parts and Service [Member]    
Inventory [Line Items]    
Inventory $ 199.5 $ 184.2